Box Compression Strength Calculator (McKee Formula, BCT and Safe Stacking)

How the calculator works

The tool splits box strength into the two questions that matter: how strong the box is in the lab, and how much of that strength survives in the real world. You describe the board and the box, then the conditions the load will face, and it works through both in one pass. Pick a board grade from the ECT presets, 23 through 71, or type a custom ECT value, and choose the ECT unit if your spec sheet reads in kN/m or kgf/cm rather than lb/in. Pick the flute, which fills in the board caliper (E at 0.06 in, B at 0.10 in, C at 0.16 in, A at 0.19 in, or BC double wall), or enter a custom caliper. Then give the box length and width, and the box weight so the tool can count the stack.

The first number it settles is the box compression strength, the BCT, from the simplified McKee formula. BCT is 5.87 times the ECT times the square root of the perimeter times the caliper, where the perimeter is twice the sum of the length and the width. In imperial units the ECT is in pounds per inch, the caliper and perimeter are in inches, and the BCT comes out in pounds. On the default, a 32 ECT C-flute box at 20 x 15 in has a perimeter of 70 in and a caliper of 0.16 in, so the BCT is 5.87 x 32 x sqrt(70 x 0.16), which is 629 lb, or 285 kgf. That is the peak strength a conditioned box would show on a compression tester, and it is the number most people stop at. It is also the number that misleads them.

The second half of the tool is where the honest answer lives. It takes the lab BCT and derates it for the conditions you set: the humidity the box will sit in, the time it will hold the load, whether the stack is column or interlocked, and whether the load overhangs the pallet. Each condition has a factor, and the safe stacking load is the lab BCT multiplied by all four factors and then divided by an extra safety factor. On the default, the humidity factor is 1.00, the creep factor is 0.72, the pattern factor is 1.00, and the overhang factor is 1.00, so the combined deration is 72%, and dividing 629 lb by 1.5 after that deration gives a safe stacking load of 302 lb, or 137 kgf.

From the safe load the tool counts the stack. The bottom box carries every box above it, so the maximum number of boxes in a stack is the floor of the safe load divided by the box weight, plus one for the bottom box itself. On the default, a 302 lb safe load and a 20 lb box give a maximum of 16 boxes. The panel also shows the real safety factor, the honest margin against the safe load, which reads 1.01, next to the naive 2.10 you would get by comparing the box weight to the lab BCT alone. The deration waterfall drawn on the canvas shows the whole descent, from the lab BCT down through humidity, creep, pattern, and overhang to the safe load, so you can see exactly which condition costs you the most. All of this feeds the wider work in the Packaging Engineering silo and the Supply Chain hub, where box strength decides how tall a pallet can safely stack.

The McKee formula explained

The McKee formula is the standard shortcut for estimating box compression strength from board properties, and it has been the working tool of corrugated engineers since the 1960s. The tool uses the simplified form, BCT = 5.87 x ECT x sqrt(perimeter x caliper), because it needs only three things you can read off a board spec and a ruler: the edge crush test value, the board caliper, and the box perimeter. The constant 5.87 is a customary-unit fit that already folds the board bending stiffness into a single number, which is what lets the formula skip the stiffness measurement the original version needs. The tool computes in imperial and then converts the result to pounds, kilograms-force, and newtons, using 1 lb equals 0.4536 kgf equals 4.448 N.

The full, original McKee formula is BCT = 2.028 x ECT^0.746 x (geometric-mean flexural stiffness)^0.254 x perimeter^0.492. It is more accurate, but it needs the four-point bending stiffness of the board, a value few buyers have on hand, so the tool uses the simplified form and mentions the full one only for completeness. If you do have measured bending stiffness and want a tighter estimate, the full formula is the one to reach for, but for planning and board selection the simplified form is what the industry uses day to day.

A few things are worth keeping straight about the inputs. The perimeter is the distance around the box footprint, twice the sum of the length and the width, not the box height, because compression strength comes from the vertical walls and the corners, and the perimeter counts how much wall there is. The caliper is the thickness of the combined board, which the flute sets, so a thicker flute gives a higher caliper and a stronger box for the same ECT. And the ECT is the property that scales the whole thing, which is why the next section is about getting the ECT right. The formula assumes a regular slotted case with the flutes running vertically and a roughly even load on the top, which is how most cases ship, so for a standard RSC the estimate is sound.

ECT versus Mullen and the board grades

The single most important input is the ECT, the edge crush test, so it is worth understanding what it measures and why it replaced the older test. ECT measures the force a short column of the board can carry on its edge before it crushes, reported in pounds per inch of load-bearing edge. That is exactly the way a box carries a stack, on its vertical edges, so ECT correlates directly with box compression strength, which is why the McKee formula is built around it. The board grades the tool presets, 23, 26, 29, 32, 44, 48, 51, 55, 61, and 71, are ECT values in lb/in, and they run from light single-wall board at the low end to heavy double-wall at the high end.

The test ECT replaced is the Mullen burst test, which measures how much pressure the board resists before it bursts, reported in pounds per square inch. Burst strength describes how hard the board is to puncture, which matters for rough handling, but it does not describe how the board carries a stacking load, and a high burst board can still crush under a tall stack. The industry moved from specifying board by Mullen to specifying it by ECT precisely because ECT predicts stacking strength and Mullen does not. If your spec still reads in burst, that is a sign the case was designed around the wrong property for stacking, and switching to an ECT spec is the fix.

If your board spec reads in metric, the tool converts for you. One pound per inch is 0.1751 kN/m, or one over 5.5997 kgf/cm, so 32 lb/in is 5.60 kN/m, or 5.71 kgf/cm. Set the ECT unit to match your spec sheet and the tool handles the conversion, so you can enter the number exactly as your board supplier states it. Single-wall board covers the lower grades, and double-wall, such as BC flute, is where the high grades live, because stacking two flutes together raises both the ECT and the caliper.

Flutes and caliper

The flute is the wavy medium sandwiched between the liners, and it sets the caliper, which is the second lever in the McKee formula. The tool presets the common flutes with their typical calipers: E flute at 0.06 in, the thinnest, used for retail and small cases; B flute at 0.10 in, a common shipping flute; C flute at 0.16 in, the default and the workhorse of shipping cases; A flute at 0.19 in, the thickest single wall, with the most cushioning; and BC, a double wall that combines B and C for heavy loads. A taller flute gives a thicker caliper, which raises the square-root term in the formula and so raises the BCT, even at the same ECT.

The reason caliper matters is that box compression strength depends on both how strong the board edge is, the ECT, and how far apart the two liners are held, the caliper. A thicker board resists buckling better, the way a thicker beam resists bending, so for a given ECT a C flute box is stronger than a B flute box of the same size. That is why the flute and the ECT are separate inputs: you can buy the same ECT in different flutes, and the flute changes the answer. The default C flute at 0.16 in is the most common shipping choice because it balances strength, cushioning, and cost.

When you choose a flute the tool fills in a typical caliper, but real board varies, so if your supplier gives you a measured caliper, enter it as a custom value. A worked example below uses a BC double wall at 0.25 in rather than the preset, because the actual board measured that thickness, and the tool takes whatever caliper you give it. Getting the caliper right matters most when you are comparing flutes or grades closely, because it is inside the square root, so a change in caliper moves the BCT less sharply than a change in ECT, but it still moves it.

The safe-stacking deration factors, one by one

The heart of this tool is the set of factors that turn the lab BCT into a safe stacking load, so it is worth walking through each one. The safe load is the lab BCT multiplied by the humidity factor, the creep factor, the pattern factor, and the overhang factor, then divided by an extra safety factor you set. Each factor is a number at or below 1.00, a fraction of the strength that survives that condition, and they multiply, so two moderate factors together can cut the strength more than either alone. The default extra safety factor is 1.5, a common margin that leaves room for the variation you cannot fully predict.

The humidity factor captures how corrugated loses strength as it takes on moisture, and it is the largest single lever. At normal warehouse humidity, 50 to 65% RH, the factor is 1.00, the baseline the lab test is run at. In dry air, 35% RH, the board is actually a little stronger, so the factor rises to 1.20. As humidity climbs the strength falls fast: 0.75 at 85% RH, 0.70 at 95% RH, and 0.50 when the board is saturated or cold and damp. These are the published corrugated-industry figures, and they mean a box near saturation carries only half of its lab strength before any other condition is applied.

The creep factor captures how corrugated weakens under a sustained load over time, a slow collapse called creep. A box that holds its rated load for an hour in a tester may sag and fail under the same load held for weeks, because the fibers keep deforming under constant stress. The factor is 0.95 for under a week, 0.72 for about a month, the default, and 0.68 for three months or more. The pattern factor is simpler: column stacking, box directly over box, keeps the corners aligned and is the baseline at 1.00, while interlocked or cross-tied stacking, which offsets the boxes for stability, trades away up to 30% of the strength and so sits at 0.70. The overhang factor is 1.00 when the boxes sit flush on the pallet, 0.90 for slight overhang, and 0.70 for significant overhang, because a box hanging past the pallet edge loses the corner support that carries most of the load.

Humidity and creep, the two slow killers

Humidity and creep deserve their own section because they are the two factors people most often forget, and together they do the most damage. Both are invisible on a spec sheet and neither shows up in a quick lab test, so a box that passes a compression test on the day it is made can still fail on a pallet three weeks later in a humid warehouse. The lab test conditions the board at 50% RH and loads it to failure in minutes; the real box lives at whatever humidity the warehouse holds and carries its load for as long as it sits, and both differences run against you.

Humidity works on the fibers directly. Corrugated is made of paper, and paper is hygroscopic, so it takes on water from humid air and gives it up in dry air, and its stiffness follows the moisture. Near saturation the board can lose half its strength, which is why the humidity factor drops to 0.50, and why cold, damp storage and refrigerated supply chains are so hard on corrugated. If your product ships into or through a humid climate, or sits in an unconditioned warehouse, the humidity factor is the first thing to set honestly, because it can halve the safe load on its own.

Creep is the time dimension of the same weakness. Under a constant load the board slowly deforms, and the longer it holds the load the less it can carry before it buckles, so a stack that is stable on day one can settle and collapse over a month. The creep factor of 0.72 for a month and 0.68 for three months reflects this, and it compounds with humidity, because a damp box creeps faster than a dry one. The practical lesson is to derate for both the humidity and the storage time your load will actually see, not the short, dry conditions the lab measures, because the safe stacking load is what keeps the bottom box standing for the whole time it holds the stack.

Column versus interlocked, and overhang

How the boxes are arranged on the pallet is a strength decision, not just a stability one. Most of a box’s compression strength lives in its four vertical corners, where the walls meet and the board is doubled, so anything that keeps the corners loaded straight down preserves the strength, and anything that shifts the load off the corners throws it away. Column stacking, box directly over box with corner over corner, keeps the load in the corners and is the baseline at a factor of 1.00. It is the strongest arrangement, and it is what the McKee number assumes.

Interlocked or cross-tied stacking offsets each layer to bind the stack together, like bricks in a wall, and it does make the pallet more stable against toppling and shifting in transit. But it puts the boxes of one layer across the seams of the layer below, so the corners no longer line up, and the load spreads onto the weaker box faces instead of the strong corners. That trade costs up to 30% of the compression strength, which is why the pattern factor for interlocked stacking is 0.70. When you interlock for stability, you have to buy back that strength with a higher board grade, and the reverse mode is the fastest way to see how much higher.

Overhang is the other corner killer, and it is the one that catches export loads. When a box hangs past the pallet edge, the overhanging corner has nothing under it, so that corner carries no load and the remaining corners take more, and the box loses strength fast. A slight overhang costs about 10%, the factor of 0.90, and a significant overhang costs about 30%, the factor of 0.70, and both compound with everything else. The fix is to keep the load flush to the pallet edge, which is also what keeps the deck-area efficiency high on the Cases Per Pallet Calculator, so a load built to tile the deck cleanly also keeps its full box strength. Bottom-case strength and pallet fit are two ends of the same problem, which is why these two tools sit next to each other.

Lab BCT, safe load, and the maximum stack

The three numbers the tool reports, the lab BCT, the safe stacking load, and the maximum boxes in a stack, answer three different questions, and confusing them is the most common mistake in box specification. The lab BCT is a material property, the peak a conditioned box shows in a compression tester, and it is useful for comparing boards but dangerous for planning a stack, because no real box ever operates at that peak. On the default it is 629 lb, which sounds like plenty for a stack of 20 lb boxes, and that impression is exactly the trap.

The safe stacking load is the number to plan around. It is the lab BCT after the humidity, creep, pattern, and overhang deration, divided by the extra safety factor, and on the default it is 302 lb, less than half the lab BCT. That gap, from 629 lb down to 302 lb, is the whole point of the tool: the conditions and the safety margin together cost more than half the lab strength, and the safe load is what the bottom box can actually hold for the life of the stack. The combined deration figure, 72% on the default, tells you how much of the lab strength survives the conditions before the safety factor is applied.

The maximum boxes in a stack turns the safe load into a count. The bottom box carries every box above it, so the maximum is the floor of the safe load divided by the box weight, plus one for the bottom box, which gives 16 on the default. The real safety factor, 1.01, is the honest margin against the safe load, and it sits next to the naive 2.10 you would get by dividing the lab BCT by the load. The naive number looks safe and is not; the real number is tight and is true. Reading the safe load and the real safety factor instead of the lab BCT is the difference between a stack that stands and one that settles.

Reverse-solving the board grade

The reverse mode answers the question most buyers actually have: not how strong is this box, but what board do I need to buy. Switch the mode to reverse and instead of a board grade you enter the load you need to carry safely, along with the same box size and conditions, and the tool works backward to the ECT and the stock grade that will hold it. It is the fastest way to turn a stacking requirement into a purchase order line, and it saves the trial and error of guessing a grade and checking it.

The math runs the forward chain in reverse. The required lab BCT is the target load times the extra safety factor, divided by the combined deration, because you have to start with enough lab strength that what survives the conditions still covers the load with the margin. Then the required ECT is that required BCT divided by 5.87 times the square root of the perimeter times the caliper, which is the McKee formula solved for ECT. Finally the tool recommends the smallest stock grade at or above the required ECT, because you buy board in standard grades, not in arbitrary values.

The fifth worked example below shows the whole flow. To carry 400 lb safely in a 20 x 15 in C-flute box at normal humidity, one month of storage, column-stacked and flush, with a 1.5 safety factor, the required lab BCT is 833 lb, the required ECT is 42.4 lb/in, and the recommended stock grade is 44 ECT, the smallest standard grade above the requirement. That single answer, buy 44 ECT board, is what the reverse mode exists to give you, and it folds the environment into the choice, so the grade you order already accounts for the humidity, the storage time, and the stacking pattern the load will see.

Standards and testing

The McKee estimate is a design tool, not a substitute for a box test, and for anything critical the box should be tested to a recognized standard. Box compression is measured by TAPPI T804 in North America and by ISO 12048 internationally, both of which press a conditioned box to failure and report the peak, which is the lab BCT the McKee formula estimates. The edge crush value the formula depends on is measured by TAPPI T811 and by ISO 3037, so when you specify an ECT grade you are specifying a number that a lab can verify against those methods.

Regional standards mirror these. In Brazil, ABNT NBR 6737 covers the edge crush test, the coluna or column test, and NBR 6739 covers box compression, so a spec written for the Brazilian market maps onto the same two properties. In Mexico, the NMX-N-007 and NMX-N-109 standards cover the corresponding tests. Whatever the market, the pair is the same: an edge crush property for the board and a compression property for the finished box, and the McKee formula is the bridge between them.

The validity of the estimate rests on a few assumptions. The simplified McKee formula assumes a regular slotted case with the flutes vertical and a roughly distributed load on the top, which is how most shipping cases work, and it assumes the board is in the condition the constant was fit for. If your case has large vents or hand holes, an unusual shape, or a concentrated load, the estimate will be off, and if the exact strength matters, for a heavy stack, a fragile product, or a regulated shipment, test conditioned samples to T804 or ISO 12048 rather than relying on the formula alone. Use the tool to choose a grade and size the stack, then confirm the critical cases by test.

Five worked examples

Example 1: default single-wall (the widget opens on this)

This is the case the tool opens on, a common single-wall shipping box in mild conditions, and it shows how far the safe load sits below the lab number even when nothing is against you. A 32 ECT C-flute box, 20 x 15 in, with a 0.16 in caliper, holds 20 lb boxes at 65% RH, about one month of storage, column-stacked, no overhang, with a 1.5 safety factor. The lab BCT is 629 lb (285 kgf). The only deration is the creep factor of 0.72, so the combined deration is 72%, and the safe stacking load is 302 lb (137 kgf), which allows a maximum of 16 boxes in a stack. The real safety factor is 1.01, against a naive 2.10 measured on the lab BCT. The lesson: even in mild conditions the safe load is well under half the lab number, so plan on the safe load, not the BCT.

Example 2: humid warehouse, interlocked stacking

This case stacks a heavy double-wall box in a humid warehouse and cross-ties it for stability, and it shows how two moderate factors compound. A 44 ECT BC double-wall box at 0.25 in caliper, 24 x 18 in, holds 30 lb boxes at 85% RH (factor 0.75), three or more months of storage (0.68), interlocked (0.70), no overhang, with a 1.5 safety factor. The lab BCT is 1,184 lb (537 kgf), a strong box on paper. But the combined deration is only 36%, so the safe stacking load is 282 lb (128 kgf), a maximum of 10 boxes in a stack, and a real safety factor of 1.04 against a naive 4.38. The lesson: humidity, long storage, and cross-stacking together cut nearly two thirds off, so the flattering four-times lab margin is really about one time in the conditions that count.

Example 3: export load with pallet overhang

This case ships a small, strong box for export, in very humid conditions, with the load overhanging the pallet, and it shows how overhang guts the usable strength. A 55 ECT C-flute box, 16 x 12 in, holds 25 lb boxes at 95% RH (0.70), three or more months of storage (0.68), column-stacked but with significant overhang (0.70), and a 1.5 safety factor. The lab BCT is 966 lb (438 kgf). The combined deration is 33%, so the safe stacking load is 215 lb (97 kgf), a maximum of 9 boxes in a stack, and a real safety factor of 1.07 against a naive 4.83. The lesson: overhang on top of export humidity strips most of the strength, so keep boxes flush to the pallet edge, because the overhang factor alone costs the same 30% as cross-stacking.

Example 4: light single-wall box

This case is a small, light box in mild conditions, and it shows that a weak box can still stack high when each layer adds little weight. A 23 ECT B-flute box at 0.10 in caliper, 12 x 10 in, holds 8 lb boxes at 65% RH, about one month of storage, column-stacked, no overhang, with a 1.5 safety factor. The lab BCT is 283 lb (128 kgf), a modest number. The only deration is the creep factor of 0.72, so the combined deration is 72%, and the safe stacking load is 136 lb (62 kgf), which allows a maximum of 17 boxes in a stack, with a real safety factor of 1.06 against a naive 2.21. The lesson: a light box still stacks high because the load grows slowly, so the stack height is set as much by the box weight as by the board grade.

Example 5: reverse solve for the board grade

This case runs the tool in reverse, starting from a load and finding the board to buy. You need to carry 400 lb safely in a 20 x 15 in C-flute box at 0.16 in caliper, at 65% RH, about one month of storage, column-stacked, no overhang, with a 1.5 safety factor. Working backward, the required lab BCT is 833 lb, the required ECT is 42.4 lb/in, and the smallest stock grade at or above that is 44 ECT. The lesson: work backward from the load and the environment to the board grade you buy, and the reverse mode folds the humidity, the storage time, and the stacking pattern into the grade, so the 44 ECT it recommends already covers the conditions the load will face.

Three expert tips

Lab BCT is not the load you can stack

The McKee number is a peak from a conditioned lab test, and treating it as the load you can carry is the mistake that crushes pallets. Real corrugated loses up to half its strength near saturation, weakens over weeks under a sustained load through creep, and loses more to cross-stacking and overhang, and those losses multiply. On the default the lab BCT is 629 lb and the safe load is 302 lb, less than half, from creep and the safety factor alone. Always derate for the actual humidity, storage time, and stacking of your load, and read the safe working load and the real safety factor, not the lab BCT, when you decide how tall to stack.

Corners carry the box, so keep them aligned and supported

Most of a box’s compression strength is in its four vertical corners, so how the load sits on the pallet decides how much of that strength you keep. Column stacking keeps corner over corner and preserves the full strength; interlocking or cross-tying offsets the layers for stability but shifts the load off the corners and costs up to 30%; and letting a box overhang the pallet edge removes a corner’s support entirely, for another loss of up to 30%. Keep the stack in columns and the load flush to the pallet when strength is tight, and if you must interlock or overhang for another reason, buy the strength back with a higher grade using the reverse mode.

Spec by ECT, not by burst (Mullen)

The industry moved from the Mullen burst test to ECT because ECT correlates with stacking strength and Mullen does not, so a box specified by burst is specified around the wrong property for stacking. Use ECT to describe the board, and use the reverse mode of this tool to turn a required stack load and the real environment into the ECT grade to order, which is how the fifth example arrives at 44 ECT for a 400 lb load. Then confirm the choice with a conditioned box compression test to TAPPI T804 or ISO 12048 for anything critical, because a design estimate is a starting point, and a test is the proof.

Limits of the method

This calculator gives a sound first pass at box strength and safe stacking, not a finished packaging engineering study. It estimates the box compression strength from the simplified McKee formula, derates it with the published humidity, creep, pattern, and overhang factors, applies your safety factor, and counts the stack, which is most of what you need to choose a board grade and plan a stack height. It treats the box as a standard regular slotted case with the flutes vertical and a roughly distributed load on the top, and it uses typical deration factors that represent common conditions rather than your exact warehouse.

What it does not do is the detail a full box design carries. The simplified McKee formula uses a single fitted constant in place of the measured board bending stiffness, so it is an estimate, not a test result, and it can be off for boxes with large vents or hand holes, unusual shapes, or concentrated loads. The deration factors are representative figures, and your real humidity, storage time, and handling may differ, so treat the safe load as a planning number and confirm the critical cases by test. Use the result to choose a grade and size the stack, then verify the box compression strength on conditioned samples to TAPPI T804 or ISO 12048, and confirm the pallet fit and the stack stability, before you commit to a design.

Common mistakes to avoid

The first mistake is planning a stack on the lab BCT. The McKee number is a lab peak, and a stack sized on it can settle or collapse in real conditions, so read the safe stacking load, which on the default is less than half the lab BCT. The second is ignoring humidity, the largest single factor, which can halve the strength near saturation, so set the humidity to the real warehouse or transit condition, not the mild lab default, whenever the load will see damp air.

A third mistake is forgetting creep, the slow weakening under a sustained load, which costs about 28% over a month even in mild air, so set the storage time honestly. A fourth is cross-stacking or overhanging without buying back the strength, since each costs up to 30% by moving the load off the corners, so if you interlock for stability or overhang to fit a pallet, raise the grade to cover it. A fifth is specifying board by burst instead of ECT, which describes puncture resistance rather than stacking strength, so spec by ECT and use the reverse mode to find the grade. Read the safe load, set the humidity and the storage time, keep the corners loaded, and spec by ECT, and the numbers the tool gives will match the pallet you build.

Where this calculator fits

It suits anyone choosing a board grade or planning a stack without opening a full packaging study. A packaging engineer designing a case can test how the BCT and the safe load move as the ECT, the flute, and the box size change, and find the grade that carries the stack in the real environment. A logistics or warehouse planner can check how tall a given box can safely stack in their conditions, and see the honest safety factor before building the first pallet. A buyer can run the reverse mode to turn a stacking requirement straight into the ECT grade to order.

Because it separates the lab strength from the safe load, it also builds intuition for where box strength really goes. You can watch the humidity factor halve the safe load, see creep and cross-stacking compound, and compare the same box across grades and flutes in a few clicks. The natural neighbor is the Cases Per Pallet Calculator, because the bottom-case strength this tool reports sets how tall a pallet can safely stack: that tool solves how many cases fit the deck and the height, and this one confirms whether the bottom case can carry them. Together they close the loop from pallet fit to stack safety. The Packaging Engineering hub gathers the case and cushioning tools around these two, and the Supply Chain hub carries the tools that turn a safe, well-built pallet into a trailer and container plan.

Frequently asked questions

Sources, disclaimer, and editorial transparency

The relationships used here follow standard corrugated packaging practice. Box compression strength is estimated with the simplified McKee formula, BCT = 5.87 x ECT x sqrt(perimeter x caliper), where the perimeter is 2 x (length + width), computed in imperial and converted with 1 lb equals 0.4536 kgf equals 4.448 N and 1 lb/in equals 0.1751 kN/m equals one over 5.5997 kgf/cm. The safe stacking load is the lab BCT times the humidity, creep, pattern, and overhang factors, divided by the extra safety factor, and the maximum boxes in a stack is floor(safe load divided by box weight) plus one. The deration factors are the published corrugated-industry figures: humidity 1.20 at 35% RH, 1.00 at 50 to 65% RH, 0.75 at 85%, 0.70 at 95%, and 0.50 saturated; creep 0.95 under a week, 0.72 about a month, and 0.68 at three or more months; pattern 1.00 column and 0.70 interlocked; overhang 1.00 flush, 0.90 slight, and 0.70 significant. The standards referenced are TAPPI T804 and T811, ISO 3037 and ISO 12048, ABNT NBR 6737 and NBR 6739, and NMX-N-007 and NMX-N-109. This calculator and guide are built and reviewed by the OpsCalculators team; see our Editorial Policy for how each tool is researched, built, and tested.

Results are accurate estimates for planning and education, not a substitute for a full packaging engineering study or an engineering review. The simplified McKee formula uses a single fitted constant in place of the measured board bending stiffness, so it is an estimate, not a test result, and it can be off for boxes with large vents or hand holes, unusual shapes, or concentrated loads. The deration factors are representative of common conditions and may differ from your exact humidity, storage time, and handling. Confirm the box compression strength on conditioned samples to TAPPI T804 or ISO 12048, and confirm the pallet fit and the stack stability, against your board specification, a packaging engineer, and the relevant standards before you commit to a design. See our full Disclaimer. OpsCalculators.com is operated by MAFHH INTERNATIONAL LTD. Your inputs are processed in your browser and are never stored; see our Privacy Policy.