Box Compression Strength Calculator: McKee Formula and Stack Height

Updated October 26, 2026McKee formulaFibre Box Handbook factors

Box compression strength is the top load a corrugated box carries before it buckles, and the McKee formula estimates it from three inputs: BCT = 5.87 × ECT × √(caliper × perimeter). A 12 x 12 x 12 inch box in 32 ECT single wall comes out near 520 lb. Real warehouses lose much of that to time under load, humidity and imperfect stacking: 30 days at 60 percent humidity in aligned columns on a gapped pallet leaves about 41 percent, and harsh storage can leave a quarter. Enter your box and conditions below to get its compression strength, usable stacking load and how high you can stack it.

605 lbcompression strength, BCT
0.41strength kept
230 lbusable load on the bottom box
8boxes per column
96 incolumn height

The McKee formula

The simplified McKee formula, published in 1963 and still the standard estimate, predicts the top-to-bottom compression strength of a regular slotted carton:

BCT = 5.87 × ECT × √(caliper × perimeter)
BCT is box compression in pounds, ECT is the board's edge crush value in pounds per inch, caliper is board thickness in inches and perimeter is 2 × (length + width) in inches.

  • Edge crush matters most. Strength rises in direct proportion to ECT.
  • Perimeter and caliper matter as a square root. A box with twice the perimeter is about 41 percent stronger, not twice as strong.
  • Height barely matters within normal shipping sizes, which is why the formula leaves it out.

Industry caliper targets run about 0.15 to 0.18 inch for C-flute single wall and 0.26 to 0.31 inch for BC-flute double wall, rising with the board grade. The calculator starts at 0.16 and 0.28 inch. Use a measured caliper when you have one, because crushed board runs thinner and weaker. McKee found most boxes landed within about 15 percent of the estimate, so treat it as a planning number.

Compression strength of common box sizes

Estimated BCT and the BCT divided by 4 planning load
Box, in Perimeter 32 ECT BCT 32 ECT BCT ÷ 4 44 ECT BCT 44 ECT BCT ÷ 4 48 ECT DW BCT 48 ECT DW BCT ÷ 4
12 x 12 x 12 48 in 521 lb 130 lb 716 lb 179 lb 1,033 lb 258 lb
16 x 12 x 12 56 in 562 lb 141 lb 773 lb 193 lb 1,116 lb 279 lb
18 x 18 x 18 72 in 638 lb 159 lb 877 lb 219 lb 1,265 lb 316 lb
24 x 18 x 12 84 in 689 lb 172 lb 947 lb 237 lb 1,366 lb 342 lb
24 x 24 x 24 96 in 736 lb 184 lb 1,012 lb 253 lb 1,461 lb 365 lb
12 x 10 x 8 44 in 498 lb 125 lb 685 lb 171 lb 989 lb 247 lb
20 x 20 x 20 80 in 672 lb 168 lb 924 lb 231 lb 1,334 lb 333 lb
14 x 14 x 14 56 in 562 lb 141 lb 773 lb 193 lb 1,116 lb 279 lb

How much strength a box loses in storage

BCT is measured on an empty box in a lab press at 50 percent humidity over a few minutes. A box in a warehouse carries its load for weeks, picks up moisture and rarely sits square on the box below it. The Fibre Box Handbook factors below estimate how much strength is left. Multiply the ones that apply.

Compression strength kept under storage conditions, Fibre Box Handbook factors
Condition Strength kept
10 days under load 63%
30 days under load 60%
90 days under load 55%
180 days under load 50%
60% relative humidity 90%
70% relative humidity 80%
80% relative humidity 68%
90% relative humidity 48%
Columns stacked out of line 85 to 90%
Interlocked layers 40 to 60%
Boxes overhanging the pallet 60 to 80%
Gapped pallet deckboards 75 to 90%
Rough or repeated handling 60 to 90%

The factors compound quickly. 90 days at 80 percent humidity in misaligned columns on a gapped pallet multiplies out to 0.55 × 0.68 × 0.85 × 0.82, about 0.26, which is where the common planning rule of dividing BCT by 4 comes from. Newer Fibre Box Association research found aligned columns lose 12 to 20 percent and interlocked loads about half, so test any load that sits near its limit.

How to raise stacking strength

  • Stack in columns, corner over corner. Interlocking the layers for stability gives up a large share of compression strength.
  • Keep boxes on the pallet. Overhang leaves the strongest part of the box, its corners, without support.
  • Fill the box. Contents that support the top panel share the load; empty space lets the walls buckle.
  • Step up the grade to 44 ECT or 48 ECT double wall when the bottom box needs more than the planning load. The weight limits by ECT grade page lists the certified ratings.

Boxes by strength

Frequently asked questions

What is the McKee formula?

BCT = 5.87 × ECT × √(caliper × perimeter). It estimates a box's compression strength in pounds from the board's edge crush value, its thickness in inches and the box perimeter in inches.

How much weight can you stack on a 32 ECT box?

A 12 x 12 x 12 inch 32 ECT box has an estimated compression strength near 520 lb. After 30 days at 60 percent humidity in aligned columns on a gapped pallet it keeps about 41 percent, roughly 210 lb on the bottom box. Ninety days at 80 percent humidity in misaligned columns leaves about 135 lb.

How much strength does a box lose in a warehouse?

About 40 percent after 30 days under load and half after 180 days. Humidity takes more: at 80 percent relative humidity a box keeps about 68 percent of its strength and at 90 percent less than half. Interlocked stacking costs 40 to 60 percent and overhang 20 to 40 percent.

Does box height affect compression strength?

Very little within normal shipping sizes, which is why the McKee formula leaves height out. Very tall, narrow boxes can buckle sooner than the formula predicts.

What is the difference between ECT and BCT?

ECT is a property of the board: the load one inch of board edge carries before crushing. BCT is a property of the finished box and depends on ECT, board thickness and box size.

Is double wall twice as strong as single wall?

Usually more than twice in compression. A 48 ECT double wall box combines higher edge crush with a thicker caliper, so a 12 x 12 x 12 box goes from about 520 lb in 32 ECT to about 1,030 lb.

Related tools and charts

Estimates use the simplified McKee formula published by McKee, Gander and Wachuta in 1963, caliper targets from the Walmart corrugated board specification published by the Fibre Box Association, and Fibre Box Handbook storage factors as cited by the Fibre Box Association. They are planning numbers, not a substitute for a compression test of your packed box.