Box Compression Strength Calculator (McKee BCT)

—Allowable load per box in the stated conditions

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From test strength to allowable load

McKee BCT and what is left after each loss. The line is the actual load on the bottom box.

Tap the chart to see the values.

Load on the bottom box versus stack height

Load on the base box for each number of layers in a single column (one pallet), compared with the allowable load.

Tap the chart to see the values.

The calculation report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. The full report also includes the charts and interpretations, in color. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Simplified McKee: BCT = 5.874 · ECT · √(t · Z); Z = 2 · (L + W); dimensionally consistent: BCT in lbf with ECT in lbf/in and t, Z in inches

Allowable load = BCT · (1 − l_humidity) · (1 − l_time) · (1 − l_pattern) · (1 − l_other) ÷ SF

Load on bottom box = (layers − 1) · W_box + (pallets − 1) · (layers · boxes/layer · W_box + W_pallet) ÷ boxes/layer

Max layers = ⌊allowable ÷ W_box⌋ + 1; safety ratio = allowable ÷ actual load

Assumptions and limits

Regular slotted container of corrugated board, load spread over the top and no structural help from the product. The simplified McKee formula applies to boxes that are not too shallow (depth above about perimeter ÷ 7) with usual proportions.
Published reference losses (Fibre Box Handbook and packaging literature): humidity 10%, 20%, 32% and 52% at 60%, 70%, 80% and 90% RH; time 37%, 40%, 45% and 50% at 10, 30, 90 and 180 days; pattern 2% to 8% (aligned column), 10% to 15% (misaligned), 40% to 60% (interlocked), 20% to 40% (overhang). Losses are multiplied together.

Standards and technical documents

McKee, R. C.; Gander, J. W.; Wachuta, J. R. Compression strength formula for corrugated boxes. Paperboard Packaging, v. 48, n. 8, 1963
Fibre Box Association. Fibre Box Handbook
ISO 3037: Corrugated fibreboard — Determination of edgewise crush resistance (unwaxed edge method)
ISO 12048: Packaging — Complete, filled transport packages — Compression and stacking tests using a compression tester
TAPPI T 839 (ECT, clamp method) and TAPPI T 804 (compression test of fiberboard shipping containers)
FEFCO Code: International fibreboard case code

U.S. regulations

OSHA 29 CFR 1910.176(b): Secure storage — stacked materials shall be stable and secure against sliding or collapse
Uniform Freight Classification Rule 41 and National Motor Freight Classification Item 222: corrugated box construction requirements (ECT or burst test)
ASTM D642: Standard Test Method for Determining Compressive Resistance of Shipping Containers

Frequently asked questions

What is the McKee formula?

A formula published in 1963 that estimates corrugated box compression strength (BCT) from three inputs: board ECT, board caliper and box perimeter. In simplified form, BCT = 5.87 × ECT × √(caliper × perimeter). It lets you choose the board before making samples and running tests.

Where do I get ECT and caliper?

ECT is in the board grade, often printed on the box maker’s certificate (for example 32 ECT, meaning 32 lbf per inch), and is tested by TAPPI T 839. Caliper is measured with a caliper gauge without crushing the board: C flute is about 5/32 in and BC double wall about 1/4 in.

How do I read the example result?

A 16 × 12 × 12 in box in 32 ECT C flute has a McKee BCT of 556 lbf. At 70% humidity, 30 days in storage and column stacking, 46% remains: 254 lbf. With 5 layers and 2 pallets high, the bottom box carries 231 lbf, a safety ratio of only 1.10.

Why does strength drop so much in the warehouse?

Board absorbs moisture from the air and creeps under constant load. Published losses are about 20% at 70% relative humidity and 40% after 30 days stacked; interlocked pallet patterns cost 40% to 60%. That is why, without test data, a BCT of 3 to 5 times the load is a common requirement.

How high can I stack the boxes?

Divide the allowable load by the weight of one box and add 1 (the bottom box itself). In the example, 254 lbf ÷ 25 lb gives 10, or 11 layers in a single column. With stacked pallets the upper pallets’ weight adds in and the limit drops; the page shows how many 5-layer pallets fit.

What are the formula’s limits?

It applies to regular slotted containers of usual proportions that are not too shallow, and assumes the product carries no load. Formula and test typically agree within about 10%. For critical packaging, run a compression test (ASTM D642, TAPPI T 804) and follow OSHA 1910.176 on stable stacking.