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Corrugated Box Compression Strength (BCT) Calculator

This calculator estimates the top-to-bottom compression strength of a corrugated shipping box from the board edge crush test (ECT), board caliper and box footprint. It then turns that strength into an allowed stack load and a number of boxes high for warehouse planning.

Your numbers

Board edge crush test (ECT)

kN/m

From the board spec or supplier certificate. A 32 ECT board is about 5.6 kN/m.

Combined board caliper

Thickness of the finished board, for example about 4 mm for a C flute single wall.

Box length (inside)

Box width (inside)

Box height (inside)

Filled box weight

Stacking safety factor

Your results

Estimated BCT (N)

2,416

Top-to-bottom compression strength of one empty-state box under lab conditions.

Boxes high (max)

The bottom box carries the boxes above it, so the stack is one more than the load divided by box weight.

ECT in lb/in

31.405

Converted because the McKee constant is published in imperial units.

Box perimeter

55.1181

Perimeter in inches, 2 x (L + W).

Estimated BCT (lbf)

543

Allowed stack load

61.59

BCT in kgf divided by the safety factor.

Stack height

1.8

Metres, inside box height only, excluding pallet.

The McKee formula is a published estimate for regular slotted containers. Real strength depends on humidity, storage time, flute type, print coverage, vents and handholes. Confirm with a lab BCT test before changing a specification.

Save results:

Estimated BCT (N) versus board ECT (kN/m)

Sweeps Board edge crush test (ECT) from 3 to 12, holding your other inputs fixed.

Check box specs against your ERP packaging data

Optional: we review how board grade and box dimensions are held in your ERP and flag where specs can drift.

Optional. The tool and every export work without this. No spam.

How to use the calculator

Take ECT and caliper from your board specification or the supplier certificate, then enter the inside length, width and height of the box and its filled weight. Pick a stacking safety factor. The tool returns the estimated box compression test (BCT) value, the load a bottom box can carry long term, and how many boxes can safely be stacked. Use the chart to see how much strength you gain by moving up one board grade. Run it for your current box and for the proposed box to compare.

ECT in kN/m: 32 ECT is about 5.6, 44 ECT about 7.7
Use inside dimensions of the finished box
Weight is the full filled box including product and dunnage
Safety factor of 4 is common for ordinary warehouse storage

The McKee formula behind the number

The widely used McKee equation estimates BCT as 5.87 x ECT x the square root of caliper x box perimeter, with ECT in lb/in and dimensions in inches. The result is in pounds force. Strength rises with the square root of perimeter and caliper, which is why a larger box loses strength per unit of board, and why a small caliper increase from a different flute helps less than people expect. The calculator converts from kN/m and millimetres so you can work in metric. The formula assumes a regular slotted container with good manufacturing quality and no major openings.

Reading the result: safety factors and stacking

A laboratory BCT is a short-term, controlled-humidity number. Boxes in a warehouse lose strength with time under load and with humidity, so the allowed load is BCT divided by a safety factor, commonly 3 to 6. A pallet sitting for weeks in a humid port warehouse deserves 5 or 6. The boxes-high figure assumes the bottom box carries every box above it and only the box weight, not the pallet or a second pallet stacked on top. If you double-stack pallets, enter the total weight above the bottom box as a larger effective box weight.

Common mistakes and ERP notes

The most common errors are using outside dimensions, mixing ECT in lb/in with kN/m, ignoring humidity, and assuming misaligned stacks keep the strength: column misalignment and overhang can cut BCT by a third or more. Vent holes and handholes reduce it further. In your ERP, keep board grade, flute and ECT on the packaging item master so purchasing does not substitute a weaker board silently. When the specification changes, update the packaging specification document and the item revision together.

Do not use outside dimensions
Overhanging cartons reduce real strength
Check printed coverage and vent holes
Verify with a lab compression test for new designs

Frequently Asked Questions

What does BCT mean on a box?

BCT stands for box compression test. It is the maximum top-to-bottom load a sealed box withstands in a lab compression tester before it fails. It is not the safe working load, which is lower after applying a safety factor for time and humidity.

How do I get ECT for my board?

ECT is printed in the board's supplier specification or certified in the box maker's stamp (the box maker certificate shows the combined board rating). If you only know the burst test, ask the supplier for the ECT equivalent, because the two are not directly interchangeable.

Is the McKee formula accurate?

It is a good first estimate for regular slotted containers and typically within a modest margin of lab results for standard boxes. It is less reliable for tall slender boxes, boxes with large cutouts, or non-standard styles, where you should rely on a physical compression test.

What safety factor should I choose?

Three suits short, controlled shipping. Four is a common choice for ordinary warehousing. Five or six suits long storage, high humidity, or poor stacking alignment. If damage claims are already high, move up a level and review stack pattern as well.

If board grade, box dimensions and pallet patterns live in your ERP, we can build a read-only agent that checks stacking limits against the packaging item master. Ask for a short review of your packaging data.

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