What is the McKee formula?

The McKee formula (BCT = 5.87 × ECT × √(Perimeter × Caliper)) is the industry-standard equation for estimating a corrugated box's compression strength from its edge crush test value, its box perimeter (2 × (length + width)), and the corrugated board's caliper (thickness).

Why apply a safety factor to the BCT result?

The McKee formula estimates compression strength under short-term, low-humidity lab conditions. Real warehouse stacking involves weeks or months of dwell time, humidity swings, and vibration during transport, all of which weaken corrugated board — a safety factor of 4 to 6 is standard practice to keep stacking loads well under the lab-tested BCT.

What ECT and caliper values should I use?

ECT is printed on the box manufacturer's certificate (BMC) stamped on the box, typically 23-51 lb/in for common single-wall grades. Caliper (board thickness) varies by flute: roughly 0.06 in for E-flute, 0.12 in for B-flute, and 0.17 in for C-flute — check your supplier's spec sheet for the exact board.

Does this calculator account for pallet overhang or humidity?

No — this calculator computes the base McKee estimate and applies your chosen safety factor only. Pallet overhang, uneven or offset stacking, cut-outs or handholds in the box, and high-humidity storage all reduce real-world stacking strength further and aren't modeled here.

What happens if I stack higher than the safe limit?

Exceeding the safe stack height risks the bottom boxes buckling or crushing under the weight above, which can collapse the stack, damage product, and create a safety hazard for anyone working nearby. Reducing stack height, upgrading to a higher-ECT board, or adding a stronger caliper are the usual fixes.

Warehouse Storage & Stacking

Box Compression (McKee/BCT) Calculator

Enter each box type's edge crush test, dimensions, and board caliper to get its box compression test strength — then check how many high you can safely stack it.

Box ECT Length Width Caliper Box Wt.

ECT (lb/in)

Caliper (in)

Length (in)

Width (in)

Loaded box weight (lb)

Box: BCT 579.57 lb

Safety factor Planned stack height boxes high

Max Stack Height (Weakest Box)

—

Enter ECT, dimensions, and caliper above to calculate compression strength

A box's BCT drops fast with humidity and storage duration — a common safety factor of 4-6 already accounts for this, but hot, humid warehouses or long dwell times may call for the higher end of that range.

What Is Box Compression Test Strength — and the McKee Formula

A corrugated box's stacking strength isn't a single number stamped on the carton — it's calculated from the board's edge crush test, the box's footprint, and the board's caliper using the McKee formula, then derated by a safety factor for real warehouse conditions.

Edge crush test (ECT)

The force per inch of width the corrugated board can withstand on its edge before crushing — printed on the box maker's certificate, typically 23-51 lb/in for common grades.

Perimeter & caliper

A larger footprint (length + width) gives the box more edge to bear load on; a thicker board (caliper) resists buckling better — both raise the McKee formula's result.

Safety factor

The McKee formula gives a lab-condition box compression test (BCT) value — real stacking strength is lower due to humidity, storage duration, and vibration, so a safety factor of 4-6 is applied before setting a stack height.

What this doesn't cover

Pallet overhang, uneven stacking, cut-outs or perforations in the box, and extreme humidity all reduce real-world strength further — this calculator covers the base McKee estimate only.

The math

BCT = 5.87 × ECT × √(Perimeter × Caliper)

Max stack height = ⌊(BCT ÷ Safety factor) ÷ Box weight⌋ + 1

What if a box has an ECT of 32 lb/in, is 16 x 12 inches, and uses 0.17 in caliper board? Perimeter = 2 × (16 + 12) = 56 in. BCT = 5.87 × 32 × √(56 × 0.17) ≈ 580 lb. At a safety factor of 4, that's a safe stacking load of about 145 lb on the bottom box — divide by the loaded box weight to get the max stack height.

Free Load Planning Tool

Done estimating? Plan the full load.

Drag and drop your cargo, auto-arrange for weight balance, and export step-by-step loading instructions - no account needed.

Open Load Planner

Frequently Asked Questions

BCT is the maximum top-to-bottom force a corrugated box can withstand before it crushes, measured in pounds. It's usually estimated with the McKee formula from the board's edge crush test, the box's perimeter, and the board's caliper, rather than tested on every single box.

Your next load, perfectly planned.

Start free. No credit card. No install.

Plan a Container Plan a Truck Load