About this calculator
Corrugated boxes crushed at the bottom of a stack are one of the most common causes of damage claims, in containers, trucks and warehouses alike. A box's ability to carry load from above is its BCT (box compression test) value. The simplified McKee formula estimates it from three inputs: the board's ECT (edge crush test), its caliper (thickness) and the box perimeter: BCT = 5.87 × ECT × √(caliper × perimeter). A larger perimeter and a thicker or stronger board therefore give a higher BCT.
BCT from the formula or a lab test describes standard conditions and a short-term load. In the field strength drops: high humidity, typical of Indonesian warehouses and ports, weakens the paper; weeks of storage make the boxes slowly creep; misaligned or interlocked stacking moves load away from the strong corners; and boxes overhanging the pallet edge lose part of their support. That is why BCT is divided by a safety factor; in practice values of 3 to 5 or more are common, depending on storage time and conditions.
The calculator returns the perimeter, BCT in newtons and kgf, the safe load on the bottom box, how many boxes may sit on top and the resulting stack height. Presets for common boards (such as C-flute 32 ECT) are included, but ask your box maker for the certified ECT of your board. McKee applies to ordinary regular slotted boxes; for important decisions, run a real BCT test.
Worked example
A 400 × 300 × 250 mm box made of board with ECT 5.6 kN/m and 4 mm caliper, 10 kg filled, safety factor 4.
- Perimeter: 2 × (400 + 300) = 1,400 mm
- BCT = 5.87 × 5,600 × √(0.004 × 1.4) ≈ 2,459.9 N = 250.8 kgf
- Safe load: 250.8 ÷ 4 ≈ 62.7 kg on the bottom box
- Boxes on top: 62.7 ÷ 10 → 6 boxes → a stack 7 high (1,750 mm)
The recommended maximum is 7 boxes high; for long storage in a humid warehouse, raise the safety factor.
How to use it
Choose the board
Pick a board preset (such as C-flute 32 ECT) or enter the ECT and caliper from your box supplier.
Enter box size and weight
Enter the outer length, width and height of the box and the weight of one filled box.
Set the safety factor
Choose a safety factor for the conditions: higher for humidity, long storage or interlocked stacking. A stack height limit is optional.
Read the stacking limit
See BCT, safe load, boxes allowed on top of the bottom box and stack height, then share the result with your supplier or warehouse.