Also known as: BCT, Box Compression Strength, Compression Test, Carton Compression Test
The box compression test (BCT) loads an empty, closed corrugated box between two platens until it collapses, recording the peak force in kgf or newtons. It is the direct measure of stacking strength. BCT can be estimated from board ECT with the McKee formula: BCT = 5.874 × ECT × √(caliper × perimeter).
The test is run on a compression tester with a fixed lower platen and a moving upper platen travelling at about 12.7 mm/min (ISO 12048, TAPPI T804, ASTM D642). The box is erected and closed exactly as it will ship — flaps taped, no contents — conditioned at 27 ± 2 °C and 65 ± 2% RH under the Indian convention, and loaded until the walls buckle. The peak force and the deflection at peak are reported; a second, lower value called the load-deflection limit is sometimes specified for boxes that must not deform even slightly.
Where no tester is available, the McKee formula gives a working estimate. In metric units BCT (N) = 5.874 × ECT (kN/m) × √(board caliper (mm) × box perimeter (mm)). Take a 5-ply carton of 500 × 400 mm footprint on BC flute board with ECT 8 kN/m and caliper 7 mm: perimeter is 2 × (500 + 400) = 1,800 mm, √(7 × 1,800) = 112, so BCT ≈ 5.874 × 8 × 112 = 5,260 N, or about 537 kgf. A 3-ply 400 × 300 mm box with ECT 5 kN/m and 4 mm caliper works out at roughly 2,200 N, about 224 kgf.
BCT is a laboratory peak, not a working load. Corrugated board creeps under sustained load and softens with humidity, so designers divide BCT by a safety factor of 3 to 5 before comparing it with the weight the bottom box will actually carry. With the 537 kgf carton above and a factor of 4, the allowable load is about 134 kg; if each loaded carton weighs 20 kg, six cartons can sit on top, so the stack can be seven high. Longer storage, monsoon conditions and double-stacked pallets push the factor toward 5.
Several things reduce the number the lab reports. Board that goes from 50% to 90% relative humidity can lose around half its compression strength. Cartons held under constant load for a month typically retain only about 60% of their short-term BCT. Overhang past the pallet edge, interlocked stacking patterns that stop corners aligning, and heavy flexo print coverage that crushes the flutes each take a further slice. Taller boxes of the same perimeter test lower because the panels buckle more easily.
BCT is a property of the finished box, so it changes with dimensions; ECT is a property of the board and does not. Buyers who need a stacking guarantee should give the converter the gross carton weight, the stack height, the storage period and whether the goods will see a monsoon warehouse or a sea container. We run BCT on prototypes for export loads and size the ply from the required value rather than from the product weight alone.
Box Compression Test (BCT) at a glance
| Measures | Peak vertical load a closed empty box carries before collapse |
|---|---|
| Units | kgf or N (1 kgf = 9.81 N) |
| Test standards | ISO 12048, TAPPI T804, ASTM D642 |
| Platen speed | About 12.7 mm/min, box conditioned 27 °C / 65% RH |
| McKee estimate | BCT (N) = 5.874 × ECT (kN/m) × √(caliper mm × perimeter mm) |
| Worked example | 5-ply 500 × 400 mm, ECT 8 kN/m, 7 mm → ≈ 537 kgf |
| Safety factor | Divide BCT by 3–5 to get the working stacking load |
| Humidity effect | Up to about 50% loss between 50% and 90% RH |
Common questions
What is the formula for box compression strength?
The McKee formula: BCT = 5.874 × ECT × √(caliper × perimeter), with ECT in kN/m, caliper and perimeter in mm and the answer in newtons. Divide by 9.81 for kgf. It assumes a regular slotted box of normal proportions and is reliable enough for ply selection, but a physical test is needed for a guarantee.
How is BCT different from ECT?
ECT is measured on a small strip of board stood on edge and is a board property in kN/m. BCT is measured on the whole erected box and depends on the box perimeter, height and caliper as well as the board. ECT is what you specify to the paper mill; BCT is what governs how high you can stack the finished carton.
How many boxes can I stack from a BCT value?
Divide the BCT by a safety factor of 3 to 5 to get the allowable load on the bottom box, then divide that by the gross weight of one loaded carton. A 537 kgf BCT with a factor of 4 allows about 134 kg, so six 20 kg cartons on top — a stack of seven. Use the higher factor for monsoon storage or sea freight.
Which Indian standard covers box compression testing?
Indian test houses generally run compression to ISO 12048 or TAPPI T804 and report it against the box grade requirements of IS 2771 (Part 1). Samples are conditioned at 27 ± 2 °C and 65 ± 2% RH rather than the 23 °C / 50% RH used in Europe, which gives slightly lower and more realistic values for Indian warehouses.
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