How it works
Board grammage = all liners + each fluting paper × its take-up factor (A 1.54, B 1.32, C 1.43, E 1.27, F 1.19), because the fluting paper is longer than the board once it is waved.
Stacking strength uses the McKee formula: BCT = 5.87 × ECT × √(board thickness × box perimeter). It's an estimate for regular slotted boxes; humidity, handholes, printing and poor stacking all lower the real strength, which is why you divide by a safety factor.
Frequently asked questions
What is the difference between 3-ply and 5-ply?
3-ply has one fluted layer between two liners (single wall). 5-ply has two fluted layers and three liners (double wall), which makes it much stronger for heavy or stacked goods.
What is ECT?
Edge crush test: how much force the board takes on its edge before it crushes, in kN per metre. Box strength depends on it more than on burst strength.
Which flute should I use?
B flute gives good printing and puncture resistance, C flute stacks better, and BC double wall is common for export cartons.