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CPP Packaging Knowledge Center — Guide 09

Carton Strength
Calculator

Estimate box compression strength (BCT) from board ECT and caliper using the McKee formula, then check it against your actual stack height and storage conditions.

McKee FormulaIndustry-standard BCT estimate
Pass / FailAgainst your real stack
Conditions-AdjustedHumidity & duration derate
Updated2026
01

Check your box against your stack

Pick a board grade (or enter custom ECT and caliper), enter your box footprint and loaded weight, and set how many high you're stacking. The calculator predicts lab BCT with the McKee formula, derates it for real-world storage conditions, and tells you if your stack passes.

lb/in
in
in
in
lbs
high
Predicted BCT (lab)
—
McKee formula, lbf
Field-Adjusted BCT
—
after conditions derate
Load on Bottom Box
—
weight of boxes above
Safety Factor
—
target: 2.0+
Result
—
at your stack height
Max Safe Stack
—
boxes high, at target SF
Uses the simplified McKee formula (BCT = 5.87 × ECT × √(caliper × perimeter)) — a widely used engineering estimate, not a lab measurement. It assumes a standard RSC box and doesn't account for box height, printing/die-cut coverage, pallet overhang, or stacking alignment, all of which can meaningfully reduce real strength. Validate anything critical with a physical compression test.
02

How the numbers are calculated

5.87McKee Constant
ECTEdge Crush (lb/in)
√(t·Z)Caliper × Perimeter
FORMULA

BCT = 5.87 × ECT × √(caliper × perimeter), where perimeter = 2 × (length + width). This predicts the lab-tested compression strength of a freshly made box under ideal, dry, short-term conditions.

Real stacking strength drops over time and with humidity — corrugated fiber loses stiffness as it absorbs moisture, and sustained static load causes creep. The "Field-Adjusted BCT" above applies a derate factor for your selected storage conditions, then the safety factor compares that adjusted strength to the actual load your bottom box will carry.

Box coming up short on strength?

We can spec a higher ECT, double wall, or different flute to hit your stacking requirement without over-building the box.

Talk to a Rep