About the Box Compression Strength Calculator
This box compression strength calculator estimates how much top load a corrugated shipping box can carry before it buckles. It uses the simplified McKee formula, which predicts box compression test (BCT) strength from the board’s edge crush test (ECT) rating, the board thickness (caliper) and the box perimeter.
It then applies a safety factor to give a safe stacking load and works out how many boxes of a given weight can be stacked on top of the bottom box. Packaging engineers, warehouse managers and e-commerce brands use it to choose between 32 ECT and 44 ECT board, check pallet stacking, or explain why a box crushed in storage.
McKee estimates apply to regular slotted containers (RSCs) tested at standard lab conditions. Real stacking strength is lower once humidity, long storage time, pallet overhang, handholes and misaligned stacking are involved — that is what the safety factor covers. A factor of 3–5 is commonly used for warehouse storage; use 5 or more for humid or long-term storage.
With the default inputs, the estimated box compression strength (bct) is 582 lb. Change any value above to recalculate instantly.
How to use the box compression strength calculator
- 1Enter the box length and width in inches.
- 2Choose the ECT rating printed on the box certificate.
- 3Choose the flute or caliper of the board.
- 4Enter the weight of one packed box and a safety factor for your storage conditions.
- 5Read BCT, the safe top load and how many boxes can be stacked.
Formula and method
The simplified McKee formula (McKee, Gander and Wachuta, 1963) estimates the top-to-bottom compression strength of a regular slotted container from three inputs: the edge crush test value of the combined board in lb/in, the board caliper t in inches, and the box perimeter Z = 2 × (length + width) in inches. The constant 5.87 applies to these US units. Box height has little effect for typical proportions and is ignored.
Because lab BCT is measured on a single new box at controlled humidity, the safe working load is BCT divided by a safety factor that covers humidity, time under load, stacking pattern and handling damage. The number of boxes that can rest on the bottom box is the safe load divided by the weight of each box, rounded down.
- BCT
- Box compression test strength, lb
- ECT
- Edge crush test of the board, lb per inch
- t
- Board caliper (thickness), inches
- Z
- Box perimeter, 2 × (L + W), inches
- SF
- Safety factor applied to lab strength
Worked examples
18 × 12 in box, 32 ECT C-flute, 25 lb each
The perimeter is 2 × (18 + 12) = 60 in. BCT = 5.87 × 32 × √(0.16 × 60) = 5.87 × 32 × 3.098 ≈ 582 lb. With a safety factor of 4 the safe load is 145.5 lb, so five 25 lb boxes can sit on the bottom one — six per stack.
24 × 16 in box, 44 ECT, 35 lb, safety factor 5
Perimeter 80 in, so BCT = 5.87 × 44 × √(0.16 × 80) ≈ 924 lb. Divided by 5 that is about 185 lb of safe load: five 35 lb boxes on top, six high in total.
Small B-flute carton, safety factor 3
A 44 in perimeter with 0.125 in board gives BCT = 5.87 × 32 × √5.5 ≈ 441 lb. With a safety factor of 3 about 147 lb is safe, so nine 15 lb cartons can rest on the bottom one.
Frequently asked questions
What is the McKee formula?+
The McKee formula estimates corrugated box compression strength from board properties. The simplified US version is BCT = 5.87 × ECT × √(caliper × perimeter), with ECT in lb/in and caliper and perimeter in inches.
What is the difference between ECT and BCT?+
ECT (edge crush test) measures the strength of a small strip of corrugated board standing on edge, in lb per inch. BCT (box compression test) measures the load a whole box can carry before collapsing, in pounds. McKee converts one into an estimate of the other.
What safety factor should I use for stacking boxes?+
Common practice is about 3 for short-term dry storage, 4–5 for normal warehouse storage, and higher for humid conditions or storage beyond a few months. High humidity alone can reduce corrugated strength by a third or more.
How much weight can a 32 ECT box hold?+
A 32 ECT single-wall box is commonly rated for up to about 40 lb of contents under the edge crush rule, but its stacking strength depends on size: a medium 18 × 12 in C-flute box has an estimated BCT of around 580 lb before safety factors.
Does box height affect compression strength?+
For typical box proportions, height has only a small effect and the McKee formula ignores it. Very tall boxes can lose strength to panel buckling, and handholes, vents and poor flap closure also reduce it.