Enter your box’s ECT rating, dimensions and storage conditions to calculate theoretical BCT strength, and the safe stack load for warehouse, shipping or display use. Uses the McKee formula, the most widely cited model for corrugated box compression.

How the calculation works

This tool uses the McKee formula, the most widely cited empirical model for corrugated box compression strength:

BCT = 5.874 × ECT × √(2(L+W)) × H^0.25

ECT is the edge crush test rating of the board in lb/in. L, W and H are box dimensions in inches. BCT is the theoretical box compression test result in pounds. The result assumes a perfect, well-made box at standard temperature and humidity. Real-world BCT measurements typically land at 80–100% of the McKee result.

ECT grades

ECT (lb/in)	Board grade	Typical use
23	Light single wall	Small parcels, lightweight mailers
32	Standard single wall	Most e-commerce shipping boxes
44	Heavy single wall	Heavier products, longer routes
48–51	Heavy single wall	Industrial and heavy retail
61–71	Double wall	Heavy, fragile or pallet-stacked products
82	Heavy double wall	Industrial goods, exports, long-term storage

Safety factors

Raw BCT is the theoretical failure point. Industry-standard safety margins:

Use case	Factor	Rationale
Warehouse stacking	4×	Static load, controlled environment
Dynamic shipping	10×	Vehicle vibration, drops, off-center stacking in transit
Retail display	2.5×	Stable, low-turnover, climate-controlled

Humidity and age degradation

Corrugated board loses strength as it absorbs moisture. Condition factors from McKee, Gander & Wachuta (1963) and TAPPI TM 804:

Condition	Factor	Reduction
New, dry (~50% RH)	1.00	None
30-day storage (~50% RH)	0.85	15%
Humid / extended (>65% RH)	0.60	40%

Worked examples

Scenario	ECT	L×W×H (in)	Condition	BCT (adj.)	Safe warehouse	Safe shipping
Standard e-commerce mailer	32	12×9×6	New	~703 lb	176 lb	70 lb
Heavy retail, warehouse	44	18×14×12	30-day	~1,410 lb	353 lb	141 lb
Pallet-stacked double wall	71	24×20×18	Humid	~2,390 lb	598 lb	239 lb

Limitations

The McKee formula is calibrated for regular slotted containers (RSC). Die-cut trays, self-locking bottoms and irregular shapes need additional correction factors. Off-center loading or point loads can reduce effective strength by 20–40%. For regulatory compliance, carrier certification or insurance, use laboratory BCT testing.

Cite this tool

Tria Packaging. Box compression calculator (McKee formula). Retrieved from triapackaging.com/tools/compression-calculator/. Formula source: McKee, R.C., Gander, J.W. & Wachuta, J.R. (1963). Compression strength formula for corrugated boxes. Paperboard Packaging, 48(8), 149–159.

Box compression calculator