McKee Box Compression (ECT to BCT) Calculator
Calculate a corrugated box's compression strength (BCT) from ECT, caliper and internal perimeter using the McKee formula, then apply a safety factor for stacking.
498.4 lb BCT
Internal perimeter 44 in — safe stacking load 124.6 lb after your safety factor.
| Metric | Value |
|---|---|
| Internal perimeter | 44 in |
| BCT | 498.4 lb |
| Safe stacking load | 124.6 lb |
The McKee formula, and what it assumes
The simplified McKee formula — BCT = 5.87 × ECT × √(h × Z) — estimates a single-wall regular slotted container's box compression strength from three lab-measurable inputs: the board's edge crush test rating, its caliper (thickness), and the box's internal perimeter (2 × length + 2 × width). McKee, Gander and Wachuta published it in 1963 specifically for single-wall RSC boxes with no holes, cutouts, hand slots or vent openings — every one of those weakens a real box well below what the formula predicts.
Why lab BCT isn't your safe stacking load
BCT is measured under short-term lab conditions at standard humidity — the test takes seconds, not the weeks a warehouse stack sustains load in variable humidity. Corrugated board loses a substantial share of its compression strength as relative humidity climbs, which is exactly why the safe stacking load divides BCT by a safety factor rather than treating the lab number itself as a safe target.
Why stacking pattern changes what a box can actually hold
Column stacking — boxes aligned directly over the ones below, corner to corner — transfers load efficiently down a box's strongest points, its vertical edges, and lets a stack approach its rated BCT. Interlocked or offset pallet patterns redistribute load across each box's top panel instead of its edges, which can reduce effective stacking strength well below the same box's BCT rating even though the box itself hasn't changed.
How to use this calculator
- Enter ECT, board caliper, and box length and width.
- Set a safety factor for your storage humidity and duration.
- See BCT and the resulting safe stacking load.
- Single-wall RSC boxes only
- The simplified McKee formula was derived for single-wall regular slotted containers with no holes, cutouts, hand slots or vent openings — any of those reduces real compression strength well below this estimate.
- BCT is a short-term lab figure
- BCT is measured at standard lab humidity over seconds, not the weeks a warehouse stack sustains load in variable humidity — the safety factor is what converts it into a realistic stacking load, not BCT itself.
Questions
Why did my boxes collapse in storage even though the BCT looked strong on paper?
Humidity is the usual culprit. Corrugated board loses compression strength as relative humidity rises — a box rated for a comfortable margin at 50% RH can lose a large share of that strength above 80-90% RH, which is common in an un-climate-controlled warehouse in summer. BCT is measured under standard lab humidity; the safety factor you apply is what's supposed to cover the gap between that lab number and your actual storage conditions.
Does stacking pattern matter, or just the box's own BCT?
It matters enormously. Column stacking (boxes aligned directly over the ones below, corner-to-corner) transfers load efficiently down the box walls and lets a stack approach its rated BCT. Interlocked or offset pallet patterns redistribute load across each box's top panel rather than its stronger edges, which can reduce effective stacking strength well below the same box's BCT rating.
Why does a wider, flatter box need a higher ECT than a narrower one for the same BCT?
Because BCT scales with the square root of perimeter, not perimeter directly — a box with double the internal perimeter needs its ECT × caliper term to roughly double too, just to hold BCT constant, since the formula's perimeter term only contributes at half power. Wide, flat boxes need proportionally more edge crush resistance than a cube-shaped box of similar volume.
Can I use this for double-wall or triple-wall corrugated?
Not reliably. The simplified McKee formula and its 5.87 constant were derived specifically for single-wall regular slotted containers. Modified versions of the formula exist for double- and triple-wall board with different constants, but published sources note they carry lower prediction accuracy than the single-wall version — treat any multi-wall estimate from a single-wall formula as rough at best.
Sources
- Box Compression Test Calculator: BCT Strength Calculator — Westpak (reviewed 2026-09-26)