Box Stacking Strength Calculator & Estimator

It depends on the box’s compression strength, how much each box weighs, and how long and where the stack sits. Take an 18 × 12 × 10 in B-flute 32 ECT box holding 25 lb: it tests at roughly 519 lb of compression, but after 30 days at 70 percent humidity in an interlocked pallet pattern it keeps only about 25 percent of that, so it can safely be stacked about 6 high. Stacking the same boxes in aligned columns raises that to about 9.

Work backwards from the stack. Multiply the weight of one box by the number of boxes above the bottom one to get the load, divide by the storage factor for your humidity, time and pallet pattern to get the compression strength the box must have, then convert that to ECT with the McKee formula. The calculator on this page does all three steps and rounds up to the next standard grade.

It is the standard estimate of a corrugated box’s top-to-bottom compression strength, published in 1963 by R.C. McKee, J.W. Gander and J.R. Wachuta. The simplified form most people use is BCT ≈ 5.87 × ECT × √(board caliper × box perimeter), with ECT in lb per inch and caliper and perimeter in inches; it works in any consistent units. It is built for regular slotted containers, and a 2006 USDA Forest Products Laboratory review found it averages about 11 percent off real tests and tends to overestimate.

Yes. "Machi formula" is another name some sources use for the simplified McKee formula, BCT ≈ 5.87 × ECT × √(caliper × perimeter). The full McKee formula also uses the board’s bending stiffness in the machine and cross directions; the simplified one assumes stiffness follows board thickness.

It is built for regular slotted containers (RSCs), FEFCO style 0201: the standard shipping carton made from one sheet, with four flaps top and bottom that meet in the middle. That is the box the McKee formula was fitted to. Mailers, die-cut trays and telescope boxes carry load through different panels, so for those styles treat the result as a rough guide and confirm with a compression test.

BCT is the load a whole empty box carries before it buckles when squeezed between two platens in a lab, measured in pounds or newtons. It is the most direct measure of stacking strength. ECT measures a small strip of the board; BCT measures the finished box, including the effect of its size, shape and construction.

Yes, more than any other factor. Corrugated board absorbs moisture from the air, and its fibers soften as they do. A box stored at 80 percent relative humidity keeps only about 70 percent of the compression strength it shows at the 50 percent test condition, and at 90 percent it keeps about 55 percent.

Paper creeps under a constant load: the fibers slowly deform, so a box that holds a load for a minute in the lab may buckle under a smaller load after weeks. As a rule of thumb, a box keeps about 63 percent of its lab strength after 10 days under load, 60 percent after 30 days and about 50 percent after six months.

Much stronger. A box carries most of its load near its four vertical corners, so stacking corner over corner in columns keeps that path straight and loses at most about 20 percent of strength. An interlocked or brick pattern sets corners over the middle of the panels below and can cost 40 to 60 percent. Interlocking makes a more stable pallet, so the usual compromise is columns with stretch wrap or a tie sheet.

Yes. When boxes hang over the edge of the pallet, their outer corners and walls lose support and can lose up to about 40 percent of their compression strength. Gaps between pallet deck boards cost another 10 to 25 percent. Size boxes so the load sits fully on the pallet.

Condition sample boxes at 50 percent relative humidity and room temperature for at least a day, then run a box compression test on a calibrated compression tester, which records the peak load before the box buckles. Divide your required stack load by your storage factor and compare. For the board alone, an edge crush test on strips cut from the box confirms the ECT. Packwire can arrange both through a testing lab on request.

Specify ECT on the board and check BCT on the box. ECT is what box makers certify and buy board against, so it is the practical spec. BCT is the true measure of the finished box and the right check when a stack is tall, heavy or stored for months. The McKee formula connects the two.