Edge crush test: the stacking number

The edge crush test, or ECT, measures how much load a short strip of corrugated board carries on its edge before the flutes crush. The result is in pounds per inch of edge. It is the board rating that best predicts how much top load a box carries in a stack, which is why stacking engineers watch it more closely than wall count.

Two numbers on one box

Boxes carry two kinds of marks. The # figure (200#, 275#, 1100#) is the older burst-style board test. ECT is the newer edge crush figure. A box may show either or both. If it shows neither, count the wall layers and ask the seller.

Reading a mark on a used box

1. Find the maker's certificate on a flap or panel: a round or square stamp with numbers.
2. Read the board test or ECT.
3. Compare to the table above for the wall count. A mismatch is a flag.
4. Check the print is legible; a wet box often smears.

From ECT to a stack height

ECT sets the baseline; grade, place and time cut it. A box with a clear ECT-90 mark and a Grade A condition keeps close to its baseline in a heated warehouse. The same box on an unheated dock keeps 65%, and past 90 days it keeps about 55% of that. Stack Check multiplies those shares into a stack height.

Back to box compression strength, or on to weight capacity and Stack Check.

A worked reading

Suppose a box carries a mark of 48 ECT. That means the board, tested on an edge, held 48 pounds per inch of edge width before it crushed. A box with a 160-inch perimeter does not carry 48 times 160 pounds, because the box does not fail as a flat sheet; the compression formula applies a factor for the box shape and for the board. That is why two boxes with the same ECT can have different box compression values and why we do not quote a single figure for all triple wall.

For a used box, the useful step is to find the mark, then lower it by the grade and the place. Stack Check does that for you, but it helps to know where the number comes from.

What the test does not tell you

The edge crush test measures the board and nothing else. It does not measure how a box was made, how well the glue joint holds or how a flap was cut. Two boxes from different makers with the same mark can differ in stacking strength. Use the mark as a starting figure and let the grade and the place do the rest.

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