The round stamp on the bottom flap is the Box Maker's Certificate, and it carries either an edge crush rating in pounds per inch of width or a bursting test rating in pounds per square inch. UPS publishes a chart pairing each rating with a gross weight the box is expected to carry, and the two common grades, 32 ECT and 200 lb test, both sit at the bottom rung of it.
The calculator prices the packed carton once you know which box it belongs in.
This is the only table on the internet that matters for choosing a grade, because it comes from the company that will handle the box. Find your gross weight in the left column and read across.
| Gross weight (lb) | Size limit (in) | Bursting test (lb per sq in) | Edge crush (lb per in width) | Walls |
|---|---|---|---|---|
| 0 to 30 | Under 75 | 200 | 32 | Single wall |
| 31 to 50 | 75 to 85 | 250 | 44 | Single wall |
| 51 to 65 | 85 to 96 | 275 | 55 | Single wall |
| 66 to 80 | 95 to 105 | 350 | Not stated | Single wall |
| 81 to 100 | 95 to 105 | 350 | 61 | Double wall |
| 101 to 120 | 105 to 110 | 400 | 71 | Double wall |
| 121 to 140 | 110 to 115 | 500 | 82 | Double wall |
| 141 to 150 | 115 to 120 | 600 | Not stated | Double wall |
Two readings jump out. The first row covers everything up to 30 lb, which is the overwhelming majority of what small sellers ship, and it is satisfied by either 200 lb bursting test or 32 ECT. That is why those two grades are what every supplier stocks by default. The second is where the ladder switches to double wall, at 81 lb, well past the 70 lb ceiling USPS applies to every one of its services.
UPS attaches two qualifications to its own chart, and both are worth repeating. It states that the box strengths are only guidelines to help assure the containment and protection of products transported through single-package distribution environments, and that all packaged products should be tested in accordance with the International Safe Transit Association Test Procedure 3A. If you sell one product in volume, that test procedure is the honest way to settle a packaging argument.
Flip a carton over and the stamp is usually near one edge of a bottom flap, printed in the same ink as the manufacturer's marks. Four things on it are worth your attention.
One more line from FedEx settles a common question. Chipboard boxes, such as gift or shoe boxes, must be packed into a corrugated outer box, which means a shoe box is a container for your product and never a shipping carton.
Grades are abstract until you line them up against parcels you would actually send. Four loads, one lane, one quote day, with the grade the UPS chart points to for each.
| Load | Carton, outer inches | Grade the UPS chart points to | Cheapest rate returned |
|---|---|---|---|
| 8 lb | 14 x 12 x 8 | 200 lb test or 32 ECT, single wall | $15.64 |
| 16 lb | 16 x 14 x 10 | 200 lb test or 32 ECT, single wall | $20.50 |
| 32 lb | 20 x 16 x 12 | 250 lb test or 44 ECT, single wall | $32.09 |
| 50 lb | 24 x 18 x 14 | 250 lb test or 44 ECT, single wall | $45.01 |
The step happens between 16 lb and 32 lb, where the chart moves from the 0 to 30 lb row into the 31 to 50 lb row. That is a real change in cardboard for a small increase in what the label costs, and it is the most common place sellers get caught, reusing a light grocery carton for a load it was never certified to carry.
Grade and price are not related at retail either. On 2026-07-28 Uline listed a 12 by 10 by 6 in box at 200 lb test for $0.94 and Staples listed the same footprint at 32 ECT for $1.26, two different rating systems at the same tier of strength and roughly the same money. The buying guide has the full price comparison, and the size ladder covers dimensions.
It is an edge crush test rating of 32 pounds per inch of width, a measure of how much downward force the board withstands standing on end. In the UPS Box Strength Guidelines, 32 ECT sits on the row covering gross weights from 0 to 30 lb in a single wall box.
They are different tests placed on the same rung. The UPS chart pairs a 200 lb per square inch bursting test with a 32 lb per inch edge crush rating on its 0 to 30 lb row, so either stamp indicates a box intended for the same weight class.
Printed on a bottom flap of most corrugated boxes, as a round or oval stamp. UPS tells shippers never to exceed the maximum gross weight limit for the box, which is printed on the Box Maker's Certificate on the bottom flap of most boxes.
The UPS chart moves to double wall at 81 lb gross weight, and FedEx asks shippers to use double-wall boxes for heavier items. Since USPS caps every service at 70 lb, double wall in practice means UPS or FedEx parcels, or a fragile item that needs a second carton.