ECT to Mullen Conversion Chart: Box Strength Equivalents and Weight Limits
A 32 ECT box is the working equivalent of a 200# Mullen box, 44 ECT pairs with 275#, and 55 ECT pairs with 350# in single wall. The chart maps every common rating in both directions with gross weight guidance, the metric kN/m and kPa values found on imported certificates, the flute profiles behind each grade, and the grades stocked here.
ECT to Mullen conversion chart
Edge crush test (ECT) and Mullen burst test measure different failure modes, so there is no formula that turns one number into the other. ECT measures the force in pounds per inch of width that a vertical edge of board withstands before buckling, which predicts stacking strength. Mullen measures the pressure in pounds per square inch that a board face withstands before it bursts, which predicts puncture resistance. What the industry uses is a set of equivalence classes: board grades that carrier classification rules (NMFC Item 222 and UPS and FedEx packaging guidelines) treat as the same minimum duty. Those pairings are what this chart shows, with the metric values that appear on imported box certificates.
| Board | ECT rating | Mullen equivalent | ECT in kN/m | Mullen in kPa | Gross weight guidance | Typical use |
|---|---|---|---|---|---|---|
| Single wall | 23 ECT | 125# | 4.0 | 862 | Up to about 20 lb | Light retail goods, inserts |
| Single wall | 26 ECT | 150# | 4.6 | 1,034 | Up to about 35 lb | Books, accessories, light parcels |
| Single wall | 29 ECT | 175# | 5.1 | 1,207 | Up to about 50 lb | Apparel, soft goods |
| Single wall | 32 ECT | 200# | 5.6 | 1,379 | Up to about 65 lb | The standard ecommerce shipping box |
| Single wall | 40 ECT | 250# | 7.0 | 1,724 | Up to about 80 lb | Denser products, heavier retail cases |
| Single wall | 44 ECT | 275# | 7.7 | 1,896 | Up to about 95 lb | Heavy or dense single items, long boxes |
| Single wall | 55 ECT | 350# | 9.6 | 2,413 | Up to about 120 lb | Heavy duty single wall |
| Double wall | 42 ECT | 200# | 7.4 | 1,379 | Up to about 80 lb | Light double wall, stacking over strength |
| Double wall | 48 ECT | 275# | 8.4 | 1,896 | Up to about 100 lb | Heavier shipments, moderate industrial parts |
| Double wall | 51 ECT | 350# | 8.9 | 2,413 | Up to about 120 lb | Dense products, machinery parts, bulk cases |
| Double wall | 61 ECT | 400# | 10.7 | 2,758 | Up to about 140 lb | Heavy industrial and palletized freight |
| Double wall | 71 ECT | 500# | 12.4 | 3,448 | Up to about 160 lb | Very heavy or high value dense freight |
| Triple wall | 67 ECT | 700# | 11.7 | 4,826 | Up to about 240 lb | Bulk bins, machinery, export crating alternative |
| Triple wall | 90 ECT | 1100# | 15.8 | 7,584 | Up to about 300 lb | Gaylord boxes, heavy bulk containers |
Read the rows as duty classes, not lab conversions. A mill can reach 32 ECT with more than one board recipe, so two 32 ECT boards can burst at different pressures. Gross weight figures are the conventional planning guidance for box plus contents in good condition, not a warranty; long transit, humidity above 50 percent and tall stacking all cut into real capacity. Note that 275# appears twice: it is the Mullen class for both 44 ECT single wall and 48 ECT double wall, so confirm the board construction before matching.
Metric conversions: ECT in pounds per inch x 0.1751 = kN/m. Mullen in psi x 6.895 = kPa. 32 ECT is 5.6 kN/m; 200# is 1,379 kPa. Reverse: kN/m / 0.1751 = ECT; kPa / 6.895 = psi.
Reading the chart in both directions
- You have a Mullen spec and need a modern box. Find the burst value, match the board type, and buy the ECT grade on that row. A legacy 200# single wall spec maps to 32 ECT.
- You have an ECT box and an old spec sheet asks for burst. Work the same row in reverse, confirming single or double wall first.
- You have a metric certificate. Divide kPa by 6.895 for psi and read the Mullen column; divide kN/m by 0.1751 for ECT. 1,380 kPa is 200#, 1,900 kPa is 275#, 2,410 kPa is 350#.
- Your packed weight sits near the top of a row. Step up a grade. The guidance figures assume a box in good condition and a single box; a bottom box in a pallet stack carries every layer above it, which the pallet loading table works through.
Flute profiles
The ECT rating tells you the board's strength; the flute tells you its thickness and how it behaves. The same 32 ECT rating can be made on B, C or E flute, and the three boxes handle very differently.
| Flute | Thickness | Flutes per foot | Where it is used | Trade-off |
|---|---|---|---|---|
| A | about 1/4 in (5 mm) | 33 | Cushioning, fragile goods, some double wall | Best cushioning, lowest print quality, bulky |
| B | about 1/8 in (3 mm) | 47 | Die-cut mailers, literature mailers, cans and bottles | High crush resistance, flat surface for print, less cushioning |
| C | about 5/32 in (4 mm) | 39 | The default RSC shipping box | Balance of stacking strength, cushioning and cost |
| E | about 1/16 in (1.5 mm) | 90 | Retail folding cartons, product boxes | Very smooth print surface, little stacking strength |
| F | about 1/32 in (0.8 mm) | 125 | Small retail packaging, cosmetics | Thinnest corrugated; rigidity over protection |
| BC (double wall) | about 9/32 in (7 mm) | B + C | 48 to 51 ECT boxes | Stacking strength plus cushioning |
| AC / BA (double wall) | about 13/32 in (10 mm) | A + C or B + A | Heavy duty double wall, 61 to 71 ECT | Maximum single-box cushioning |
Stocked grades and when to buy each
| Stocked grade | Board | Mullen class | Pick it when | Examples |
|---|---|---|---|---|
| 32 ECT | Single wall, C flute | 200# | Packed weight up to about 65 lb, parcel shipping, single box per shipment | 4 x 4 x 4, 6 x 4 x 4 |
| 44 ECT | Single wall, heavier liners | 275# | Long boxes, dense contents, parcels that get stacked in transit | 12 x 6 x 12.75, 6 x 6 x 6 |
| 48 ECT | Double wall, BC flute | 275# | Over 65 lb, palletized, or anything that must survive a drop with a corner hit | 8 x 8 x 8, 6 x 6 x 6 |
| 51 ECT | Double wall | 350# | Machinery parts, castings, bulk cases to 120 lb | 30 x 17 x 17, 36 x 22 x 22 |
| 71 ECT | Double wall, heavy | 500# | Very dense freight to 160 lb per box | 14 x 14 x 14, 12 x 12 x 12 |
| 90 ECT | Triple wall | 1100# | Bulk bins and heavy containers to 300 lb | 14 x 14 x 14, 24 x 24 x 24 |
For a head-to-head on the two most common grades see 32 ECT vs 44 ECT; for the decision logic behind choosing a test at all, the ECT vs Mullen box strength guide.
How to read the box maker's certificate
Every certified corrugated box carries a round box maker's certificate printed on a bottom flap. It names the test the box was certified under and the limits for that rating, which makes it the fastest way to place any box on the chart.
- Test line. An ECT certified box reads Edge Crush Test with a value in lb per inch of width. A Mullen certified box reads Bursting Test with a value in lb per square inch. A box is certified under one test, not both.
- Board construction. Single wall, double wall or triple wall. Match this before comparing strength numbers.
- Size limit. The maximum combined length plus width plus depth, in inches, for the certified rating. 32 ECT boxes are typically certified to 75 inches combined.
- Gross weight limit. The maximum weight of box plus contents for the rating.
Which rating to buy on
ECT is the default for parcel and palletized shipping because stacking compression is what those shipments stress, and ECT board uses less fiber for the same stacking strength, so it weighs and costs less. Mullen still earns its place when a single box takes repeated point impacts, such as loose heavy cartons handled by hand, or when a customer spec or an older NMFC classification calls for a burst value. If the shipment is over 50 lb, go straight to double wall regardless of the test.
Reference values: NMFC Item 222 minimum board combinations; ISTA and FBA packaging guidelines for the gross weight classes; TAPPI T 811 (ECT) and T 810 (Mullen) test methods. Weight guidance figures are industry planning values, not test results.
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Frequently asked questions
Is there an exact formula to convert ECT to Mullen?
No. ECT measures edge compression and Mullen measures burst pressure, two different failure modes. The pairings in the chart are equivalence classes used by carrier rules, not mathematical conversions.
What is the Mullen equivalent of a 32 ECT box?
200# burst is the accepted single wall equivalent. Both grades are treated as the standard parcel class, with weight guidance of about 65 lb for box plus contents.
What does 275# mean on a box?
A Mullen burst rating of 275 pounds per square inch. It is the class for 44 ECT single wall and 48 ECT double wall, so check the board construction on the certificate before assuming which one you have.
What is 32 ECT in kN/m?
About 5.6 kN/m. Multiply ECT in lb/in by 0.1751. 44 ECT is 7.7 kN/m and 48 ECT is 8.4 kN/m.
Is a higher ECT number always a stronger box?
Stronger in stacking, yes. But a 48 ECT double wall box and a 55 ECT single wall box are not interchangeable: the double wall resists punctures and corner drops far better at a lower ECT figure. Compare within the same board construction.