The short answer
The Mullen test measures burst strength — the hydraulic pressure corrugated board withstands before it ruptures, reported in pounds per square inch. The edge crush test (ECT) measures stacking strength — the compressive load the board’s edge carries before it collapses, reported in pounds per lineal inch.
They describe two properties of one sheet, so neither value converts into the other. The charts that seem to convert them come from freight rules and map compliance rather than strength.
Two numbers certify the strength of a corrugated box, yet they do not measure the same thing. One records what it takes to push through the board, the other what it takes to crush it from above. So comparing ECT vs Mullen means weighing puncture resistance against stacking strength.
This guide covers the test methods behind both ratings, the freight rules requiring them, and the tables that seem to translate one into the other. It also covers what neither number predicts, since both describe board, not a packed and stacked box.
Buyers, shippers and quality staff will find the selection logic here, as will anyone holding a stamped box and unsure what it promises.
What Each Test Physically Measures
The Mullen test, or burst test, clamps a board sample between two plates with round openings. A rubber diaphragm then pushes up through the lower opening under water pressure until the board tears, and the machine records peak pressure at the moment it gives way.
Edge crush works along a different axis. A short column of board stands on edge between two plates, flutes vertical. The plates close until the edge folds, and the result is force per inch of width.
The two methods load the same board along different axes. Burst pushes through the face; edge crush presses down the flute walls. A sheet can perform well under one and poorly under the other.
The loads run in different directions, so the numbers predict different failures. A burst rating covers something pushing through the wall — a corner hitting concrete, a sharp item shifting inside, a hand truck blade catching a panel. An edge crush rating covers weight pressing down, which is what the bottom box on a six-layer pallet feels.
Both figures belong to the board, not the finished box, and scores, flap fit, the joint and moisture all come between the test bench and the loading dock.
The Standards Behind the Two Ratings
Neither test is a supplier invention. Standards bodies publish, number and maintain both, so a box plant and its customer can settle a dispute against a written procedure.
TAPPI T 810 defines burst strength for corrugated board. It sets out the instrument, the clamping and the steps, and states a working range of 100 to 700 psi. It also excludes triple wall, because multiple facings rarely burst at the same moment.
TAPPI T 811 defines edge crush, and the trade calls it the short column test. A lab cuts the sample to size, conditions it, then loads it on edge with the force running along the flutes.
ISO methods cover both properties, and ISO 3037 reports edge crush in kilonewtons per meter rather than pounds per inch. So a buyer weighing a domestic quote against an overseas one faces two unit systems, not two quality levels.
Conditioning is part of the method. Labs hold samples at a set temperature and humidity before loading, because fiber strength shifts with moisture. A rating quoted with no conditioning reference is a number with no context.
Why the Equivalency Chart Is Not a Conversion
Almost every guide carries a table lining up 200# burst with 32 ECT, 275# with 44 ECT, and so on. The table is real, though most guides get its meaning wrong.
An accredited lab describing the burst method puts it plainly: the two are separate material properties, and neither derives from the other. Board with a high burst rating can still crush under a modest edge load. Board built for edge crush can puncture under a blow a heavier sheet would shrug off.
So where does the table come from?
It comes from freight rules. Item 222 of the National Motor Freight Classification and Rule 41 of the Uniform Freight Classification both set minimum board grades by a box’s gross weight and size. Table A runs through burst strength plus a minimum combined weight of facings. Table B runs through edge crush. Both tables end at the same allowed gross weight.
| Table A route | Table B route | |
|---|---|---|
| Property certified | Bursting strength | Edgewise compressive strength |
| Reported in | Pounds per square inch | Pounds per lineal inch |
| Also mandates | Minimum combined weight of facings | No facing-weight minimum |
| Predicts | Resistance to puncture and rupture | Resistance to vertical compression |
| Shared endpoint | The same certified gross weight and size limit | |
That shared endpoint is what lines the two columns up. A 200# burst board and a 32 ECT board are not equal in strength. Each one is enough, on its own, to certify the same size and weight class. So the chart maps compliance rather than converting strength.
The result is direct: swap a Mullen board for its charted edge crush partner and the shipment stays compliant, although what the box resists has changed. If the lane punishes puncture, that swap trades away the very property guarding the product. And the stamp still reads as compliant.
Industry records trace the change to proposals filed in 1990, after which carriers approved edge crush as an alternate route.
What Carriers Require and What the Certificate States
The Box Maker’s Certificate is the printed record of whichever route the board took, and it shows up as a round or square stamp on an outside bottom panel.
Illustrative certificate. Values shown are examples, not a published specification.
A certificate gives the maker’s name and location, a size limit in inches, and a gross weight limit. A burst stamp also carries the minimum combined weight of facings. An edge crush stamp carries the ECT value.
The gross weight limit deserves a second look, since it caps contents plus packaging and industry guidance treats it as an approximate figure rather than a firm load rating. It names the class the board qualified for, though it does not promise what a packed box survives in a given lane.
Parcel carriers stack their own rules on top. Published guidance has commonly called for Item 222 compliance and a floor of 200# burst or 32 ECT on lighter packages, with heavier grades above that weight. Carrier tariffs change, so check the current rule before locking a spec.
Not every box carries a stamp. Many shippers skip it and spec board by tested performance instead, which shifts the burden of proof to the shipper when a claim lands.
Matching the Rating to the Handling Profile
Selection follows the shipping environment, not the product category. The question is not what ships, but what happens on the way.
-
Palletized freight favors edge crushBoxes travel unitized, move by forklift and sit in stacks for days or weeks, so compression dominates and the ECT value speaks to it directly. -
Individually handled parcel favors burstPackages ride conveyors, get sorted by hand and take hits at corners and edges, so rupture resistance matters more than stacking strength a lone parcel never sees. -
Mixed lanes meet both hazardsA load palletized for line haul, split at a cross-dock, then delivered loose meets both hazards in turn. Spec for the harsher leg. -
Contents change the calculationDense, rigid goods carry part of the top load themselves, while loose items with hard edges push toward burst and light, high-cube goods stacked deep push toward edge crush.
Cost belongs at the end of that list, not the start. Edge crush allows lighter board at the same weight class, so the swap reads as a saving. Whether it saves anything depends on whether that property was working.
Where Each Rating Stops Predicting Performance
Both numbers describe flat board in a controlled lab, and plenty happens to that board afterward.
Crushing is the sharpest example. Damage in converting or handling lowers a board’s edge crush value, while burst ratings shrug off the same knocks. A sheet that left the corrugator at 32 ECT can reach the packing line weaker, under a stamp that still reads 32. Box plants control for this, yet it still explains a common complaint: boxes that test right and collapse anyway.
Moisture pushes the same way, since fiber strength falls as moisture rises and stacking performance in a humid warehouse sits below the conditioned lab figure. Published loss figures vary too widely by exposure and grade to quote one percentage.
The McKee relationship, published in 1963, estimates top-to-bottom strength of an empty regular slotted container from edge crush, caliper and box perimeter. Even so, it stays an estimate for one box style under set conditions.
Some factors sit outside both ratings entirely. Pallet pattern and overhang cut real stacking strength whatever the stamp says, long storage brings creep, and drops, vibration and forklift strikes appear in neither number.
Which is why transit testing exists separately. Board ratings qualify a material, while test protocols qualify a packed shipment. A stamp never covers the second.
Common Questions
Is 32 ECT the same as 200# Mullen?
No. They measure different properties, and neither converts into the other. Both appear against the same gross weight class in carrier tables, which is why guides print them side by side. That shared limit is a compliance link rather than equal strength.
Which rating is stronger overall?
Neither, because they answer different questions. A higher burst rating means more resistance to puncture, while a higher edge crush rating means more resistance to vertical compression. The right choice depends on whether the box gets stacked or handled singly.
Where is the rating printed on a box?
On the Box Maker’s Certificate, a round or square stamp on an outside bottom panel. It names the maker and location, gives a size limit as length plus width plus depth, then sets a gross weight limit and reports one test result.
Does a crushed board keep its rating?
Not reliably. Crushing the board in converting or handling lowers edge crush values, while burst values hold up. A stamp reflects the board as tested, not the board that reaches the packing line.
Do both tests apply to double and triple wall board?
Edge crush applies to single, double and triple wall, but the burst method excludes triple wall, because multiple facings rarely rupture together consistently enough for a valid result.
Reading the Numbers Correctly
This guide has covered both ratings, the methods behind them, the freight tables that seem to connect them, and the point where both stop predicting performance. Three things matter most: each test measures a separate property, freight rules set two routes to one weight limit, and a material rating is never a packed box in transit.
Board ratings make a sound starting point when read for what they are. Anyone comparing quotes or chasing a claim should check current carrier rules, hold the stamp against the real handling profile, and consider transit testing.
Key takeaways
- Burst measures the pressure to tear the face, while edge crush measures the load on the board edge.
- TAPPI T 810 and T 811 define the methods, with ISO covering both in different units.
- Equivalency charts show two routes to one certified gross weight, not a strength conversion.
- The Box Maker’s Certificate gives a size limit, a gross weight limit and one test result, and that weight figure is only approximate.
- Crushing and moisture both lower edge crush values, and neither rating covers pallet pattern, creep or impact.