If you’ve ever flipped a shipping box over and squinted at the small stamped circle on the bottom, you’ve seen a Box Maker’s Certificate — and it probably listed either an ECT number or a Mullen (burst) rating. The two numbers look similar but measure completely different things, and picking the wrong one can mean crushed pallets, rejected freight claims, or paying for more strength than you need.
This guide breaks down what ECT and Mullen actually test, how their ratings compare, and how to decide which certification fits your product, your pallet setup, and your carrier’s requirements.
Quick Answer
Use ECT (Edge Crush Test) if your boxes are palletized and stacked in a warehouse or truck — it measures how much stacking weight the box can bear before the edges buckle. Use Mullen (burst test) if your product ships as an individual parcel handled by many hands, since it measures puncture and burst resistance. Most e-commerce and general freight today runs on ECT because it’s the industry standard for palletized shipping and is typically lighter (and cheaper) for equivalent performance.
What Each Test Actually Measures
The Mullen test, also called the burst test, is the older of the two methods. A hydraulic diaphragm presses against a flat sample of corrugated board until it ruptures, and the rating (like 200# or 275#) reflects how many pounds per square inch of pressure the board can take before bursting. It’s a good proxy for how a box holds up against punctures, drops, and rough individual handling as it moves hand-to-hand through a supply chain.
ECT, short for Edge Crush Test, measures something different: how much compressive force the board can withstand when squeezed along its edge, in the direction of the flutes. The result (like 32 ECT or 44 ECT) is reported in pounds per linear inch and directly reflects stacking strength — the weight a box can support from other boxes and pallets stacked on top of it. ECT became the dominant standard as supply chains shifted toward palletized, automated warehousing, since stacking strength matters more than puncture resistance in that environment.
A widely used reference point: 32 ECT and 200# Mullen are treated as roughly equivalent for stacking capacity under carrier rules, but the 200# Mullen board will generally have notably higher burst/puncture resistance than the 32 ECT board, even though its stacking performance is similar. That’s the core trade-off — ECT board is usually lighter for the same stacking spec, while Mullen board holds up better against puncture and rough handling.
How to Decide Which Certification You Need
Start with how the product actually travels. If it moves on a pallet, gets stacked in a truck or warehouse, and is handled mostly by machinery (forklifts, conveyors, automated sorters), ECT is the right spec — it’s what most freight and LTL (less-than-truckload) shipping is built around today, and it tends to cost less for equivalent stacking performance. If the product ships as a single parcel that passes through multiple hands and sorting facilities — think small parcel carriers, high-touch fulfillment, or fragile individual items — a Mullen-rated box adds a margin of burst and puncture protection that ECT doesn’t directly certify.
Check the freight classification rules that actually govern box certification. Motor freight shipments fall under NMFC Item 222, and rail freight falls under Uniform Freight Classification Rule 41 — both require a Box Maker’s Certificate specifying a minimum ECT or Mullen rating based on the box’s gross weight and dimensions when it’s part of palletized or LTL freight. This is where the certificate requirement really lives, not in small-parcel shipping: major parcel carriers like UPS don’t require a Box Maker’s Certificate for a standard package. UPS does require you to attach a bright yellow heavy package sticker once a package exceeds 70 lbs, so carrier staff and recipients know to handle it with extra care, but that’s a handling label, not a box strength certification. If you’re shipping retail or big-box freight, ask the retailer directly — many have their own minimum ECT specs in their packaging compliance guides.
When in doubt, ask your box supplier for both numbers. Reputable corrugated manufacturers can tell you the ECT rating, the flute type, and the wall construction (single, double, or triple wall) for any board, and many will stamp both ECT and Mullen equivalents on request if your customer or carrier accepts either.
Tips / Common Mistakes
Don’t assume a higher number is automatically “stronger” across test types — a 32 ECT and a 200# Mullen box are not comparing the same property, so you can’t rank them on the same scale. Compare ECT-to-ECT or Mullen-to-Mullen only.
Don’t overlook flute type and wall count. Two boxes with the same ECT rating can behave differently in transit if one uses a different flute profile (like B-flute vs. C-flute) or if one is double-wall versus single-wall — ECT alone doesn’t capture cushioning or puncture resistance.
Don’t confuse a handling label with a strength certification. A UPS heavy package sticker tells handlers a package is over 70 lbs; it says nothing about the box’s ECT or Mullen rating. Freight-level box strength requirements come from NMFC Item 222 and UFC Rule 41, not from parcel carrier weight thresholds.
Don’t over-spec by default. Jumping to a higher ECT or Mullen rating than your product needs adds material cost and shipping weight for no real benefit — test with your actual product weight and handling conditions before locking in a spec.
Explore more: More packaging compliance guides.
ECT vs. Mullen Test FAQs
Can a box have both an ECT and a Mullen rating?
Yes. Some manufacturers list both on the certificate, especially when a customer’s compliance rules accept either standard. It’s not automatic, though — ask your supplier if you need both documented.
Is ECT always better than Mullen?
Not universally — it depends on how the box is handled. ECT is generally the better fit for palletized, stacked freight, while Mullen provides more direct assurance against puncture and burst damage for individually handled parcels.
Does UPS require a Box Maker’s Certificate for heavy packages?
No. UPS requires a heavy package sticker on packages over 70 lbs so handlers know to take extra care, but that’s a handling label, not a box strength certification. Box Maker’s Certificates with ECT or Mullen minimums are tied to motor and rail freight classification rules (NMFC Item 222 and UFC Rule 41), which apply to palletized and LTL freight rather than standard small parcel shipments.
What does the number in “32 ECT” or “200# Mullen” actually mean?
32 ECT means the board withstands 32 pounds of force per linear inch when compressed edgewise. 200# Mullen means the flat board can withstand 200 pounds per square inch of hydraulic burst pressure before rupturing.
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Photo: Trougnouf / CC BY 4.0, via Wikimedia Commons.