ECT vs. Mullen: How to Read a Corrugated Box Rating

What is the stamp on the bottom of a box?

The round or oval mark printed on a bottom flap is the Box Maker's Certificate. It identifies the plant that made the box and states what the board was tested at.

The certificate carries one of two ratings. Either a bursting test figure written in pounds, such as 200#, 275#, or 350#, or an edge crush figure written as a number followed by ECT, such as 32 ECT. It also lists a size limit, given as length plus width plus depth, and a gross weight limit.

Those limits are carrier requirements, not suggestions. A box shipped over its stated size or weight limit can lose a freight damage claim on the certificate alone, regardless of how well the box actually performed.

What does ECT measure?

Edge crush test measures the force a board edge holds before it buckles, reported in pounds per inch of edge.

Stacking strength runs almost entirely through the vertical edges and corners of a box. ECT tests that directly, which makes it the number that predicts whether the bottom box in a six-high stack holds up the five above it.

A 32 ECT board is the common single wall workhorse. 44 ECT single wall carries meaningfully more. Double wall starts around 48 ECT and climbs from there.

What does Mullen measure?

Bursting strength, the Mullen test, measures the pressure needed to rupture the board face, reported in pounds per square inch.

That predicts a different failure. A corner impact, a forklift tine, a strap cutting in, a box dropped off the end of a conveyor. Rough handling and single-parcel shipping punish the face of the board, and Mullen is the number that describes how much punishment it takes.

Heavy, dense, irregular products in a rough network favor a Mullen rating. So does anything with a hard corner inside pressing outward.

What are the ECT and Mullen equivalents?

Carrier classification rules publish a substitution table so a shipper can swap one rating for the other. The common pairs:

Read that table as permission, not as a conversion. The two tests measure different failures, so an equivalent box satisfies the carrier without behaving identically on a dock. A 32 ECT box and a 200# box are both legal for the same freight and they will not fail the same way.

Board weight is where the practical difference shows. ECT-rated board generally uses less fiber to reach the same carrier acceptance, which cuts material cost and shipping weight. That is the reason most stacking-oriented packaging has moved to ECT.

Does flute size matter?

Flute size sets thickness, cushioning, and printability, and it changes the answer independently of the rating.

Double wall combines two flute profiles, and B/C is the most common pairing, to get thickness and stacking strength together.

C flute, around 3/16 inch

The default for shipping boxes. Good stacking strength and good all-round cushioning.

B flute, around 1/8 inch

Thinner and more rigid on the flat. Resists face crush better and takes print better, which is why it shows up in retail-ready packaging and internal partitions.

A flute, around 1/4 inch

The thickest common single wall. More cushioning and more stacking strength, at the cost of space.

E and F flute, 1/16 inch and thinner

Print-quality flutes for small retail boxes, not for freight.

Why does a box that passed the test still crush?

Because the rating is a lab number and a warehouse is not a lab.

ECT is measured on conditioned board at controlled temperature and humidity. Three things pull real performance below it. Moisture, because corrugated loses strength as it takes on humidity, and a box stored in an unconditioned warehouse through an Alabama summer is not the box that was tested. Time, because board under constant load creeps, and a stack that held on day one can sag by day thirty. And alignment, because stacking strength lives in the corners, so a box overhanging the pallet edge or a column stacked out of register has already given away most of its rating.

The practical fix is margin. Spec above the calculated requirement when loads sit for weeks, when storage is unconditioned, or when stacks get built by hand.

Should the box be single wall or double wall?

Single wall handles most freight. Double wall earns its cost in four situations.

• Heavy contents, generally past about 65 pounds, where single wall runs out of edge strength
• Tall stacks, where the bottom box carries several hundred pounds for a long time
• Long or repeated transit, including export freight and anything handled more than twice
• Products with hard internal corners that push outward against the wall

Triple wall exists past that, for machinery, engines, and loads that would otherwise need a wooden crate. The line between heavy triple wall corrugated and a wooden crate is worth pricing both ways, since crates win on very heavy or very long items and on anything needing an internal frame.

What should be sent with a box quote request?

Enough to size the board without a second call.

• Internal dimensions in length, width, and depth, measured to the product rather than to an old box
• Product weight
• Whether loads stack, and how high
• Whether loads go into racking
• Where boxes are stored, and whether that space is climate controlled
• How they ship: full truckload, LTL, or parcel
• Whether print is needed

A photo of the current box with its Box Maker's Certificate visible answers half of it at once.