Box strength is printed on the box maker's certificate, a round stamp on a flap, as an Edge Crush Test (ECT) rating or a Mullen burst rating plus a size limit and a gross weight limit. ECT describes how well the board resists crushing from above; burst describes the pressure it takes to rupture the board. For a heavy parcel, never pack past the stamp's weight limit, then check your carrier's chart, because the UPS parcel chart asks for a stronger box at most weights than the freight rule behind the stamp.

What do ECT and Mullen ratings measure?

They are separate lab tests: ECT for stacking strength, Mullen for burst strength. According to IPS Testing's T 811 summary, the Edge Crush Test, TAPPI method T 811, measures the edgewise compressive strength of a short column of single-, double- or triple-wall corrugated board. That number relates to the top-to-bottom compression strength of the finished box, which keeps a carton from folding when other parcels sit on it.

The Mullen test, TAPPI method T 810, clamps a sample of board and pushes a diaphragm through it with hydraulic pressure until it ruptures, according to IPS Testing's summary of the T 810 bursting strength method.

The units differ too. UPS prints burst strength in pounds per square inch and ECT in pounds per inch of width, per its packaging guidelines PDF.

How do you read a box maker's certificate?

Find the round stamp on a flap and read three things: the strength rating, the size limit and the gross weight limit. Sample stamps in a Walmart supplier standard hosted by the Fibre Box Association come in two styles. An ECT stamp reads "EDGE CRUSH TEST 32 LBS/IN, SIZE LIMIT 75 INCHES, GROSS WT LT 65 LBS". A burst stamp reads "BURSTING TEST 200 LBS PER SQ INCH" and also lists a minimum combined weight of facings, a size limit and a gross weight limit.

Strength rating. ECT or burst, in the units above.
Size limit. Length, width and depth added together, in inches. This is not the length-plus-girth measurement UPS uses, so see UPS additional handling and large package surcharges for how UPS measures size.
Gross weight limit. The most the packed box, box plus contents, may weigh. According to the same document, that limit is set for full-truckload and less-than-truckload freight carriers.

That last line is where UPS's own documents disagree. The UPS heavy items support page says BMC weight limits "are intended for palletized freight shipments, not for shipments with small parcel carriers". The UPS packaging guidelines PDF says "Never exceed the maximum gross weight limit for the box, which is printed on the Box Maker's Certificate". The reading that satisfies both: the stamp limit is a ceiling you never cross, and for parcels you follow the stricter UPS chart.

NMFTA's packaging manager puts it bluntly: "If the box is only rated for 65 pounds, don't try putting 100 pounds in the box." She adds that freight damage claims are likely to be denied when contents exceed the certificate.

Is a 32 ECT box the same as a 200 lb burst box?

Not automatically. IPS Testing's T 810 page says burst strength and ECT "measure different things" and "cannot be directly related one to the other". The freight table below pairs 200 lb burst with 32 ECT because both grades get the same limits on single wall, not because one converts into the other.

Per the same T 810 page, the burst method does not apply to triple-wall board, and burst results on double-wall board are "of questionable accuracy".

If a supplier lists "200 lb test" and your chart asks for ECT, ask for the ECT grade rather than converting.

How much weight can single, double and triple wall boxes carry?

It depends on the document, and the UPS parcel chart is stricter than the freight table on the stamp. These figures were read on October 21, 2026. The freight table (Item 222, as reprinted in the Walmart standard) pairs each grade with a gross weight limit:

Wall	Grade (burst or ECT)	Gross weight limit
Single	200 lb or 32 ECT	65 lb
Single	250 lb or 40 ECT	80 lb
Single	275 lb or 44 ECT	95 lb
Single	350 lb or 55 ECT	120 lb
Double	200 lb or 42 ECT	80 lb
Double	275 lb or 48 ECT	100 lb
Double	350 lb or 51 ECT	120 lb
Double	400 lb or 61 ECT	140 lb
Double	500 lb or 71 ECT	160 lb
Double	600 lb or 82 ECT	180 lb
Triple	67 to 112 ECT (puncture rated)	240 to 300 lb

The four single-wall rows carry size limits of 75, 85, 95 and 105 inches.

The UPS packaging guidelines chart sets a higher bar for parcels: 32 ECT / 200 lb burst single wall for 0 to 30 lb only, 44 ECT / 250 lb for 31 to 50 lb, 55 ECT / 275 lb for 51 to 65 lb, and 350 lb burst for 66 to 80 lb. From 81 lb it moves to double wall, starting at 61 ECT / 350 lb and reaching 600 lb burst at 141 to 150 lb. UPS calls these strengths "only guidelines" and "not to be considered packaging specifications", and says packaged products should be tested to ISTA 3A.

USPS uses pound-test terms for high-density items such as tools, hardware, and machine and auto parts. Per USPS Domestic Mail Manual section 601, 20 to 45 lb needs at least 200-pound test fiberboard, and 45 to 70 lb needs at least 275-pound test, or equivalent wood, metal or plastic containers.

FedEx's general packaging guidelines say "Use double-wall boxes for heavier items" without a weight number, so for FedEx the stamp is the hard ceiling.

Can you reuse a box for a heavy shipment?

For the heaviest parcels, UPS says no. For packages over 70 lb, UPS asks for new boxes "that have not been compromised by humidity or the wear and tear of previous use", preferably with stitched or stapled seams, "not just glued". FedEx says to remove old labels from reused boxes and check for "holes, tears, or corner dents". The stamp describes the box when new, not its condition today.

How do you choose a box grade for a heavy parcel?

Start from the packed weight and apply the strictest of three checks: the stamp, the carrier chart and the content rule. Box grade is one step in our guide to shipping heavy packages; padding and taping are in how to pack a heavy box.

Weigh the packed box with its padding.
Compare length plus width plus depth with the stamp size limit.
Confirm the packed weight is under the stamp's gross weight limit.
Find your row on the carrier chart: UPS guidelines, or DMM 601 for high-density items by USPS.
Move to double wall where the chart says so, which is 81 lb on the UPS chart.
Over 70 lb on UPS, use a new box with stitched or stapled seams.
Measure the final box for billing, since a stronger box can be a bigger one and dimensional weight may change the price.

Check	Example A: 58 lb hand tools, 20 x 16 x 14 in	Example B: 85 lb machine parts
Freight table, single wall	32 ECT / 200 lb (65 lb limit)	44 ECT / 275 lb (95 lb limit)
UPS parcel chart	55 ECT / 275 lb single wall	Double wall, from 61 ECT / 350 lb
USPS high-density rule	At least 275-pound test	Covers up to 70 lb only

In Example A, a 32 ECT stock box passes its stamp (50 inches and 58 lb against 75 inches and 65 lb) but falls short of the UPS 51 to 65 lb row. A 275 lb burst single-wall box matches the burst figure in that row and meets the USPS 275-pound rule, so one box works for either carrier.

Example B needs double wall on the UPS chart. Split into two 43 lb boxes, each sits in the UPS 31 to 50 lb row (44 ECT / 250 lb) and the USPS 20 to 45 lb row (200-pound test). Two labels can cost more or less than one, so price both. Our comparison of UPS vs USPS for heavy packages lays out the trade-offs.

Where GoatLabels fits

GoatLabels does not sell boxes or check box grades; it buys the label once you know the packed weight and size. It rate-shops USPS, UPS, FedEx and DHL from one account with one all-in price per rate, so you can quote one 85 lb box against two 43 lb boxes before you pack. If you split, each box can get its own label carrying the same reference. Re-weighs and dimension corrections after delivery post to the same prepaid wallet, and GoatLabels finds and drafts refund claims for disputed re-weigh and dimensional weight surcharges. Free covers 50 shipments a month; Pro is a flat $40 a month for unlimited shipments (pricing).

The limit that matters here: GoatLabels is parcel only and does not book LTL or freight, so a load that belongs on a pallet needs a freight carrier.