Three numbers decide whether a big box can travel as a parcel: its weight, its longest side, and its length plus girth. Stay inside the ceilings and the job is ordinary; cross one and the shipment belongs to freight. This guide walks the measuring, the limits and the volume billing. The companion question of which carrier charges least for large cartons has its own page, linked below.
Every rule on this page keys off three dimensions taken after the box is packed and closed.
Lay the packed box down and call its longest side the length. Girth is the distance around the other two sides: twice the width plus twice the height, which matches the formula UPS publishes at ups.com, checked 2026-07-27. Add length to girth and you have the combined figure every ceiling below refers to. Our 40 by 20 by 20 inch test carton works out to 80 inches around plus 40 along, or 120 combined.
Cardboard bulges once foam and contents push the walls out, so numbers taken from an empty box run small. Hubs pass every parcel through automated measuring machines, and a declared size that disagrees with the scanned one turns into an extra charge on the back end. Taking the tape to the sealed carton, bulges included, is the cheap way to keep the first price the final one.
A big box takes its drops on the corners, so fill every void; a wall that can flex will crease under the parcel stacked on top of it. USPS asks for 2-inch-wide packing tape across all seams and warns against string or twine, which snags sorting equipment, per usps.com/ship/packages.htm, checked 2026-07-27. Double-walled cardboard earns its price on anything past 30 lb.
| Carrier | Weight ceiling | Length rules | Length plus girth |
|---|---|---|---|
| USPS | 70 lb | 108 in cap on Priority Mail and Priority Mail Express | 130 in maximum |
| UPS | 150 lb | 108 in maximum length | 165 in maximum |
| FedEx | 150 lb before freight | oversize charge past 96 in | oversize charge past 130 in, freight past 165 in |
Read the table from the left. A bathroom scale answers the first column, the tape measure answers the other two, and the smallest ceiling your box clears decides which carriers can even bid on it.
Postal rules price any parcel bigger than one cubic foot at whichever is greater, the scale weight or the dimensional weight, computed as length times width times height divided by 139 and rounded up, per pe.usps.com/text/dmm300/123.htm, checked 2026-07-27. To show what that does to real money, we quoted two oversize cartons from Newark, NJ (07102) to Atlanta, GA 30303 through the SMKlog calculator on 2026-07-27. Each figure below is the complete checkout amount for that service on that box.
| Carrier and service | 40 × 20 × 20 in, 25 lb | 30 × 24 × 24 in, 45 lb |
|---|---|---|
| UPS Ground | $98.81 | $105.87 |
| UPS Ground Saver | $101.79 | $108.15 |
| FedEx Ground Economy | $118.68 | $126.98 |
| UPS 3 Day Select | $144.69 | $154.04 |
| FedEx Ground | $169.94 | $178.47 |
| USPS | no rate returned | no rate returned |
The 25 lb carton on the left is billed like a parcel weighing well past 100 lb, purely on volume. For contrast, a compact 50 lb box roughly 24 by 18 by 14 inches quoted $45.01 on UPS Ground over the identical route that same day, with 40 lb at $38.45 and 30 lb at $31.52. Twice the scale weight, less than half the price, because those boxes stay dense. Shrinking a carton by an inch per side does more for the total than shaving pounds ever will.
Price your carton by its true size Compare large-box carriers by cost
We also fed the calculator a genuinely huge box on 2026-07-27: 60 by 30 by 30 inches at 60 lb, which stands at 180 combined inches. No parcel rate came back. The system moved the request into a freight review, where a person prices it as cargo. That refusal is the honest answer; a parcel label sold for that box would be intercepted at the first hub. Anything past 150 lb, past 108 inches of length, or past 165 combined inches should start life as a freight quote, and palletizing gets cheaper than cardboard sooner than most people expect.
Girth is twice the width plus twice the height, and carriers judge a box by its length plus that girth. A 40 by 20 by 20 inch carton carries 80 inches of girth, so it stands at 120 combined inches, per the formula on ups.com, checked 2026-07-27.
USPS tops out at 70 lb and 130 combined inches, with a 108-inch cap on Priority Mail. UPS runs to 150 lb, 108 inches of length and 165 combined inches. FedEx adds oversize charges beyond 96 inches of length or 130 combined inches and treats anything past 150 lb or 165 combined inches as freight. All per the carriers' own pages, checked 2026-07-27.
Volume billing. USPS rules price any parcel over one cubic foot at the greater of its scale weight or length times width times height divided by 139, per pe.usps.com, checked 2026-07-27, and our quotes show the ground carriers behaving the same way. A roomy carton is bought by the cubic inch, not the pound.
It leaves the parcel network. When we quoted a 60 by 30 by 30 inch box at 60 lb on 2026-07-27, the calculator returned no parcel rates at all and routed the job to a freight review with a person instead. That beats buying a label a hub would refuse or reprice.