Carriers bill whichever is greater: the pounds on the scale or the pounds a box "should" weigh for its size. The formula behind that second number is public and simple, so here it is as a tool you can use right now, no download and no signup.
A truck fills up by volume long before it maxes out on weight, which is the whole reason this formula exists.
Enter the outer box in inches and the packed weight in pounds. The tool divides the cubic volume by 139, the standard retail divisor at USPS, UPS and FedEx, then compares the result to the scale and reports which number the carrier will charge on.
Fractions round up, not to the nearest inch. The Postal Service instructs you to “round up each measurement to the nearest whole inch,” then divide by 139 and round up a second time, so a side measuring 12.2 in is rated as 13. The tool does both steps for you; type what the tape measure actually reads (pe.usps.com, DMM section 123, read 2026-08-15).
| Carton | Scale weight | Size-based weight | Billed on | Cheapest quote |
|---|---|---|---|---|
| 18 x 14 x 12 in | 20 lb | 3,024 ÷ 139 ≈ 22 lb | size | $23.00 UPS Ground Saver |
| 24 x 18 x 14 in | 50 lb | 6,048 ÷ 139 ≈ 44 lb | scale | $47.42 UPS Ground |
The first carton is the classic trap: light enough that the volume math wins, so two phantom pounds ride along on the invoice. The second is dense enough that the scale stays in charge. Quotes from the SMKlog calculator already run this comparison for you, which is why the price you see is the price that holds.
Dimensional weight is the gentle version of this. The expensive version is a threshold, and the gap between the two boxes below is the clearest measurement we have of it. Same 35 lb of contents, same lane, same day, two boxes that would look identical stacked in a hallway.
| Box | Volume | Length plus girth | Billed weight | Carrier charge |
|---|---|---|---|---|
| 54 x 28 x 8 in | 12,096 cu in | 126 in | 88 lb, dimensional | $100.86 |
| 54 x 30 x 9 in | 14,580 cu in | 132 in | 105 lb, dimensional | $183.48 |
Neither box comes near the 17,280 cu in ceiling, and the scale reads 35 lb both times. What separates them is a single line: length plus girth of 130 inches. The smaller box lands at 126 and stays in the additional handling tier. The larger one lands at 132, two inches over, and becomes a large package — a tier that swaps the smaller charge for a much bigger one and rates the box at a minimum of 90 lb whatever it weighs. Two inches of width and one of height moved it across, and the invoice went up $82.62.
This is the argument for measuring the box before you buy it rather than weighing the contents after you pack them. A scale tells you 35 lb in both cases. Nothing about the weight predicts the second number.
The divisor encodes an assumed density, roughly how many cubic inches carriers grant per pound before charging for air. Contract shippers sometimes negotiate a gentler divisor, but 139 is the number retail rates are built on.
The Postal Service skips the volume comparison until a parcel tops one cubic foot, 1,728 cubic inches. Under that threshold your scale weight stands, which quietly favors USPS for compact shipments.
Bubble wrap weighs almost nothing, so protection itself is cheap; it's the box growing around it that costs. Snug interior padding in the smallest safe carton beats a roomy box full of loose fill on both damage risk and price.
It's a carrier's way of charging for the space a box occupies rather than what the scale says. Multiply length by width by height in inches, divide by 139, and if that number tops the actual pounds, the bigger number is what gets billed.
For retail domestic parcels the working divisor at these carriers is 139, and that's the one this page's tool uses. USPS only applies the size-based math once a box exceeds one cubic foot, while UPS and FedEx apply it broadly.
When the box is large for its contents. Our 18 x 14 x 12 in test carton holds 3,024 cubic inches, which divided by 139 gives about 21.8, so it rates as 22 lb even when the scale reads 20. Dense boxes escape: a 24 x 18 x 14 in carton at a true 50 lb calculates to only 44, so the actual weight governs.
Shrink the air, not the protection. Choose the smallest box the padded item fits, avoid doubling boxes without reason, and never ship a small heavy thing in a big pretty carton. An inch trimmed from each side often drops a whole billable pound.
No, and that is where big boxes get expensive. Above 10,368 cubic inches a package picks up an additional handling charge, and above 17,280 cubic inches, or 130 inches of length plus girth, it becomes a large package billed at a minimum of 90 lb. On 2026-08-21 a 54 x 30 x 9 in box at 132 inches drew $183.48 in carrier charges while a 54 x 28 x 8 in box at 126 inches drew $100.86, with the same 35 lb inside.
Carriers revise these figures without notice, and the thresholds above moved twice in 2026. The dates are printed so you can tell how fresh the reading is; check the carrier's own page before committing to an expensive box.