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.
Round each dimension to the nearest whole inch the way carriers do if you want the strictest answer.
| 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 | $45.01 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.
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.