Cubic feet is length times width times height in inches divided by 1,728. In parcel pricing that single number decides two separate things: whether a USPS parcel can be priced by cube instead of by pound, and whether USPS applies dimensional weight to it at all.
Half a cubic foot is where cube pricing ends. A full cubic foot is where volume billing begins. Both lines run off the same arithmetic.
Enter the outside of the carton in inches. The tool multiplies the three figures, divides by 1,728 and tells you which side of each USPS line the box falls on.
Cube pricing also requires the parcel to weigh 20 pounds or less and its longest dimension to stay at 22 inches or under, so the volume answer above is a necessary condition rather than a full verdict.
Between those two lines sits an unremarkable middle band, from half a cubic foot to a full one, where the parcel is simply priced on its weight and zone. Most everyday boxes live there.
Cube pricing is banded, so a parcel at 0.11 cubic feet and one at 0.20 pay the same. The published tiers below show why measuring carefully is worth a minute of your time.
| Tier | Zone 1 | Zone 2 | Zone 8 |
|---|---|---|---|
| 0.10 cu ft | $7.45 | $7.51 | $10.13 |
| 0.20 cu ft | $7.82 | $7.89 | $11.84 |
| 0.30 cu ft | $8.39 | $8.45 | $14.67 |
| 0.40 cu ft | $9.07 | $9.13 | $17.29 |
| 0.50 cu ft | $9.59 | $9.66 | $18.90 |
| Carton | Cubic inches | Cubic feet | Which side of the lines | Cheapest quote |
|---|---|---|---|---|
| 6 x 5 x 5 in, 2 lb | 150 | 0.09 | inside the smallest cube tier on volume, weight and longest side | $8.48 |
| 12 x 10 x 6 in, 5 lb | 720 | 0.42 | inside the 0.50 tier on volume, longest side 12 in | $13.52 |
| 12 x 12 x 12 in, 15 lb | 1,728 | 1.00 | exactly on the cubic foot line, so volume billing has not engaged | $17.01 |
| 18 x 14 x 10 in, 5 lb | 2,520 | 1.46 | past the cubic foot line, volume billing applies | $20.50 |
| 24 x 18 x 14 in, 5 lb | 6,048 | 3.50 | far past it | $36.75 |
The mug box at 0.09 cubic feet quoted $8.48 on USPS Ground Advantage, and the 12 x 12 x 12 in carton holding 15 lb quoted $17.01, both on the Newark to Atlanta lane that morning. Note the last two rows: identical contents at 5 lb, and the volume alone put $16.23 between them. That is what a cubic figure is really telling you.
One thing to be plain about: we sell United States domestic parcel labels with no account needed, and our quotes already reflect whatever pricing the carrier applies to a given box. There is no cube option to tick at checkout, and we do not sell international labels, warehousing or marketplace integrations.
Cube eligibility rounds each measurement down to the nearest quarter inch. The dimensional weight rule rounds each measurement to the nearest whole inch. Same box, two different sets of arithmetic, and using the wrong one at the wrong moment gives you the wrong answer.
Prices move at the tier boundaries and nowhere else. Shaving a box from 0.31 to 0.30 cubic feet is worth real money; shaving it from 0.29 to 0.28 is worth nothing at all.
A poly mailer or padded envelope is measured on length plus width, with a combined cap of 36 inches, rather than on three dimensions. Flat goods often qualify where a rigid box of the same contents would not.
Multiply length by width by height in inches, then divide by 1,728. A 12 x 10 x 6 in carton is 720 cubic inches, which comes to 0.42 cubic feet, and a 12 x 12 x 12 in carton lands on exactly 1.00.
Half a cubic foot. The rule requires each cubic mailpiece to measure .50 cubic foot or less and weigh 20 pounds or less, with a longest dimension not exceeding 22 inches and each measurement rounded down to the nearest quarter inch.
Above one cubic foot, meaning more than 1,728 cubic inches, for parcels going to Zones 1-9. Below that line the scale weight governs and the volume comparison is never run.
On the Newark to Atlanta lane on 2026-07-28, the identical 5 lb of contents quoted $20.50 in an 18 x 14 x 10 in carton at 1.46 cubic feet and $36.75 in a 24 x 18 x 14 in carton at 3.50 cubic feet, a gap of $16.23 bought with nothing but air.