Most postage is priced by weight and distance. Cubic pricing flips that around: for qualifying packages, USPS prices by how much space the box takes up, not how heavy it is. If you sell anything small and dense (candles, coffee, books, tools, sauces, ceramics) this is one of the most useful tricks in the hold.
How it works
Your package is sorted into a cubic tier based on its volume. To find the volume, measure length, width and height in inches, round each down to the nearest quarter inch, multiply them together, and divide by 1,728 (the number of cubic inches in a cubic foot).
- Tiers: 0.1, 0.2, 0.3, 0.4 and 0.5 cubic feet. Your package goes in the smallest tier it fits under.
- Weight: the package must weigh under 20 lb.
- Size: no side can be longer than 18 inches.
- Services: cubic pricing is available for USPS Priority Mail and USPS Ground Advantage through commercial shipping software like Octoship. It isn't offered at the retail counter.
A worked example
Say you ship a set of three ceramic mugs in a 10" x 8" x 4" box, and the whole thing weighs 6 lb. Multiply: 10 x 8 x 4 = 320 cubic inches. Divide by 1,728 and you get 0.185 cubic feet, so the package lands in the 0.2 tier.
At normal weight-based pricing, those 6 lb get charged as 6 lb. At cubic pricing, the 6 lb don't matter at all (they just have to be under 20). Here's how that can look across zones:
| Zone | Priority Mail, 6 lb | Priority Mail Cubic, 0.2 tier | Difference |
|---|---|---|---|
| Zone 2 (nearby) | $12.40 | $8.95 | -$3.45 |
| Zone 5 (mid-country) | $19.85 | $11.60 | -$8.25 |
| Zone 8 (cross-country) | $31.10 | $15.20 | -$15.90 |
The farther the package travels and the heavier it is, the bigger the gap. A light package in a big box, on the other hand, usually does better on regular weight-based pricing.
When cubic is not the answer
- Light, bulky items. A pillow in a 0.5 cubic foot box weighing 1 lb is cheaper priced by weight.
- Long, skinny items. Anything over 18 inches on a side doesn't qualify.
- Oversized boxes. If you use a bigger box than you need, you pay for the empty air. Right-size first.
The short version
If your packages are small and heavy, cubic pricing can save you real money per label, and those savings compound fast over a busy month. Measure your most common boxes carefully, save them as presets, and let the software pick. The box is the same; the bill is smaller.
