USPS cubic pricing explained
For small, heavy packages, USPS cubic pricing can be one of the cheapest ways to ship in the US. Instead of charging by weight, it charges by the package's volume tier, as long as the package stays within size and weight limits.
How the tier is calculated
- Measure the outside of the package in inches.
- Round each side down to the nearest quarter inch. A side of 8.9 inches becomes 8.75.
- Multiply and divide by 1,728 to get cubic feet.
- Round up to the next tier. Tiers step in tenths of a cubic foot: 0.1, 0.2, 0.3, 0.4 and 0.5.
An example
A package measures 10.1 × 8.2 × 6.3 inches and weighs 8 lb. Rounded down: 10 × 8 × 6.25 = 500 cubic inches. 500 ÷ 1,728 = 0.289 cubic feet, which falls in the 0.3 tier. The price is the same whether the package weighs 2 lb or 18 lb.
Eligibility
The usual limits are a maximum of 0.5 cubic feet, 20 lb, and 18 inches on the longest side. Cubic pricing is typically available through commercial shipping software rather than at the post office counter, and the services and tiers offered change from time to time. Check the current USPS rules and your software before relying on it.
Who benefits
Dense products in small boxes: supplements, cosmetics, tools, parts, coffee, books. If a 5 lb product fits in a 0.2 cubic foot box, cubic pricing can be much cheaper than weight-based pricing.
Light, bulky products don't benefit, because they'd be cheap by weight anyway and often exceed the size limit.
How to drop a tier
Because sides round down to the quarter inch, a small change can move you into a lower tier. A box that calculates at 0.302 cubic feet sits in the 0.4 tier; shaving a quarter inch off one side might bring it under 0.3. Boxes designed for cubic pricing can save a meaningful amount at volume.