USPS Priority Mail Cubic: How It Works and When to Use It

Priority Mail Cubic is a USPS pricing option that charges by a package's outer size instead of its weight. For small, dense items like a box of hardware or a bottle of supplements, that can cut shipping costs sharply compared with standard weight-based rates.

How Cubic pricing works

USPS sorts Cubic packages into tiers based on outer volume, from the smallest tier up. The smaller the box, the lower the tier and the cheaper the label, and weight doesn't factor in at all as long as you stay under the limit. That's why Cubic wins for anything heavy for its size: you pay for the space the box takes up, not the pounds inside it.

Size and weight limits

To qualify, a package has to measure half a cubic foot (0.5 cubic feet) or less, weigh no more than 20 lbs, and be no longer than 22 inches on its longest side, according to the USPS Domestic Mail Manual. Rolls and tubes don't qualify. You can ship Cubic in your own box, but not in a USPS Flat Rate box or envelope, which can only be used at the Flat Rate price. Anything larger or heavier moves to standard Priority Mail.

Poly mailers and padded envelopes qualify too, under the same limits. USPS sets their tier by adding the length and width of the original mailer, and that total can't go over 36 inches. For the tier cutoffs, see our guide to how USPS cubic tiers work, including Ground Advantage Cubic.

How you actually get Cubic pricing

Here's the part shippers miss: you can't buy Priority Mail Cubic at the Post Office counter. It's commercial-only pricing, available through shipping software, online postage providers, and 3PLs. Print a label at retail and USPS charges the standard weight-based rate instead, so with Cubic, how you buy the label matters as much as the box you put the product in.

When Cubic is the right call

Reach for Cubic when your product is small and dense and ships across the country, where weight-based and zone-based rates climb fastest. It's also the replacement most shippers moved to after USPS discontinued Priority Mail Regional Rate Boxes in 2023.

Shipping small, heavy products at volume? A 3PL sets up the commercial pricing, so services like Cubic are simply how your orders go out. See how Simpl prices fulfillment.