How is USPS cubic size calculated?

For a rigid box, round each dimension down to the nearest quarter inch, multiply length by width by height, then divide by 1,728 to get cubic feet. That cubic-foot figure sets the tier. A 10 by 8 by 6 inch box is 480 cubic inches, or 0.278 cubic feet, which falls in the 0.30 cubic-foot tier.

What qualifies for USPS Ground Advantage cubic pricing?

As of July 2026 a rigid package qualifies for Ground Advantage cubic pricing when its longest side is 22 inches or less, it weighs 20 pounds or less, and its cubic size is 1.0 cubic foot or less. Cubic pricing is priced by size, not weight, so it often beats standard weight-based pricing for small, dense items.

What are the USPS Ground Advantage cubic tiers?

Ground Advantage cubic has ten tiers in tenth-of-a-cubic-foot steps: 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, and 1.00 cubic feet. A package is charged at the tier its measured cubic size falls into. Anything over 1.0 cubic foot uses standard weight-based Ground Advantage pricing instead.

USPS Cubic Pricing Calculator

Small and dense? USPS cubic pricing charges by size, not weight — often the cheapest way to ship it. Enter a box and get its Ground Advantage cubic tier and whether it qualifies, measured the USPS way (rounded down to the quarter inch).

USPS cubic pricing is a US, inches-and-pounds rule — enter your box in inches and its weight in pounds.

Length (in)

Width (in)

Height (in)

Weight (lb)

Cubic pricing caps at 20 lb — over that, it’s weight-based.

Cubic pricing

—

Cubic tier — cu ft

Cubic size — cu ft measured

Longest side — inches

Weight — pounds

Cubic eligibility

Requirement	Limit	This package

Check the billable weight instead →

What USPS cubic pricing is

Most parcels are priced by weight and zone. Cubic pricing throws weight out and prices small packages purely by how much space they take up — so a small, heavy box (a case of screws, a stack of books) can ship for far less than its weight-based rate. It’s a USPS Ground Advantage option for compact packages, and knowing your tier tells you whether it’s worth using.

Cubic pricing rewards dense freight. The heavier a small box is for its size, the more you save shipping it by the cubic foot instead of the pound.

How the cubic size is measured

For a rigid box, USPS rounds each dimension down to the nearest quarter inch, multiplies length × width × height, then divides by 1,728 to get cubic feet. That figure lands in one of ten tiers. A 10 × 8 × 6″ box is 480 cubic inches — 0.278 cubic feet — which bills at the 0.30 tier.

Requirement (as of July 2026)	Limit
Longest side	≤ 22 in
Weight	≤ 20 lb
Cubic size	≤ 1.0 cu ft
Tiers	0.10 – 1.00 cu ft (0.1 steps)

July 2026 raised the maximum length from 18″ to 22″, so more boxes now qualify. Go over any limit — longest side, weight, or 1.0 cu ft — and the package falls back to standard weight-based Ground Advantage pricing.

A note on soft packs and rates

This tool measures rigid boxes. Poly bags and padded envelopes use a different, length-plus-width tier method and were trimmed to five tiers in July 2026. And because published rates change with every increase, this tool gives you the tier and eligibility — the piece that decides whether cubic is even on the table — not the dollar figure. Take the tier to USPS or your shipping platform for the current price.