The Short Answer
USPS Priority Mail Cubic Pricing uses 5 volume-based tiers for packages under 0.5 cubic feet and 20 lbs. Tiers are: 0.1 (up to 0.1 cu ft), 0.2 (up to 0.2 cu ft), 0.3 (up to 0.3 cu ft), 0.4 (up to 0.4 cu ft), and 0.5 (up to 0.5 cu ft). Pricing ignores weight, making Cubic ideal for small, dense items that would cost significantly more at weight-based Priority rates.
What is Cubic Pricing?
USPS Cubic Pricing is an alternative rate structure that charges based on package volume rather than weight. Its available only through Commercial Plus pricing, meaning shippers need an account with a USPS-approved software platform or shipping aggregator.
The genius of Cubic is simple: if your package fits in a small space, you pay a small rate regardless of weight. A 15 lb product in a qualifying package ships at the same Cubic rate as a 5 lb product of identical dimensions. For dense products like hardware, tools, supplements, and books, this can cut shipping costs by 40-60% compared to weight-based Priority Mail.
However, Cubic has strict limits. Maximum volume is 0.5 cubic feet (864 cubic inches). Maximum weight is 20 lbs. Packages exceeding either limit dont qualify and must use standard Priority Mail rates.
The 5 Cubic Tiers
USPS defines cubic volume as Length × Width × Height in inches, then divided by 1,728 to convert to cubic feet. The result determines your tier:
Tier 0.1: Up to 0.10 cubic feet (up to 173 cubic inches). Example: 6×6×4.8 inch box.
Tier 0.2: 0.10 to 0.20 cubic feet (174-346 cubic inches). Example: 7×7×7 inch box.
Tier 0.3: 0.20 to 0.30 cubic feet (347-518 cubic inches). Example: 8×8×8 inch box.
Tier 0.4: 0.30 to 0.40 cubic feet (519-691 cubic inches). Example: 9×9×8.5 inch box.
Tier 0.5: 0.40 to 0.50 cubic feet (692-864 cubic inches). Example: 9.5×9.5×9.5 inch box.
When Cubic Wins
Example: 12 lb product in 7×7×5 inch box
Volume: 245 cubic inches = 0.14 cubic feet = Tier 0.2. Commercial Plus Cubic rate Zone 5: approximately $9.50. Standard Priority Mail at 12 lbs Zone 5: approximately $18.50. Savings: $9.00 per package (49%).
Example: 8 lb product in 8×6×4 inch box
Volume: 192 cubic inches = 0.11 cubic feet = Tier 0.2. Cubic rate Zone 7: approximately $11.75. Standard Priority at 8 lbs Zone 7: approximately $21.00. Savings: $9.25 (44%).
When Cubic loses: Light products in qualifying boxes. A 2 lb product in a 7×7×5 box still pays the Tier 0.2 Cubic rate. At 2 lbs, standard Priority might be cheaper. Always compare both calculations.
Optimization Strategy
Right-size your packaging: The tier boundaries create cliffs. A box at 0.21 cubic feet pays Tier 0.3. Reducing to 0.19 drops to Tier 0.2. Target the lower bound of each tier rather than the upper.
Calculate the break-even weight: For each tier, determine when Cubic beats Priority. Generally, if your product weighs more than 4 lbs in Tier 0.1 or more than 6 lbs in Tier 0.2, Cubic likely wins. Test with your specific zones.
Combine with poly mailers: Soft goods shipped in poly mailers that can be compressed may qualify for lower tiers than their unpacked volume would suggest. USPS measures the actual package, not theoretical box size.
Getting Started
1. Check Eligibility: Cubic requires Commercial Plus pricing through USPS-approved platforms like Pirate Ship, ShipStation, or Stamps.com. Retail counter rates dont include Cubic.
2. Measure Your Products: Record L×W×H for your top sellers. Calculate cubic feet. If under 0.5 cu ft and 20 lbs, Cubic may apply.
3. Compare Rates: Price each product at both Cubic tier rate and standard Priority weight-based rate. Use the lower option.
4. Stock Smart Packaging: Design or source boxes that stay just under tier boundaries. An 8×8×5 inch box (0.18 cu ft, Tier 0.2) beats 8×8×6 inch (0.22 cu ft, Tier 0.3).
5. Automate Selection: Most shipping platforms auto-select Cubic when its cheaper. Verify this is enabled in your account settings.
Pro Insights
Savvy shippers design products and packaging together to hit Cubic tier sweet spots. A product manufacturer might reduce packaging by 0.5 inches to drop from Tier 0.3 to 0.2, saving $2-3 per shipment. At 10,000 packages annually, thats $20,000-30,000 in savings from a minor design change.
Looking Ahead
USPS periodically adjusts Cubic tier pricing in their annual rate updates. The tier boundaries (0.1, 0.2, 0.3, 0.4, 0.5) have remained stable, but per-tier costs increase each year. Some industry observers expect USPS to add a 0.6 tier or adjust boundaries as package sizes evolve. Monitor annual USPS rate announcements for changes.
Find Your Cheapest Rate
Compare Cubic vs Priority vs other carriers for your package instantly.
Launch CalculatorCubic Tier Quick Reference
| Tier | Cu Ft Range | Cu In Range | Example Box |
|---|---|---|---|
| 0.1 | 0-0.10 | 0-173 | 6×6×4 |
| 0.2 | 0.10-0.20 | 174-346 | 7×7×7 |
| 0.3 | 0.20-0.30 | 347-518 | 8×8×8 |
| 0.4 | 0.30-0.40 | 519-691 | 9×9×8 |
| 0.5 | 0.40-0.50 | 692-864 | 9.5×9.5×9.5 |
Technical Details
USPS calculates cubic volume using outside package dimensions. For irregular shapes like poly mailers, they use the smallest box that would contain the package. Soft packages are often manually measured, which can introduce variability.
The maximum dimension for any single side is 18 inches under Priority Mail. Combined with the 0.5 cu ft limit, this creates practical constraints on package shape. Flat or elongated packages may hit the 18-inch limit before reaching volume limits.
Cubic rates include insurance up to $100, same as standard Priority Mail. Additional insurance can be purchased. Delivery time is identical to Priority Mail for the destination zone, typically 1-3 business days.
Frequently Asked Questions
Disclaimer: This content is for educational purposes only.