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There's some pretty simple rules of thumb when it comes to knowing what will be the cheapest way to ship:

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If your package is 15.9 ounces or less, then using your own packaging with the First Class Package service will always be cheapest.

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If your package is over 1lb but it fits into any Flat Rate Envelope or the Small Flat Rate Box, those are the cheapest ways to go.

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If your package is over 1lb and doesn't fit into those, then it's cheapest to use as small of a box as possible to qualify for Priority Mail Cubic.

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Only if your package weighs more than 20lbs would Medium or Large Flat Rate Boxes ever be a good deal. Regional Rate Boxes are never a good deal.

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One benefit of Priority Mail Cubic is you can use your own packaging, but USPS also offers free boxes you can use with Priority Mail Cubic:

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Priority Mail Box - 1096L | OD: 9-7/16" x 6-7/16" x 2-3/16" | Cubic Tier 1

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Priority Mail Box - 4 | OD: 7-1/4" x 7-1/4" x 6-1/2" | Cubic Tier 2

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Priority Mail Box - 1097 | OD: 11.625" x 2.5" x 13.4375" | Cubic Tier 3

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Priority Mail Box - 1092 | OD: 12.25" x 2.875" x 13.6875" | Cubic Tier 3

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Priority Mail Box - 1095 | OD: 12.5" x 3.125 x 15.375" | Cubic Tier 4

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Priority Mail Box - Shoe Box | OD: 7-3/8" x 5-1/4" x 14-7/8" | Cubic Tier 4

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Learn More: USPS First Class Package and Other USPS Mail Classes

Calculating Your Package's Cubic Tier

For Boxes and Rigid Packages:

Measure the outer dimensions (length, width, height) in inches
Round each measurement down to the nearest quarter inch
Multiply length x width x height
Divide the result by 1728 (cubic inches in a cubic foot)
The result is your package's cubic feet measurement

Example: For a box measuring 12.3" x 9.5" x 3":

Round down to 12.25" x 9.5" x 3"
12.25 x 9.5 x 3 = 349.125
349.125 / 1728 = 0.202 cubic feet
This package falls into the 0.3 cubic feet pricing tier

Note: Maximum size for any single dimension is 18 inches

For Envelopes and Soft Packs:

Measure the length and width of the empty envelope
Round down to the nearest quarter inch
Add the length and width together
Use the total to determine the pricing tier:
- Tier 0.1: Up to 21"
- Tier 0.2: 21.01" to 27"
- Tier 0.3: 27.01" to 31"
- Tier 0.4: 31.01" to 34"
- Tier 0.5: 34.01" to 36"

Restrictions

Tubes and rolls are not eligible for Cubic pricing
Maximum combined length and girth is 84 inches

Benefits of Using Priority Mail Cubic

Cost savings: Cheaper than weight-based pricing for small, heavy items
Predictable pricing: Easy to calculate shipping costs based on box dimensions
Fast delivery: 1-3 business day delivery to any US address, including PO boxes
Free tracking: Monitor shipments at no extra cost
Included insurance: $100 of coverage with every shipment
No surcharges: Residential and fuel surcharges included in the price

Tips for Maximizing Savings

Optimize packaging: Use the smallest box possible that safely fits your items
Consider breaking up large orders: Sometimes shipping in multiple smaller boxes can be cheaper
Use cubic feet calculators: Online tools can help quickly determine your package's tier
Compare rates: Always compare Cubic rates with other shipping methods
Use Commercial Pricing: Accessing Commercial Pricing rates can lead to even more savings

2024 USPS Priority Mail Cubic Rates

Accessing Priority Mail Cubic Rates

Priority Mail Cubic isn't available at regular Post Office counters. However, many online shipping platforms and 3PL providers offer access to these rates. By partnering with a fulfillment provider, you can easily access these discounted rates and streamline your shipping process. For small-scale sellers not yet ready for a 3PL, Pirate Ship offers free access to USPS Priority Mail Cubic rates with no monthly fees or minimums.

What's the Cheapest Way to Ship with USPS?

We will be the first to admit it - shipping is complicated! One of the first questions that any online retailer faces is: what is the most cost-effective way to ship my packages? While there is no one-size-fits-all answer, we understand that controlling your bottom line is vital.