Priority Mail Cubic

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If your package is around the size of a shoebox or smaller, it's perfect for this secret dimension-rated service that lets you ship up to 20 pounds for no additional cost.


Max Any Dimension: 18" inches

Max Cubic Volume: 0.50 cubic ft

Max Weight: 20 pounds

Calculate your Priority Mail Cubic pricing tier

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Get rates BELOW Commercial Pricing for Priority Mail Cubic

Pirate Ship offers Priority Mail Cubic rates BELOW Commercial Pricing® (the deepest discounts publicly available from USPS) for shipments to and from the lower 48 states, with no markup, per-label costs, or monthly fees.

We can't advertise the highlighted rates because they're UP TO $2.98 CHEAPER than Commercial Pricing!
Only for shipments to/from the lower 48 states. Create a free account & get a rate quote to see the rates.
Cubic Feet
Up to, not over
0-50 miles
Zone 1
51-150 miles
Zone 2
150-300 miles
Zone 3
301-600 miles
Zone 4
601-1000 miles
Zone 5
1001-1400 miles
Zone 6
1401-1800 miles
Zone 7
1801+ miles
Zone 8
Zone 9
Cubic 0.1
Less than
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Cubic 0.2
Less than
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Cubic 0.3
Less than
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Cubic 0.4
Less than
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Cubic 0.5
Less than
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Priority Mail Cubic is one of the best kept secrets for shipping packages that weigh up to 20 lbs

Priority Mail Cubic (or "Cubic" as it's commonly called) is a secret sub-service of Priority Mail® that shippers of small but relatively heavy packages use to optimize their shipping. With Pirate Ship you can access even deeper discounts than Commercial Pricing for Priority Mail Cubic with no minimum volume requirements.

The pricing for Priority Mail Cubic is based on the outer dimensions of your package, rather than the weight. There are 5 different tiers of pricing that your package could be in depending upon its size, and any box that's under half of a cubic foot in volume can qualify, as long as none of the dimensions go over the 18" maximum. You can ship up to 20 pounds for no extra cost, making it one of the cheapest shipping services available for small boxes.

To calculate which Cubic pricing tier your package is in:

  1. Measure the outer length, width, and height of your package (18" max in any dimension). If any of the measurements are over the nearest quarter inch, round down to the nearest quarter inch (for example, 5.6" inches turn into 5.5" inches);
  2. Multiply the Length x Width x Height, then divide the result by 1728;
  3. The resulting number is the size of the package in cubic feet, and you can use the Priority Mail Cubic® rate chart above to determine the pricing. Round up fractional results to the next tier (so for example, a package that is 0.24 cubic feet will be in the 0.3 tier).

Note that Priority Mail Cubic packages cannot be larger than 0.5, half of a cubic foot, but larger packages can be sent with weight-based Priority Mail.

Get the cheapest Priority Mail Cubic Softpack pricing

You can also ship soft pack poly or padded envelopes with Priority Mail Cubic Softpack. Cubic Softpack pricing is the same as if you were shipping a box, but the formula for calculating the pricing tier is different.

To calculate which pricing tier a softpack envelope is in:

  1. Measure the Length and Width of the empty envelope or softpack mailer (in inches);
  2. If the measurements exceed the nearest quarter inch, round down to the nearest 1/4" inch (for example, 9.9" inches turns into 9.75" inches);
  3. Add the measurements together and determine which pricing tier your envelope is in:

    Cubic Softpack Tier 0.1 - Length plus Width measures more than 0" up to 21"
    Cubic Softpack Tier 0.2 - Length plus Width measures more than 21" up to 27"
    Cubic Softpack Tier 0.3 - Length plus Width measures more than 27" up to 31"
    Cubic Softpack Tier 0.4 - Length plus Width measures more than 31" up to 34"
    Cubic Softpack Tier 0.5 - Length plus Width measures more than 34" up to 36"

Note that neither dimension can be over 18", and the total size for Priority Mail Cubic Softpack cannot exceed 36". Larger envelopes can be sent via weight-based Priority Mail.

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