What Is Priority Mail Cubic? - Pirate Ship Support

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Learn about this "secret" dimension-based USPS shipping service that's perfect for sending small and heavy packages

Updated over 6 months agoTable of contentsPriority Mail Cubic features specially discounted dimension-based pricing

Priority Mail Cubic (or "Cubic," for short) is a special version of Priority Mail that’s priced based on the outer dimensions of your package instead of the weight, which can save you massive amounts off what it would otherwise cost to ship.

• Cheapest and fastest service: Cubic is the cheapest, fastest way to ship small packages with USPS that weigh over 1 pound, with delivery in 1-3 days and $100 in insurance included for free.

• Qualifying for cubic rates: You can ship packages that are up to half of a cubic foot in volume with this service (which is 12x12x6 inches, or other combinations that make the same volume), and your packages can weigh up to 20 lbs. Note that to qualify for Cubic rates, the maximum size for any dimension is 18", and the maximum volume is 0.5 cubic feet. If your package's volume exceeds 0.5 cubic feet, it won't qualify for cubic rates.

• Different pricing tiers: Depending on the size of your package, there are 5 pricing tiers it will fall into. It’s a good idea to plan your packaging to fit within the smallest tier possible for your package’s content, because it can drastically lower your costs.

Priority Mail Cubic pricing tiers

To calculate which of the 5 tiers your package will be in, you can use our Priority Mail Cubic Pricing Calculator or the manual formula below:

1. Measure your package’s outer dimensions

2. If any of the measurements exceed the nearest quarter inch, round down to the nearest quarter inch (for example, 5.6 inches turns into 5.5 inches)

3. Multiply Length x Width x Height, and divide the result by 1728

For example, to calculate the Cubic tier for a box that is 12.3" x 9.5" x 3":

1. Round down to 12.25" x 9.5" x 3"

2. Multiply 12.25" x 9.5" x 3" = 349.125

3. Divide 349.125 / 1728 = .202

4. Because this package is just over .2 cubic feet, it would be in the 0.3 cubic feet pricing tier.

5. If it were a slightly smaller box (such as 12x9.5x3), it would be in the 0.2 pricing tier instead and would cost less to ship.

You can use Priority Mail Cubic with an envelope or poly mailer

• Envelopes and softpacks: You can also use a padded envelope or poly softpack to ship with Priority Mail Cubic, but it changes the formula used to determine your pricing tier.

  • Learn more about Priority Mail Cubic Softpack here.

You cannot use Priority Mail Cubic with tubes

• Tubes don't qualify: Rolls or tubes do not qualify for Priority Mail Cubic rates.​

USPS Priority Mail Cubic rates

• Calculating your shipping cost: Once you know which USPS Cubic pricing tier your package is in, you can use our Priority Mail Cubic rate chart to determine what it will cost to ship your package, or use our USPS rates calculator.

• Getting the cheapest rates: Generally speaking, Priority Mail Cubic offers the cheapest rates when shipping to Zones 1-4, and becomes more expensive when shipping to Zones 5-8.

  • Our software always displays the cheapest service option for you, no matter what you're shipping, and where. Sometimes it's Cubic, and sometimes it isn't!

• Package size matters: One final thing to remember is that the size of your box or envelope matters! The smaller your package is, the less it will cost to ship it with Priority Mail Cubic.

Pirate Ship offers BELOW Commercial Pricing for Priority Mail Cubic

• Get the deepest discounts: Pirate Ship passes through the deepest discounts available for Priority Mail Cubic for free, with no markup, fees, or minimum volume required.

Learn more about Priority Mail Cubic

Additional resources

• How do I contact Pirate Ship?

• Free USPS Mailing Boxes for Priority Mail Cubic

• Can I use Priority Mail Cubic with a softpack or padded envelope?

• How to cut down your boxes to size

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