Maximum Box Dimensions for Shipping With USPS

Before you start shipping or start to create custom packaging for your business, it’s good to be aware of the maximum box dimensions to ship with the US Postal Service. We’ve listed all of that information below for the corresponding mail classes. First, you’ll need to know what the girth of your package is.

How to Calculate the Girth of Your Package

To calculate your package’s girth, Measure the width and the height, and add those two numbers together. Then, multiply that sum by 2 to get the girth. The formula should look like this:

Girth = 2*(Width + Height)

Domestic Mail Classes

First Class Package

• Maximum Dimensions: 22″ x 18″ x 15″

Priority Mail

• Maximum Length + Girth: 108 inches

Priority Mail Express

• Maximum Length + Girth: 108 inches

Priority Mail Cubic

• Maximum Dimensions: No one dimension can exceed 18 inches, and total volume can’t exceed 0.5 cubic feet

Cubic Poly Mailers or Envelopes

• Maximum Length + Width: 36 inches

Retail Ground (or Parcel Select Ground)

• Maximum Length + Girth: 130 inches

International Mail Classes

First Class Package International

• Maximum Dimensions: 24″ in Length, Length + Width + Height: 36″

Priority Mail International

• Maximum Length + Girth: 108 inches (varies by country—check USPS’ individual country codes for restrictions)

Priority Mail Express International

• Maximum Length + Girth: 79 inches (varies by country—check USPS’ individual country codes for restrictions)

What About Minimum Dimensions?

They say good things come in small packages…but USPS does have a minimum package size requirement! When shipping a box, the minimum dimensions are 6x3x0.25″. This rule applies for any USPS mail class you decide to ship in.

USPS could technically reject your package if your box is smaller than those dimensions, but the chances of that happening are pretty rare. However, we always suggest using a box that’s at least as big as the minimum dimensions. If there’s no way for you to do that, we recommend putting the box into a small poly mailer envelope when you ship it.

Pro Tip: If you use free USPS boxes, you won’t have to worry about minimum or maximum dimensions at all! Check out our guide that lists the dimensions of all the free USPS boxes.