NOTE: We are NOT the United States Post Office and cannot answer your questions about personal shipments, rates, or other post office related matters. We make shipping software that’s compatible with USPS.  For postage related questions please visit the USPS at: https://www.usps.com/

The popular Priority mail flat rate sizes offer you a wide range of packaging options that assure that your customers’ orders arrive in good shape, and without costing you too much money. While using an integrated shipping program can help streamline your costs and fulfillment operations; it doesn’t do much to help you with the real time cost of packaging supplies and postage. The USPS Priority mail service can deliver the right rates, and the right supplies, and can help you improve your bottom line.


The first thing you should know is that Priority mail service offers more than just boxes. There are envelopes and tubes too (more info on getting flat rate boxes). The tubes and envelopes follow a standard sizing that can accommodate legal, letter and portfolio size shipments. Most of the focus of the Priority service lies in the flat rate shipping of boxes. Most commercial shipping is going to relay on boxes, although envelopes are serviced with bulk discounts too.


When shipping USPS Priority Mail, there are also two forms of Priority shipping available – standard and Express. Express can offer fast track delivery for certain Priority mail flat rate sizes. The sizes that can use Priority Express are restricted to most mediums and some large boxes. Small boxes aren’t eligible, as there is a larger padded envelope size that will cover that shipping need under Priority Express. Two sizes of flat rate boxes may be shipped by Priority Express. The boxes must way no more than 20 pounds apiece.

  • 11 x 8-1/2 x 5-1/2 inner dimension and 11-1/4 x 8-3/4 x 6 inches outer dimension
  • 13-5/8 x 11-7/8 x 3-3/8 inner dimension and 14″ x 12″ x 3-1/2 inches outer dimension

You can also use Priority Express to ship:

  • Flat Rate Envelope – 12-1/2 x 9-1/2 inches
  • Padded Flat Rate Envelope – 12-1/2 x 9-1/2 inches
  • Legal Flat Rate Envelope – 15 x 9-1/2 inches
  • Each of these has a 4-pound limit.


There are more sizes available for flat rate service for small packages than many people are aware of when they ship. Many of the envelope sizes can work better than the small box side. The small flat rate envelope sizes are:

  • Flat Rate Envelopes – 12-1/2 by 9-1/2 inches or smaller
  • Legal Flat Rate Envelope – 15 by 9-1/2 inches
  • Padded Flat Rate Envelope – 12-1/2 by 9-1/2 inches

And the small flat rate box size is:

  • Inside Dimension = 8-5/8 by 5-3/8 by 1-5/8 inches

Envelopes must weight under 4 lbs. and the small box weight must be under 20.


The Priority medium boxes are available in side and top loading versions. The large boxes only come in top loading.

Medium Flat Rate Boxes

  • Inside Dimension = 11 by 8-1/2 by 5-1/2 inches
  • Inside Dimension = 13-5/8 by 11-7/8 by 3-3/8 inches

Large Flat Rate Boxes

  • Inside Dimension = 12 by 12 by 5-1/2 inches
  • Inside Dimension = 23-11/16 by 11-3/4 by 3 inches

For boxes being shipped to military or federal addresses with the APO/FPO designation, the available box size is:

  • Inside Dimension = 12 by 12 by 5-1/2 inches