USPS Box Sizes: Free Priority Mail Boxes That Are Not Flat Rate
USPS gives away more Priority Mail boxes than the flat rate ones. The free weight-priced boxes include the Small Box at 9 1/4 x 6 1/4 x 2 inches inside, the Medium Cube-Shaped Box, once called Box 4, at 7 x 7 x 6, the Large Box, once Box 7, at 12 x 11 3/4 x 8, two side-loading boxes and a shoe box. They ship at Priority Mail prices by weight and zone, up to 70 lb, and they can only be used for Priority Mail: USPS Ground Advantage needs your own box. The last column lists a stocked box with the same inside room.
USPS Priority Mail box sizes
| Box | Former name or number | Outside | Inside | Service | Own box for Ground Advantage |
|---|---|---|---|---|---|
| Priority Mail Small Box | 1096L | 9 7/16 x 6 7/16 x 2 3/16 | 9 1/4 x 6 1/4 x 2 | Priority Mail | 10 x 8 x 2 |
| Priority Mail Medium Cube-Shaped Box | Box 4 | 7 1/4 x 7 1/4 x 6 1/2 | 7 x 7 x 6 | Priority Mail | 7 x 7 x 6 |
| Priority Mail Large Box | Box 7 | 12 1/4 x 12 x 8 1/2 | 12 x 11 3/4 x 8 | Priority Mail | 12 x 8 x 12 |
| Priority Mail Side-Loading Medium Box | 1092 | 13 11/16 x 12 x 2 7/8 | 13 3/8 x 11 7/8 x 2 3/4 | Priority Mail | 14 x 12 x 3 |
| Priority Mail Side-Loading Large Box | 1095 | 15 x 12 x 3 1/8 | 14 7/8 x 11 7/8 x 3 | Priority Mail | 15 x 12 x 3 |
| Priority Mail Shoe Box | Shoe Box | 14 7/8 x 5 1/4 x 7 3/8 | 14 3/8 x 5 1/8 x 7 1/4 | Priority Mail | 14.5 x 8.75 x 6 |
| Small Tube Box | 1098S | 25 9/16 x 6 x 5 1/4 | 25 7/16 x 5 7/8 x 5 1/8 | Priority Mail or Priority Mail Express | 26 x 6 x 6 |
| Priority Mail Express Medium Box | Top-loading | 11 3/4 x 8 3/4 x 5 3/4 | 11 1/2 x 8 1/2 x 5 1/2 | Priority Mail Express | 12 x 9 x 6 |
| Priority Mail Express Side-Loading Medium Box | 1093 | 15 x 12 x 3 1/8 | 14 7/8 x 11 7/8 x 3 | Priority Mail Express | 15 x 12 x 3 |
USPS lists outside dimensions first on its product pages and inside dimensions in the specifications. A few pages disagree with themselves by a fraction of an inch, such as the Shoe Box inside height at 7 1/4 or 7 1/2 inches, so measure before you commit a tight fit. The Priority Mail Poly Envelope is 15 x 11 5/8 inches.
Priority Mail boxes vs flat rate boxes
Flat rate boxes ship for one price to any US address regardless of weight. The boxes on this page are free too, but they are priced like any Priority Mail parcel: by weight and distance. They make sense when a package is light or travels a short distance, and the USPS flat rate box sizes and prices page covers the flat rate side.
Discontinued Priority Mail boxes
USPS has renamed several boxes, so older guides list numbers you will not find on the Postal Store today: Box 4 is now the Medium Cube-Shaped Box, Box 7 the Large Box, 1092 and 1095 the side-loading boxes and 1096L the Small Box. Box 1097, the large tube, the DVD box and the separate video box are not on the current free supplies list.
Shipping USPS Ground Advantage? Use your own box
USPS packaging must be used only for the product it is printed for. A Priority Mail box shipped with Ground Advantage postage is charged Priority Mail prices, even with the markings covered. For Ground Advantage, use a plain box: each one in the last column is the smallest stocked 32 ECT box at least as large inside as the USPS box, so the same item fits. Parcels over one cubic foot are billed on dimensional weight, which the dimensional weight calculator works out.
Frequently asked questions
What are the USPS Priority Mail box sizes?
The free weight-priced boxes are the Small Box, 9 1/4 x 6 1/4 x 2 in inside; the Medium Cube-Shaped Box, 7 x 7 x 6 in; the Large Box, 12 x 11 3/4 x 8 in; the Side-Loading Medium, 13 3/8 x 11 7/8 x 2 3/4 in; the Side-Loading Large, 14 7/8 x 11 7/8 x 3 in; and the Shoe Box, 14 3/8 x 5 1/8 x 7 1/4 in.
What size is a Priority Mail Box 4?
Box 4 is now called the Medium Cube-Shaped Box. It is 7 x 7 x 6 inches inside and 7 1/4 x 7 1/4 x 6 1/2 inches outside.
What size is a Priority Mail Box 7?
Box 7 is now the Priority Mail Large Box: 12 x 11 3/4 x 8 inches inside and 12 1/4 x 12 x 8 1/2 inches outside.
Can I use a Priority Mail box for Ground Advantage?
No. USPS packaging must be used for the product printed on it, so a Priority Mail box is charged Priority Mail prices even with the markings covered. Use your own box for Ground Advantage.
Are Priority Mail boxes flat rate?
Only the boxes marked Flat Rate. The other free Priority Mail boxes are priced by weight and zone like any Priority Mail parcel.
How much weight can a Priority Mail box hold?
USPS accepts Priority Mail packages up to 70 lb in its own boxes.
Related tools and charts
- USPS flat rate box sizes and prices
- FedEx box sizes
- UPS box sizes
- USPS, UPS and FedEx size and weight limits
- Envelope size chart
- Shoe box dimensions and the shipping box for each
- Mailing tube size chart and poster tube finder
Sources: USPS Postal Store product pages and Domestic Mail Manual section 601.6.1, checked October 26, 2026. USPS and Priority Mail are trademarks of the United States Postal Service; Packrift is not affiliated with USPS.