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USPS Flat Rate Box Sizes: Interior Is 1/16–1/2 in Smaller Than Exterior
The four boxes compared
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USPS Priority Mail
USPS Small Flat Rate Box
Exterior 8 11/16 × 5 7/16 × 1 3/4 in, interior 8 5/8 × 5 3/8 × 1 5/8 in
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USPS Priority Mail
USPS Medium Flat Rate Box 1 (top-loading)
Exterior 11 1/4 × 8 3/4 × 6 in, interior 11 × 8 1/2 × 5 1/2 in
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USPS Priority Mail
USPS Medium Flat Rate Box 2 (side-loading)
Exterior 14 × 12 × 3 1/2 in, interior 13 5/8 × 11 7/8 × 3 3/8 in
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USPS Priority Mail
USPS Large Flat Rate Box
Exterior 12 1/4 × 12 × 6 in, interior 12 × 11 3/4 × 5 1/2 in
The short answer
- SmallInterior 8 5/8 × 5 3/8 × 1 5/8 in (219.1 × 136.5 × 41.3 mm)
- Medium 1Interior 11 × 8 1/2 × 5 1/2 in (279.4 × 215.9 × 139.7 mm) — top-loading
- Medium 2Interior 13 5/8 × 11 7/8 × 3 3/8 in (346.1 × 301.6 × 85.7 mm) — side-loading
- LargeInterior 12 × 11 3/4 × 5 1/2 in (304.8 × 298.5 × 139.7 mm)
- GapMedium 1 and Large boxes lose 1/4 in total in width/depth and 1/2 in total in height (Medium 2: 3/8, 1/8, 1/8)
See the sizes in AR (shown as boxes)
The four sizes as boxes at their published dimensions
Preparing 3D…
1 USPS Small2 USPS Medium 13 USPS Medium 24 USPS Large
On a phone, the "View in your room (AR)" button places the boxes in your room. The boxes are locked to the published dimensions and cannot be resized. They show size only and are not 3D replicas of the products.
Drawn to the same scale
Dimensions side by side
| Box | Exterior (in) | Interior (in) | Interior (mm) |
|---|---|---|---|
| Small | 8 11/16 × 5 7/16 × 1 3/4 | 8 5/8 × 5 3/8 × 1 5/8 | 219.1 × 136.5 × 41.3 |
| Medium 1 (top-load) | 11 1/4 × 8 3/4 × 6 | 11 × 8 1/2 × 5 1/2 | 279.4 × 215.9 × 139.7 |
| Medium 2 (side-load) | 14 × 12 × 3 1/2 | 13 5/8 × 11 7/8 × 3 3/8 | 346.1 × 301.6 × 85.7 |
| Large | 12 1/4 × 12 × 6 | 12 × 11 3/4 × 5 1/2 | 304.8 × 298.5 × 139.7 |
Dimensions in W × D × H order. Millimeters converted at 1 in = 25.4 mm and rounded to one decimal place.
Interior vs exterior: how much is lost to the walls
| Box | Width loss (total) | Depth loss (total) | Height loss (total) |
|---|---|---|---|
| Small | 1/16 in (1.6 mm) | 1/16 in (1.6 mm) | 1/8 in (3.2 mm) |
| Medium 1 | 1/4 in (6.4 mm) | 1/4 in (6.4 mm) | 1/2 in (12.7 mm) |
| Medium 2 | 3/8 in (9.5 mm) | 1/8 in (3.2 mm) | 1/8 in (3.2 mm) |
| Large | 1/4 in (6.4 mm) | 1/4 in (6.4 mm) | 1/2 in (12.7 mm) |
"Loss" is exterior minus interior for the full dimension (both walls combined). Fractions are exact per USPS specs; mm values rounded.
What the numbers mean
The exterior dimensions tell you how much space the box takes up. The interior dimensions are what your items must fit inside.
For Medium Box 1 and the Large box, the walls take 1/2 in (12.7 mm) out of the height. The exterior is 6 in tall, but the interior is only 5 1/2 in. An item taller than 5 1/2 in does not fit inside either box, even though the outside measures 6 in.
Medium Box 2 is 2 5/8 in wider than Medium Box 1 on the interior and 3 3/8 in deeper, but only 3 3/8 in tall against 5 1/2 in.
The Large box and Medium Box 1 share the same interior height of 5 1/2 in. The Large gives 1 in more in width and 3 1/4 in more in depth on the interior.
Product links
9×6×2 shipping boxes on Amazon
11×8×6 shipping boxes on Amazon
FAQ
What are the interior dimensions of the USPS Medium Flat Rate Box?
There are two Medium Flat Rate Boxes. Medium Box 1 (top-loading) has an interior of 11 × 8 1/2 × 5 1/2 in (279.4 × 215.9 × 139.7 mm). Medium Box 2 (side-loading) has an interior of 13 5/8 × 11 7/8 × 3 3/8 in (346.1 × 301.6 × 85.7 mm). Both ship at the same flat rate.
How much smaller is the interior of a USPS Flat Rate Box compared to the exterior?
For the Small box the difference is about 1/16 in (1.6 mm) total in width and depth, and 1/8 in (3.2 mm) total in height. For Medium Box 1 and the Large box the difference is 1/4 in (6.4 mm) total in width, 1/4 in in depth, and 1/2 in (12.7 mm) total in height. Always pack to the interior dimensions, not the exterior.
What is the difference between USPS Medium Flat Rate Box 1 and Box 2?
Medium Box 1 is top-loading with an interior of 11 × 8 1/2 × 5 1/2 in. Medium Box 2 is side-loading with an interior of 13 5/8 × 11 7/8 × 3 3/8 in. Box 2 is 2 5/8 in wider and 3 3/8 in deeper, and Box 1 is 2 1/8 in taller.
How does the USPS Large Flat Rate Box interior compare to Medium Box 1?
The Large Flat Rate Box interior is 12 × 11 3/4 × 5 1/2 in (304.8 × 298.5 × 139.7 mm). Medium Box 1 interior is 11 × 8 1/2 × 5 1/2 in (279.4 × 215.9 × 139.7 mm). They share the same 5 1/2 in height. The Large is 1 in wider and 3 1/4 in deeper.
See also: Carry-on vs Personal Item Size — airline size limits drawn to the same scale. FedEx vs UPS Express Box Sizes — how FedEx and UPS branded express boxes compare to each other and to USPS Flat Rate boxes.
Sources
- USPS Priority Mail Flat Rate box dimensions: USPS.com, "Priority Mail", checked 2026-09-29. The page lists "Outside" and "Inside" dimensions for each box in inches and fractions; the millimeter values here are conversions.
- Credit card reference: ISO/IEC 7810 ID-1, 85.60×53.98 mm (rounded to 85.6×54 mm in the figure).