Free shipping reference
USPS box sizes and Flat Rate box dimensions
Every USPS Priority Mail Flat Rate box and envelope with inside and outside dimensions and current prices, the USPS size and weight limits, and a finder that shows which box fits your item.
Which USPS box fits my item?
Boxes that fit
Enter your item’s size to see which boxes fit.
USPS Flat Rate box and envelope sizes
Prices: USPS holiday prices, in effect through 2027-01-17. “Online” is the commercial price available through shipping platforms such as ShipWave; “Post Office” is the retail counter price.
| Box or envelope | Inside dimensions | Outside dimensions | Online | Post Office |
|---|---|---|---|---|
| Small Flat Rate Box | 8-5/8" × 5-3/8" × 1-5/8" | 8-11/16" × 5-7/16" × 1-3/4" | $12.95 | $14.65 |
| Medium Flat Rate Box (top-loading) | 11" × 8-1/2" × 5-1/2" | 11-1/4" × 8-3/4" × 6" | $22.02 | $25.80 |
| Medium Flat Rate Box (side-loading) | 13-5/8" × 11-7/8" × 3-3/8" | 14" × 12" × 3-1/2" | $22.02 | $25.80 |
| Large Flat Rate Box | 12" × 11-3/4" × 5-1/2" | 12-1/4" × 12" × 6" | $32.75 | $36.10 |
| Large Flat Rate Board Game Box | 23-11/16" × 11-3/4" × 3" | 24-1/16" × 11-7/8" × 3-1/8" | $32.75 | $36.10 |
| Flat Rate Envelope | 12-1/2" × 9-1/2" | — | $11.97 | $13.90 |
| Legal Flat Rate Envelope | 15" × 9-1/2" | — | $12.51 | $14.25 |
| Padded Flat Rate Envelope | 12-1/2" × 9-1/2" | — | $12.84 | $15.00 |
USPS size and weight limits
- Maximum weight: 70 lb (Priority Mail, Flat Rate and USPS Ground Advantage).
- Maximum size: 108 inches in length plus girth; USPS Ground Advantage accepts oversized parcels up to 130 inches at the oversized price.
- Packages over 1 cubic foot are billed by dimensional weight when it is more than the actual weight, and long or very large parcels pay nonstandard fees.
- Flat Rate boxes must close normally; you may tape them but not enlarge or rebuild them.
Dimensions and prices verified 2026-10-07 from USPS Notice 123 — Price List and USPS Priority Mail (Flat Rate packaging and dimensions).
How to choose a USPS box
Measure the item
Measure the longest side, width and height, and add room for padding.Check Flat Rate
If it fits a Flat Rate box and is heavy or going far, Flat Rate is often the best deal.Compare weight-based prices
For light or nearby shipments, compare USPS Ground Advantage and weight-based Priority Mail with the Flat Rate vs. cheapest rate calculator.Buy the label online
Online (commercial) postage costs less than the Post Office counter for most services.
USPS box sizes FAQ
- What are the USPS Flat Rate box sizes?
- Inside dimensions: Small Flat Rate Box 8-5/8" × 5-3/8" × 1-5/8"; Medium Flat Rate Box 11" × 8-1/2" × 5-1/2" (top-loading) or 13-5/8" × 11-7/8" × 3-3/8" (side-loading); Large Flat Rate Box 12" × 11-3/4" × 5-1/2"; Large Flat Rate Board Game Box 23-11/16" × 11-3/4" × 3".
- How much weight can a Flat Rate box hold?
- Up to 70 lb for domestic shipments. The price is the same for any weight up to 70 lb and any US destination, as long as the box closes normally.
- Are USPS Flat Rate boxes free?
- Yes, Priority Mail Flat Rate boxes and envelopes are free from the Post Office or USPS.com, but you can only use them for Priority Mail (Flat Rate or weight-based) postage.
- What is the largest box USPS will take?
- Priority Mail and commercial USPS Ground Advantage packages can measure up to 108 inches in length plus girth (the longest side plus twice the width plus twice the height) and weigh up to 70 lb. Ground Advantage accepts oversized parcels up to 130 inches at a higher price.
- Is Flat Rate always cheaper?
- No. Flat Rate is cheapest for heavy, compact items going far. Light packages usually cost less with USPS Ground Advantage or weight-based Priority Mail, especially to nearby zones. Compare both for your package.
Ready to ship?
Compare discounted USPS, UPS and FedEx labels for your package.
Free tools
More free shipping tools
Cheapest Way to Ship
Compare USPS Flat Rate with Ground Advantage and Priority Mail prices by zone.
Box Size Calculator
Find the smallest box for your items, its DIM weight and Flat Rate boxes that fit.
Dimensional Weight Calculator
Find the billable DIM weight carriers will charge before the invoice lands.
Want us to find the money leaking out of your shipping? Get a free profit-leak audit.
Request a free auditShipWave is not affiliated with or endorsed by the United States Postal Service. USPS, Priority Mail, Flat Rate and USPS Ground Advantage are trademarks of the United States Postal Service, used here only to identify its products. Dimensions and prices are from USPS publications and may change; check usps.com before shipping.