USPS Flat Rate Box Sizes, Dimensions, and Prices (Live Reference)
Reference last refreshed May 3, 2026. Pulls live from our
flat-rate-prices table — never out of sync with USPS.
Quick reference for every USPS Priority Mail Flat Rate box and envelope: name, internal dimensions, weight cap, and current discounted commercial price. Use the table to pick the right box for your item, then book through any shipping software that has USPS commercial rates (Stamps.com, ShippingEasy, Shippo, ShipStation, etc.).
Flat rate boxes
| Box | Inside dimensions | Weight limit | Priority Mail price |
|---|---|---|---|
| Small Flat Rate Box | 8.625×5.375×1.625" | 70 lb | $10.59 |
| Medium Flat Rate Box (top-loading) | 11×8.5×5.5" | 70 lb | $19.98 |
| Medium Flat Rate Box (side-loading) | 13.625×11.875×3.375" | 70 lb | $19.98 |
| Large Flat Rate Box | 12×12×5.5" | 70 lb | $27.76 |
Flat rate envelopes
| Envelope | Outside dimensions | Weight limit | Priority Mail price |
|---|---|---|---|
| Flat Rate Envelope | 12½ × 9½" | — lb | $9.62 |
| Small Flat Rate Envelope | 10 × 6" | — lb | $9.62 |
| Legal Flat Rate Envelope | 15 × 9½" | — lb | $10.91 |
| Padded Flat Rate Envelope | 12½ × 9½" | — lb | $11.02 |
About these prices
Prices above are discounted commercial rates — what you get from shipping software, not the post office counter. Retail counter prices are approximately 10–15% higher; see USPS Notice 123 for retail. USPS adjusts both retail and commercial rates twice a year (typically January and July).
Priority Mail Express versions of each flat rate envelope and box exist with the same dimensions and weight cap but faster delivery (1–2 days vs Priority Mail's 1–3 days) at higher cost. See our companion guide USPS Flat Rate Boxes — when they save money for the Express prices alongside Priority Mail.
Important notes
- The boxes are free. Order from USPS.com free shipping supplies or grab them at any post office.
- You must use the official USPS box. Putting flat rate postage on a generic box won't work — USPS scans confirm the box template.
- Flat rate is one price regardless of weight (up to 70 lb) or zone. That's the whole point — you're trading weight-based pricing for predictability.
- USPS retired Regional Rate Boxes (Type A and Type B) in January 2023. If you find an old box at the post office, it can no longer be shipped at flat rate.
When flat rate beats by-weight pricing
See our companion guide for the math. Quick rule of thumb: if you're shipping heavy items (5+ lb) across multiple zones, flat rate usually wins. If you're shipping light items (under 1 lb) to nearby zones, by-weight Priority Mail is cheaper.
For exact comparisons against by-weight Priority Mail, UPS Ground, and FedEx Ground for your specific shipment, use the calculator on our home page.
Methodology
Both this page and the flat rate boxes
overview guide read from the same Supabase flat-rate-prices table,
populated by a Netlify cron that calls Shippo's API monthly with each USPS flat
rate template. The data sources can never disagree because they're the same data,
just rendered differently.