USPS Flat Rate Box Sizes, Dimensions and Prices (2026)
All five USPS Priority Mail flat rate boxes and five envelopes with outside dimensions, inside dimensions, cubic inches and the retail and commercial prices in force since July 12, 2026, plus the weight and zone points where a flat rate box stops being the cheaper choice.
USPS Priority Mail flat rate box sizes and prices
Flat rate means one price for anything that fits in the box, up to 70 lb, to any US address, with Priority Mail service (1 to 3 business days), tracking and $100 of included insurance. USPS supplies the boxes free. The prices below are the retail and commercial base rates effective July 12, 2026, from USPS Notice 123. Commercial pricing applies to postage bought online through USPS Click-N-Ship or a shipping platform.
| USPS flat rate box | Outside dimensions | Inside dimensions | Inside cubic inches | Retail | Commercial |
|---|---|---|---|---|---|
| Small Flat Rate Box | 8 11/16 x 5 7/16 x 1 3/4 in | 8 5/8 x 5 3/8 x 1 5/8 in | 75 | $13.65 | $12.10 |
| Medium Flat Rate Box 1 (top loading) | 11 1/4 x 8 3/4 x 6 in | 11 x 8 1/2 x 5 1/2 in | 514 | $24.80 | $21.17 |
| Medium Flat Rate Box 2 (side loading) | 14 x 12 x 3 1/2 in | 13 5/8 x 11 7/8 x 3 3/8 in | 546 | $24.80 | $21.17 |
| Large Flat Rate Box | 12 1/4 x 12 x 6 in | 12 x 11 3/4 x 5 1/2 in | 776 | $34.00 | $31.00 |
| Large Flat Rate Board Game Box | 24 1/16 x 11 7/8 x 3 1/8 in | 23 11/16 x 11 3/4 x 3 in | 835 | $34.00 | $31.00 |
| APO/FPO/DPO Large Flat Rate Box | 12 1/4 x 12 x 6 in | 12 x 11 3/4 x 5 1/2 in | 776 | $32.55 | $29.59 |
Inside dimensions are what matters for fit; the USPS spec sheet lists outside dimensions, and the two differ by about 1/4 inch per side. The cubic inch column is the inside volume. Note that both Medium boxes are the same price and nearly the same volume; Medium 1 is deep and Medium 2 is flat.
Flat rate envelopes
Envelopes follow the same rule: one price up to 70 lb as long as the envelope closes on its own adhesive without being reshaped or taped into a box. The padded envelope is the only one with cushioning built in.
| USPS flat rate envelope | Size | Retail | Commercial |
|---|---|---|---|
| Flat Rate Envelope | 12 1/2 x 9 1/2 in | $12.90 | $11.12 |
| Window Flat Rate Envelope | 12 1/2 x 9 1/2 in (10 x 5 in window) | $12.90 | $11.12 |
| Small Flat Rate Envelope | 10 x 6 in | $12.90 | $11.12 |
| Legal Flat Rate Envelope | 15 x 9 1/2 in | $13.25 | $11.66 |
| Padded Flat Rate Envelope | 12 1/2 x 9 1/2 in | $14.00 | $11.99 |
When flat rate beats your own box
Flat rate is a good deal when the contents are heavy for their size and traveling far. It is a poor deal for light items, local shipments, and anything that rattles around inside the box. The crossover points below are approximate and assume retail Ground Advantage and Priority Mail zone pricing as of July 2026; confirm with the USPS price calculator for an exact shipment.
| Flat rate box | Flat price (retail) | Ships cheaper on Ground Advantage when | Ships cheaper at flat rate when |
|---|---|---|---|
| Small, $13.65 | $13.65 | The item is under about 4 lb, or going to zones 1 to 5 | The item is over 4 lb and dense, or going coast to coast |
| Medium, $24.80 | $24.80 | Under about 8 lb to nearby zones, under 5 lb to far zones | 10 lb and up, or anything heavy going to zones 7 to 9 |
| Large, $34.00 | $34.00 | Under about 12 lb to nearby zones | 15 lb and up; the box holds up to 70 lb at one price |
Own-box equivalents for each flat rate size
If the flat price loses, you still need a box. The sizes below are the stocked corrugated sizes closest to each flat rate box, so the packout stays the same and only the postage changes. All of them are under one cubic foot, which means USPS charges on actual weight only; see the dimensional weight chart for the cubic inch figures.
| Flat rate box | Closest stocked own-box size (inside) | Why you would switch |
|---|---|---|
| Small (8 5/8 x 5 3/8 x 1 5/8) | 9 x 6 x 2 or 8 x 6 x 4 | Anything under about 2 lb ships cheaper on Ground Advantage than the $13.65 flat price |
| Medium 1 (11 x 8 1/2 x 5 1/2) | 11 x 8 x 6 or 12 x 9 x 6 | Local and zone 1 to 4 parcels under 10 lb are usually cheaper on zone pricing |
| Medium 2 (13 5/8 x 11 7/8 x 3 3/8) | 14 x 12 x 4 or 14 x 12 x 3 | Folded apparel and books pack flat; a shallow own box avoids the fixed $24.80 |
| Large (12 x 11 3/4 x 5 1/2) | 12 x 12 x 6 or 12 x 12 x 5 | Under one cubic foot, so no USPS dimensional weight on your own box |
| Board Game (23 11/16 x 11 3/4 x 3) | 24 x 12 x 4 | Only worth the flat price for heavy flat items; light ones ship cheaper by zone |
Rules that catch people
- The box must close normally. Overstuffing until the flaps bow or taping a bulging lid voids the flat rate, and the parcel is repriced on weight and zone at the counter.
- Flat rate boxes are Priority Mail only. You cannot use a flat rate box with Ground Advantage postage, even if you pay the Ground Advantage rate.
- 70 lb is the ceiling. Above that nothing ships USPS. The carrier size and weight limits page covers UPS and FedEx above 70 lb.
- Regional Rate boxes are gone. USPS retired them in 2023. Anything still labeled Regional Rate A or B is not valid for postage.
- Cubic pricing is the other flat-ish option. Commercial shippers can use Priority Mail Cubic for parcels under 0.5 cubic feet and 20 lb, priced by volume tier rather than weight. It is often cheaper than flat rate for dense small items and works with your own box.
Need the dimensions of a non-USPS carton? The box size chart lists the common corrugated sizes, and the Uline model cross-reference maps S-numbers to stocked equivalents.
Sources: USPS Notice 123 Price List effective July 12, 2026 (pe.usps.com); usps.com Priority Mail page (dimensions, retrieved August 2026).
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Frequently asked questions
What are the USPS flat rate box sizes?
Small 8 11/16 x 5 7/16 x 1 3/4 in; Medium 1 (top loading) 11 1/4 x 8 3/4 x 6 in; Medium 2 (side loading) 14 x 12 x 3 1/2 in; Large 12 1/4 x 12 x 6 in; Large Board Game 24 1/16 x 11 7/8 x 3 1/8 in. Those are outside dimensions; subtract about 1/4 inch per side for the inside.
How much does a USPS flat rate box cost in 2026?
At the Post Office: Small $13.65, Medium $24.80, Large $34.00 since July 12, 2026. Commercial (online) pricing is $12.10, $21.17 and $31.00. The boxes themselves are free from USPS.
What is the weight limit for a flat rate box?
70 lb for every flat rate box and envelope. The price does not change with weight, which is why flat rate wins for dense, heavy items and loses for light ones.
Is the medium flat rate box cheaper than the large?
Yes. Both medium boxes are $24.80 retail and the large is $34.00. The two medium boxes hold almost the same volume (514 and 546 cubic inches inside); pick by shape, not by size.
Can I use my own box at the flat rate price?
No. Flat rate pricing only applies to USPS-supplied flat rate packaging. Your own box is priced by weight and zone, or by Priority Mail Cubic if you ship commercially.