USPS Flat Rate boxes charge one fixed price no matter what the package weighs or how far it travels, up to a 70 lb limit. Retail prices run from $12.90 for the Flat Rate Envelope to $34.00 for the Large Flat Rate Box, and the packaging itself is free.
That single price is only a bargain in one situation: the package is heavy and going far. A 3 lb box crossing two states is cheaper by weight. A 20 lb box crossing the country is dramatically cheaper flat rate. The tables below show the exact weight where the math flips, by zone.
Every USPS Flat Rate Box and Envelope: 2026 Prices and Dimensions
These are the domestic Priority Mail Flat Rate prices effective July 12, 2026, taken from the USPS Notice 123 price list. Retail is what you pay at the post office counter. Commercial is what you pay through Click-N-Ship or shipping software.
| Container | Interior dimensions | Retail | Commercial | Realistic use |
|---|---|---|---|---|
| Flat Rate Envelope | 12-1/2” x 9-1/2” | $12.90 | $11.12 | Documents, thin goods |
| Legal Flat Rate Envelope | 15” x 9-1/2” | $13.25 | $11.66 | Legal-size paperwork |
| Padded Flat Rate Envelope | 12-1/2” x 9-1/2” | $14.00 | $11.99 | Books, apparel, soft goods |
| Small Flat Rate Box | 8-5/8” x 5-3/8” x 1-5/8” | $13.65 | $12.10 | Coins, jewelry, parts |
| Medium Flat Rate Box (top) | 11” x 8-1/2” x 5-1/2” | $24.80 | $21.17 | Shoes, clothing, tools |
| Medium Flat Rate Box (side) | 13-5/8” x 11-7/8” x 3-3/8” | $24.80 | $21.17 | Frames, books, flat items |
| Large Flat Rate Box | 12” x 11-3/4” x 5-1/2” | $34.00 | $31.00 | Bulk, dense heavy items |
| APO/FPO/DPO Flat Rate Box | 12” x 11-3/4” x 5-1/2” | $32.55 | $29.59 | Military and diplomatic |
Two details people get wrong. The Large Flat Rate Box is not 12” x 12” x 5-1/2” — the interior is 11-3/4” on one side, which matters if you are fitting something square. And the APO/FPO/DPO box is the same size as the Large box at a $1.45 discount, but USPS only distributes that one through USPS.com, not the counter.
USPS also gives away a smaller 10” x 6” Flat Rate Envelope. It ships at the same $12.90 retail rate, because Notice 123 prices the Flat Rate Envelope at 12-1/2” x 9-1/2” or smaller.
Usable volume, if you are trying to work out what fits:
- Small Flat Rate Box: about 75 cubic inches
- Medium Flat Rate Box (top-loading): about 514 cubic inches
- Medium Flat Rate Box (side-loading): about 546 cubic inches
- Large Flat Rate Box: about 776 cubic inches
What the Flat Rate Price Actually Includes
Every domestic Flat Rate shipment travels as Priority Mail, and Priority Mail carries these features:
| Feature | What USPS provides |
|---|---|
| Delivery speed | 2-3 business days, no money-back guarantee |
| Weight limit | 70 lbs — the same limit for every box and envelope |
| Tracking | USPS Tracking included at no extra cost |
| Insurance | Up to $100 included on most shipments |
| Packaging | Free from USPS; ordered online, delivered in about 7 to 10 days |
| Pickup | Free Package Pickup from your home or office |
The 70 lb limit applies identically across the range. You can legally load 70 lbs into the Small Flat Rate Box and pay $13.65, which is why dense, small, heavy items — coin rolls, fasteners, bearings, tile samples — are the classic flat rate play.
The “flat” part means zone-blind pricing. Regular Priority Mail and Ground Advantage, the service that replaced Parcel Select Ground, both price by distance band: zone 1 is local, zone 8 is coast to coast, zone 9 covers offshore and military destinations. Flat rate ignores all of it.
When Flat Rate Beats USPS Ground Advantage
Ground Advantage is the default cheap option — 2 to 5 business days, 70 lb limit, $100 insurance included, starting at $7.90 retail. For most light parcels it wins outright. Flat Rate only takes over above a certain weight, and that weight drops sharply as the package travels farther.
The table shows the weight at which each Flat Rate container becomes cheaper than retail Ground Advantage:
| Flat Rate container | Zone 1 (local) | Zone 4 | Zone 6 | Zone 8 (cross-country) |
|---|---|---|---|---|
| Small Flat Rate Box ($13.65) | 7 lbs | 3 lbs | 2 lbs | 2 lbs |
| Padded Flat Rate Envelope ($14.00) | 7 lbs | 4 lbs | 2 lbs | 2 lbs |
| Medium Flat Rate Box ($24.80) | 21 lbs | 17 lbs | 9 lbs | 5 lbs |
| Large Flat Rate Box ($34.00) | 27 lbs | 21 lbs | 14 lbs | 9 lbs |
Read it as a threshold. A 6 lb parcel in zone 6 costs $21.05 by retail Ground Advantage against $24.80 in a Medium Flat Rate Box, so Ground Advantage wins. Push the same box to 10 lbs and Ground Advantage jumps to $27.50 while the Medium box stays at $24.80.
The pattern that actually matters: cross-country shipments flip to flat rate at surprisingly light weights. At zone 8, a Medium Flat Rate Box is the cheaper choice from 5 lbs up. At zone 1 you need 21 lbs before it pays.
When Flat Rate Beats Weight-Based Priority Mail
If you need Priority Mail speed either way, the comparison is different — you are choosing between two Priority Mail prices. Retail Priority Mail starts at $11.00 for 1 lb in zone 1, so the flat rate boxes catch up much faster:
| Flat Rate container | Zone 1 | Zone 4 | Zone 6 | Zone 8 |
|---|---|---|---|---|
| Small Flat Rate Box ($13.65) | 5 lbs | 2 lbs | 1 lb | 1 lb |
| Padded Flat Rate Envelope ($14.00) | 5 lbs | 2 lbs | 1 lb | 1 lb |
| Medium Flat Rate Box ($24.80) | 19 lbs | 14 lbs | 4 lbs | 3 lbs |
| Large Flat Rate Box ($34.00) | 24 lbs | 21 lbs | 8 lbs | 5 lbs |
At zone 6 and beyond, almost anything you would put in a Small Flat Rate Box or Padded Envelope is cheaper flat rate than by weight. Weight-based Priority Mail only wins on short hops.
Both tables use retail prices. Commercial pricing narrows the gaps because it discounts the flat rate containers by roughly 9% to 15% and discounts weight-based Priority Mail too — the ranking rarely flips, but confirm in your shipping software before a big run.
When Flat Rate Is the Wrong Choice
This is the part most guides skip.
A 1 lb parcel traveling one zone from home costs $9.95 by retail Ground Advantage. Put the same item in a Medium Flat Rate Box and you pay $24.80 — nearly two and a half times the price, on a package that was never heavy enough to need flat rate.
Three traps worth naming:
- Sizing up because the box is free. The box costs nothing; the postage does not. Reaching for a Medium box when a Small box or a padded envelope would do costs you $11.15 per shipment.
- Light parcels in nearby zones. Under 4 lbs inside zones 1 through 4, Ground Advantage beats the Medium and Large boxes every time, and usually beats the Small box too.
- Bulky but light items. Flat rate rewards density. Pillows, foam, and packing-heavy items waste the one advantage you are paying for.
If you are shipping regularly, the practical routine is to price both options per package before you buy postage. Across fifty shipments a month, guessing wrong costs real money.
How to Get Free Flat Rate Boxes
USPS charges nothing for the packaging itself. You have two options:
- Order online. The USPS Postal Store lists Flat Rate envelopes and boxes at $0.00 in singles and packs of 10 or 25. They arrive with your regular mail in about 7 to 10 days.
- Pick them up. Any post office lobby stocks the common sizes. The APO/FPO/DPO box is the exception — that one is USPS.com only.
The boxes remain USPS property until you mail something in them. They are not packing material, not storage, and not resalable.
Flat Rate Rules That Trip People Up
- Priority Mail only. USPS-produced Priority Mail packaging is restricted to Priority Mail. Anything mailed in a Flat Rate box is charged the Priority Mail price no matter what class you requested, and USPS treats misuse as a federal-law issue.
- No rebuilding the box. Notice 123 requires the flaps to close within their normal folds. Tape may reinforce flaps and seams, but you cannot open the sides, turn the box inside out, or enlarge it in any way.
- Regional Rate is gone. USPS discontinued Priority Mail Regional Rate Boxes A and B effective January 22, 2023. Leftover boxes still ship, but they are processed as ordinary weight-and-zone parcels. Any guide still recommending Regional Rate is out of date.
- Restricted contents still apply. The 70 lb limit does not override the USPS shipping restrictions on hazardous materials, lithium batteries, and other prohibited goods.
Faster Flat Rate: Priority Mail Express Envelopes
Flat rate is not only a Priority Mail feature. Priority Mail Express, the fastest domestic tier, offers three flat rate envelopes — but no flat rate boxes:
| Priority Mail Express container | Size | Retail | Commercial |
|---|---|---|---|
| Flat Rate Envelope | 12-1/2” x 9-1/2” | $35.90 | $31.11 |
| Legal Flat Rate Envelope | 15” x 9-1/2” | $36.20 | $31.43 |
| Padded Flat Rate Envelope | 12-1/2” x 9-1/2” | $36.90 | $31.70 |
If you need a contract or a passport moved as fast as USPS moves anything, these are the flat rate option. Put the same paperwork in a box and Express charges by weight and zone instead.
International Flat Rate
Priority Mail International keeps flat rate pricing but tightens the weight limits severely, and prices vary by country price group:
| Container | Max weight | Retail price range |
|---|---|---|
| Flat Rate Envelope | 4 lbs | $32.65 – $62.35 |
| Small Flat Rate Box | 4 lbs | $33.75 – $64.45 |
| Medium Flat Rate Box | 20 lbs | $61.80 – $133.10 |
| Large Flat Rate Box | 20 lbs | $75.15 – $155.00 |
The low end of each range is the Canada price group; the high end is the most expensive group. Check Notice 123 for your destination country’s group before quoting a customer.
FAQ
How much does a USPS flat rate box cost?
At the retail counter in 2026: $13.65 for the Small box, $24.80 for the Medium box (both shapes), and $34.00 for the Large box. Envelopes run $12.90 to $14.00. Commercial rates through Click-N-Ship or shipping software are lower — $12.10, $21.17, and $31.00 for the three boxes.
What are the dimensions of a USPS flat rate box?
Interior dimensions: Small is 8-5/8” x 5-3/8” x 1-5/8”. Medium comes in two shapes — 11” x 8-1/2” x 5-1/2” top-loading, and 13-5/8” x 11-7/8” x 3-3/8” side-loading. Large is 12” x 11-3/4” x 5-1/2”. Every one of them accepts up to 70 lbs.
Is it cheaper to use your own box or a Priority Mail flat rate box?
Your own box, shipped by Ground Advantage, is cheaper for most parcels — anything light, and anything traveling a short distance. The flat rate box wins once the package is heavy for its zone: roughly 5 lbs and up going coast to coast in a Medium box, or 21 lbs and up going across town. Weigh the package, check the zone, then compare.
Is it cheaper to ship USPS flat rate or UPS?
USPS is usually cheaper for small, heavy parcels. UPS does offer its own flat rate product, UPS Simple Rate, which lets you use any box up to 1,728 cubic inches and 50 lbs, with published starting prices from $12.20 (Extra Small) to $30.95 (Extra Large). The two limits are the deciding factor: UPS caps Simple Rate at 50 lbs where USPS allows 70 lbs, and USPS Flat Rate boxes are supplied free. Above 50 lbs, or for anything under about 5 lbs, USPS generally wins.
Are USPS flat rate boxes really free?
Yes. Order them from the USPS Postal Store at $0.00, in singles or packs of 10 and 25, or take them from any post office lobby. You only pay when you ship. If you are trying to cut packaging costs further, note that the rules on reusing shipping packaging do not let you reuse a Flat Rate box for another class of mail.
Can I use a flat rate box for Ground Advantage or Media Mail?
No. USPS-produced Priority Mail packaging is reserved for Priority Mail, and USPS charges the Priority Mail price on anything mailed in it regardless of the class you selected. Turning the box inside out does not change that — the pricing override applies either way.
What happens if a flat rate package goes missing?
Priority Mail includes up to $100 of insurance, so file a claim rather than absorbing the loss. If the parcel was marked delivered but never arrived, the USPS stolen package process covers the order of operations: check with neighbors, open a Missing Mail search, then file the insurance claim.
Once the box is packed and labeled, you can hand it to your letter carrier, use a free Package Pickup, or drop off the USPS package at a post office or a blue collection box that accepts parcels. After it moves, follow it on the USPS tracking hub.
