USPS Flat Rate Box Sizes, Dimensions, and Prices (2026)
USPS Flat Rate boxes let you ship anything up to 70 pounds for a single fixed price, regardless of weight or distance. A 2-pound book going across town and a 40-pound box of tools going coast-to-coast cost exactly the same if they fit in the same flat rate box.
That makes flat rate one of the simplest ways to ship with USPS. You don’t need a scale, you don’t need to look up zone charts, and you don’t need to guess what the final price will be. If it fits, you already know the cost. USPS provides the boxes for free, so the only expense is the postage itself.
The real advantage shows up with heavier items. Regular Priority Mail prices climb as weight increases and distance grows, but flat rate stays the same. If you’re shipping something dense — tools, canned goods, books, auto parts — flat rate can save you a significant amount compared to weight-based pricing.
All USPS Flat Rate Box Sizes
USPS offers four flat rate box options (five if you count the military variant). Each one has specific dimensions, and you must use the official USPS flat rate packaging — you can’t use your own box and pay the flat rate price.
| Box Name | Dimensions | Retail Price | Commercial Price | Best For |
|---|---|---|---|---|
| Small Flat Rate Box | 8⅝” × 5⅜” × 1⅝” | $10.40 | $8.45 | Jewelry, small electronics, phone accessories |
| Medium Flat Rate Box (top-loading) | 11” × 8½” × 5½” | $17.10 | $14.25 | Books, shoes, tools, small appliances |
| Medium Flat Rate Box (side-loading) | 13⅝” × 11⅞” × 3⅜” | $17.10 | $14.25 | Board games, flat electronics, clothing |
| Large Flat Rate Box | 12” × 12” × 5½” | $22.45 | $19.30 | Heavy items, tools, multiple books |
| APO/FPO/DPO Large Flat Rate Box | 12” × 12” × 5½” | $22.45 | $19.30 | Military addresses only |
A few things worth noting about this table. The two medium boxes cost the same but have different shapes — the top-loading version is deeper and works better for bulky items, while the side-loading version is flatter and wider, which suits board games, framed photos, or stacks of clothing. Pick whichever fits your item better.
The “Commercial Price” column is the discounted rate available when you buy postage through shipping software rather than at the post office counter. Commercial prices are available through services like Stamps.com, saving you 17-18% on every shipment.
The APO/FPO/DPO box is the same size as the regular Large Flat Rate Box but is restricted to military addresses. If you’re shipping care packages to service members overseas, it’s the same price as domestic shipping — a genuinely good deal for something going halfway around the world.
When Does Flat Rate Save Money?
Flat rate isn’t always the cheapest option. For lightweight packages — say a t-shirt or a small accessory — regular Priority Mail almost always costs less than flat rate because the price is based on actual weight.
The breakeven point depends on two factors: how much your package weighs and how far it’s going. A 3-pound package shipped to a neighboring state might be cheaper with regular Priority Mail, but that same 3-pound package going from New York to California could cost more than the flat rate price.
The general rule: the heavier the item and the farther it’s going, the more likely flat rate will save you money. Use the calculator below to see exactly where the breakeven point falls for your situation.
Flat Rate Breakeven Calculator
Rates shown are retail USPS prices for the route 10001 → 90210. Commercial rates may differ.
Enter your ZIP codes and click Calculate to see personalized breakeven points.
How to Order Free Flat Rate Boxes
One of the best parts of flat rate shipping is that USPS provides all the boxes and envelopes completely free. You don’t need to buy packaging or hunt for the right box size at a store.
There are two ways to get them:
Order online for free delivery. Go to usps.com/store and search for “flat rate boxes.” You can order them in quantities of 10 or 25, and USPS will deliver them right to your door. Delivery usually takes 1-2 weeks, so plan ahead if you need them for upcoming shipments. You can also order through Stamps.com if you already have an account there.
Pick them up in person. Every post office keeps flat rate boxes in stock. Walk in, grab what you need from the self-service area, and walk out. No purchase required, no questions asked. This is the faster option when you need boxes right away.
A word of caution: flat rate boxes are clearly marked with “Priority Mail” branding. You cannot use them for other shipping services. If you try to send a flat rate box via Ground Advantage or any non-Priority Mail service, it will either be returned or charged the Priority Mail rate.
Tips for Using Flat Rate Boxes
Pack strategically with the medium boxes. The top-loading and side-loading medium boxes have the same price but different internal volumes. The side-loading box (13⅝” × 11⅞” × 3⅜”) actually has a slightly larger total volume than the top-loading version and works surprisingly well for clothing shipments. Try both to see which fits your items better.
Consider Regional Rate for mid-weight packages. USPS also offers Regional Rate boxes, which are priced by shipping zone but still use free USPS-provided packaging. Regional Rate can be cheaper than flat rate for packages going shorter distances, and cheaper than regular Priority Mail for heavier packages going medium distances. They’re a middle ground worth knowing about.
Flat rate shines for cross-country shipments. Regular Priority Mail prices increase dramatically with distance — a package going from zone 1 to zone 8 can cost two to three times what the same package costs going to zone 2. Flat rate eliminates that distance penalty entirely. If you regularly ship coast-to-coast or to Alaska and Hawaii, flat rate is almost always your best option for anything over a few pounds.
Don’t forget about flat rate envelopes. Beyond boxes, USPS offers flat rate padded envelopes and legal-size flat rate envelopes at lower prices than the small box. If your item is flexible enough to fit in an envelope, you can save even more.