Sellers on marketplaces and anyone mailing a gift ask the same thing: will it fit in a flat rate box, and is that the best deal? This flat rate box calculator answers both at once using the official USPS Notice 123, effective October 4, 2026 prices. Online label prices apply when you buy postage on USPS.com or in a shipping app; Post Office counter prices are higher, so switch the price type if you pay in person.
Flat rate box sizes and prices
USPS sells seven domestic Priority Mail Flat Rate options. The sizes below are inside dimensions, which is what matters for fitting an item. Each one ships anywhere in the U.S. for one price up to 70 lb, and the boxes are free at the Post Office or from USPS.com.
| Flat rate option | Inside size | Online label | Post Office |
|---|---|---|---|
| Flat Rate Envelope | 12-1/2" x 9-1/2" | $11.97 | $13.90 |
| Legal Flat Rate Envelope | 15" x 9-1/2" | $12.51 | $14.25 |
| Padded Flat Rate Envelope | 12-1/2" x 9-1/2" | $12.84 | $15.00 |
| Small Flat Rate Box | 8-5/8" x 5-3/8" x 1-5/8" | $12.95 | $14.65 |
| Medium Box (Top-Loading) | 11" x 8-1/2" x 5-1/2" | $22.02 | $25.80 |
| Medium Box (Side-Loading) | 13-5/8" x 11-7/8" x 3-3/8" | $22.02 | $25.80 |
| Large Flat Rate Box | 12" x 11-3/4" x 5-1/2" | $32.75 | $36.10 |
The two Medium boxes cost the same: the top-loading one is deeper, the side-loading one is wider and flatter. When people compare flat rate box sizes and prices, the Medium box is usually the one that surprises them — it is almost twice the price of the Small box, so it pays to check whether an envelope works first.
Cheapest way to ship a package
The cheapest way to ship a package depends on two numbers: weight and distance. Flat Rate ignores both, so it wins for heavy items going far. Ground Advantage charges by weight and zone, so it wins for light items and short trips. For example, a small 12 oz parcel costs $7.34 by Ground Advantage to a nearby zone and $8.95 to zone 8, while the Small Flat Rate Box is always $12.95. A 20 lb box the size of the Large Flat Rate Box costs $18.11 by Ground Advantage to zone 2 but $42.14 to zone 8 — compared with $32.75 for the Large Flat Rate Box.
Size matters too. Once a parcel is bigger than one cubic foot, USPS bills the higher of the real weight and the dimensional weight (L × W × H ÷ 166). That is why the cheapest way to ship large packages is often a flat rate box or a tighter box with less padding. For a small package, start with the envelopes: they are the least expensive Priority Mail option and take more than most people think.
Cheapest way to ship clothes
A folded hoodie (11 × 9 × 2.5 in, about 22 oz) is a classic case. In our calculator at zone 5, the cheapest pick is Ground Advantage in your own box at $10.50. Soft items squash, so a Padded Flat Rate Envelope or a poly mailer by Ground Advantage usually beats a box. For the cheapest way to ship clothes, roll or vacuum-pack them, weigh the finished package, and compare — a pair of jeans and a T-shirt often still fits in one envelope.
Shoes are different: the retail box is rigid. A typical shoebox (13 × 8 × 5 in) is too long for the top-loading Medium box, so the best pick here is Ground Advantage in your own box at $12.12.
Cheapest way to ship books
Books are small and heavy — the case where Flat Rate is most worth checking. Three hardcovers (9 × 6 × 3.5 in, about 3.8 lb) come out cheapest as Ground Advantage in your own box at $13.89 for zone 5, and as Ground Advantage in your own box at $19.06 for zone 8. A heavier stack tips the balance toward the Medium or Large box, so for the cheapest way to ship books, weigh the box and run the numbers each time. A single paperback usually fits the regular Flat Rate Envelope. USPS Media Mail can be even cheaper for books only, but it is slower and has content rules, so it is not part of this comparison.
USPS Ground Advantage vs Priority Mail cost
Ground Advantage takes 2–5 business days and is priced by weight and zone; Priority Mail Flat Rate takes 1–3 days with one price everywhere. The USPS Ground Advantage vs Priority Mail price difference is biggest at the ends of the range: under 1 lb, Ground Advantage is usually several dollars cheaper, while for heavy, long-distance parcels Flat Rate can come out ahead — the 20 lb example above is $42.14 by Ground Advantage to zone 8 vs $32.75 flat. A boxed phone (6.5 × 3.5 × 1.5 in) is a good middle example — at zone 5 the best pick is Ground Advantage in your own box at $8.24.
Both services include USPS Tracking and up to $100 of insurance for most packages. If speed matters, the Priority Mail shipping time is the deciding factor even when Ground Advantage is a little cheaper. See each flat rate box size for more examples.
Frequently asked questions
How do I know if my item fits in a flat rate box?
Measure the item's longest, middle and shortest sides and compare them with the box's inside length, width and height, longest to longest. The item can be turned any way. For envelopes, the item's thickness reduces the usable length and width, and the flap must close normally.
Can I put anything in a flat rate box as long as it closes?
Yes. Any mailable item that fits with the flaps closing normally ships for the flat price, up to 70 lb. You can't reshape or tape the box to make it bigger.
Is flat rate always cheaper than regular shipping?
No. For light items or short distances, Ground Advantage or Priority Mail by weight is often cheaper. Flat Rate wins mostly for items over about 2–3 lb going to far zones.
What is dimensional weight?
For parcels over one cubic foot (1,728 cubic inches), USPS charges the greater of the actual weight and length × width × height ÷ 166, rounded up to the next pound. Flat Rate boxes are never charged by dimensional weight.
Where do the prices come from?
From the official USPS price list, USPS Notice 123, effective October 4, 2026. Online label = Commercial prices; Post Office = Retail prices. Prices were last reviewed on 2026-11-10.