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For a small seller, first compare Flat Rate packaging with regular weight-and-zone pricing; eligible commercial shipments can also use Priority Mail Cubic. Flat Rate uses USPS-produced boxes and envelopes that ship for one price anywhere in the U.S. up to 70 lb, as long as the flaps close within their normal folds. Regular Priority Mail can go in any sturdy box (including your own) and is priced by weight and zone. The packed parcel must weigh 70 lb or less and measure no more than 108 inches in length plus girth. The four main Flat Rate boxes are Small (8 11/16 x 5 7/16 x 1 3/4 in outside), 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) and Large (12 1/4 x 12 x 6 in).
USPS gives Priority Mail boxes away free, so the things you buy are the labels, tape and a scale: 4x6 thermal labels , tape selected to the carrier’s width requirement for your own boxes, and a shipping scale to price regular Priority Mail by weight. For box sizes and dim-weight rules on other services, see the USPS box sizes chart .
Priority Mail Flat Rate box sizes
These are the dimensions USPS lists on its Priority Mail page , checked October 3, 2026. Inside dimensions are what your item and padding must fit into. Outside dimensions are what you store on the shelf.
| Flat Rate box | Outside (in) | Inside (in) | Packing note |
|---|---|---|---|
| Small Flat Rate Box | 8 11/16 x 5 7/16 x 1 3/4 | 8 5/8 x 5 3/8 x 1 5/8 | Only 1 5/8 in of depth, so it suits flat, dense items that need little wrap |
| Medium Flat Rate Box 1 (top-loading) | 11 1/4 x 8 3/4 x 6 | 11 x 8 1/2 x 5 1/2 | The deeper medium box; room for a layer of cushioning around a compact item |
| Medium Flat Rate Box 2 (side-loading) | 14 x 12 x 3 1/2 | 13 5/8 x 11 7/8 x 3 3/8 | Same price as Medium 1 but wide and shallow, for flatter items |
| Large Flat Rate Box | 12 1/4 x 12 x 6 | 12 x 11 3/4 x 5 1/2 | Largest standard Flat Rate box |
| APO/FPO/DPO Flat Rate Box | 12 1/4 x 12 x 6 | 12 x 11 3/4 x 5 1/2 | Same size as the Large box, priced lower for military and diplomatic addresses only |
| Discontinued Board Game Box (existing stock) | 24 1/16 x 11 7/8 x 3 1/8 | 23 11/16 x 11 3/4 x 3 | Production discontinued in 2022; USPS said existing boxes retain domestic/APO/FPO/DPO Flat Rate pricing |
USPS discontinued Board Game Box production in 2022 while retaining the stated domestic Flat Rate treatment for existing boxes. Do not plan a new supply order around that legacy row.
USPS also lists Flat Rate envelopes. Per DMM 123 , any USPS-produced Priority Mail envelope smaller than the 12 1/2 x 9 1/2 in EP14F qualifies for the Flat Rate Envelope price.
| Flat Rate envelope | Size (in) | Typical use |
|---|---|---|
| Flat Rate Envelope (paperboard) | 12 1/2 x 9 1/2 | Documents, thin flat goods |
| Padded Flat Rate Envelope | 12 1/2 x 9 1/2 | Clothing, books and other soft items (costs more than the paperboard version) |
| Legal Flat Rate Envelope | 15 x 9 1/2 | Legal-size documents |
| Small Flat Rate Envelope | 10 x 6 | Gift cards, small documents |
| Window Flat Rate Envelope | Listed at 10 x 5 and 12 1/2 x 9 1/2 | Pre-addressed items |
A paperboard Flat Rate envelope is not a rigid mailer. If you are looking for rigid mailer sizes for prints or cards, see the Etsy fragile packing guide and ship those as regular Priority Mail or Ground Advantage in your own mailer.
What “If It Fits, It Ships” actually allows
“If It Fits, It Ships” is the USPS slogan for Flat Rate (2009 Postal Bulletin ). The current rules behind it:
- Weight: any weight up to 70 lb, same price to any domestic address (DMM 123.1.4 ).
- Fit: the container flaps must be able to close within the normal folds. Bulging boxes and flaps held down with extra tape do not qualify (USPS Priority Mail page ).
- Fees: Flat Rate packaging is exempt from the nonstandard fees for length, volume and shape, and dimensional weight does not apply.
- No bundling: two Flat Rate boxes cannot be taped together and mailed as one parcel (DMM 101.3.3 ).
- USPS packaging only: you cannot get Flat Rate pricing in your own box, and any USPS-produced Priority Mail box must ship as Priority Mail, even if it is reconfigured or its markings are covered (DMM 601.6.1 ). A free Priority Mail box cannot be used with Ground Advantage postage.
Flat Rate pays off when the item is heavy for its size or is going far. A light item going a short distance can cost less as regular Priority Mail or Ground Advantage priced by weight, so compare in the USPS price calculator before you pack.
Rules for regular Priority Mail in your own box
When you use your own carton or a non-Flat-Rate Priority Mail box, these limits apply to the packed, taped parcel:
- Maximum weight: 70 lb (DMM 101.3.2 ).
- Maximum size: 108 inches in length plus girth. Length is the longest side; girth is the distance around the thickest part, or 2 x width + 2 x height for a box. A 20 x 12 x 10 in box is 20 + 2 x (12 + 10) = 64 inches.
- Dimensional weight: since July 12, 2026, a parcel over 1 cubic foot (1,728 cubic inches) is priced at the greater of actual weight and dimensional weight. For a qualifying rectangular parcel, round each dimension up, multiply, divide by 139 and round the DIM result up to a pound; DIM above 70 uses the 70-pound price (DMM 123.1.3 ).
- Nonstandard fees: extra fees apply to parcels more than 22 inches long, more than 30 inches long, or more than 2 cubic feet (3,456 cubic inches), and to tubes, rolls and some other shapes.
The USPS box-size and dimensional-weight guide has worked examples of the 139 divisor, the fee thresholds and the July 2026 dimension-reporting rules, so they are not repeated here.
Commercial shippers using PC Postage or Click-N-Ship can also get Priority Mail Cubic pricing, which charges by package volume and zone instead of weight. Under DMM 223.1.3 , a cubic piece must be 0.50 cubic foot or less, weigh 20 lb or less, and have no side longer than 22 inches. For small, heavy parcels it is worth pricing cubic against the Flat Rate box before you pack.
How to choose between Flat Rate and your own box
- Pack the item first with the cushioning it needs, then compare it with the inside dimensions above.
- If it fits a Flat Rate box with the flaps closing flat, price that box against the weight-and-zone price for the same parcel. Also compare available commercial cubic pricing; no option wins without the actual service quote.
- If it does not fit cleanly, use the smallest sturdy box that holds the item and its padding, weigh it on a shipping scale , measure the outside, and check the 70 lb and 108-inch limits.
- If the item is flat, a Flat Rate envelope or a mailer usually costs less to pack than a box.
If you reuse any box, USPS asks that old logos, barcodes, shipping labels and addresses be covered or blacked out, and that seams be closed with 2-inch-wide packing tape (USPS: How to Prepare & Send a Package ). A reused Priority Mail box still has to ship as Priority Mail. Check the current USPS supply-order delivery estimate and keep enough suitable packaging for upcoming shipments; do not treat a historical delivery estimate as a guarantee.
Sources
- USPS Priority Mail page : Flat Rate box and envelope dimensions, the 70 lb and 108-inch limits, the “flaps must close within the normal folds” rule, the 139 DIM divisor, nonstandard fee thresholds, the Flat Rate fee exemption and supply delivery time.
- DMM 123, Priority Mail retail prices and eligibility : Flat Rate pricing up to 70 lb, Flat Rate envelope eligibility, dimensional weight rules effective July 12, 2026, and nonstandard fees.
- DMM 101, Physical Standards : 108-inch length-plus-girth maximum, 70 lb maximum, and the rule that Flat Rate boxes cannot be mailed fastened together.
- DMM 601, Mailability : USPS-produced packaging may be used only for its product.
- DMM 223, Priority Mail commercial prices and eligibility : Priority Mail Cubic size and weight limits.
- USPS: How to Prepare & Send a Package : reused box markings, 2-inch packing tape, and the rule that Priority Mail packaging requires Priority Mail postage.
- USPS Postal Bulletin 22265 (2009) : origin of the “If It Fits, It Ships” Flat Rate slogan.
FAQ
What are the USPS Priority Mail Flat Rate box sizes?
Outside dimensions listed by USPS are: 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; and the discontinued Board Game Box 24 1/16 x 11 7/8 x 3 1/8 in for identifying existing stock, not a currently offered supply.
Can I use my own box for Priority Mail?
Yes. Regular Priority Mail uses weight and zone, with dimensional rules where applicable; eligible commercial shipments can also use cubic pricing. The packed parcel must weigh 70 lb or less and measure no more than 108 inches in length plus girth, and dimensional weight applies above 1 cubic foot. Flat Rate prices are available only in USPS-produced Flat Rate boxes and envelopes.
Does Flat Rate depend on weight?
No. A USPS Flat Rate box or envelope ships for the same price to any domestic address up to 70 lb, as long as the flaps close within their normal folds. Flat Rate packaging is also exempt from the Priority Mail nonstandard fees.
Can I use a free Priority Mail box for Ground Advantage?
No. USPS rules say its produced packaging may be used only for the product it is made for, and you must pay the Priority Mail price for anything shipped in a Priority Mail box, however the box is reconfigured and even if its markings are covered.