9×12 vs 10×13 envelope: which do you need?

Use a 9 × 12 for flat Letter or A4 pages, and step up to a 10 × 13 when the stack is thick, the contents are bigger than about 8½ × 11½ in, or you're putting a 9 × 12 inside. Both are too big for letter postage, so in the US both mail as large envelopes (flats) and pay the same price for the same weight.

Side-by-side sizes

A 9 × 12 holds Letter paper with ¼″ of room on each side across and ½″ lengthwise, which is plenty for a thin set of pages. Thickness eats into that room: a stack bulges the envelope, so the usable width shrinks as the stack grows. That is when the extra 1½″ across a 10 × 13 helps. A 9 × 12 also fits inside a 10 × 13 with 1 in to spare each way (10 − 9 and 13 − 12), which is handy for a return envelope.

A4 fits a 9 × 12 but only just lengthwise (8 mm in total). Outside North America the matching size is the ISO C4 (229 × 324 mm), the same width as a 9 × 12 but 324 − 305 = 19 mm longer.

Booklet or catalog style?

Both sizes come in two styles. Catalog (open end) envelopes open on the short side, good for dropping in a thick stack. Booklet (open side) envelopes open on the long side, easier for inserting bound booklets. The style doesn't change the size or the postage.

Postage: both are flats

A USPS letter can be at most 6⅛″ high (DMM 101), so 9″ and 10″ are both too tall. Flats can be up to 12″ high, 15″ long and ¾″ thick, so both sizes qualify (9 and 10 are under 12; 12 and 13 are under 15), as long as the piece is flexible and uniformly thick.

As of October 25, 2026 (Notice 123, effective October 4, 2026), retail First-Class Mail large-envelope prices are:

The price rises by $0.29 or $0.30 per ounce (for example $1.98 − $1.69 = $0.29) and the First-Class flat list stops at 13 oz. A 10 × 13 envelope is a little heavier than a 9 × 12, so with a stack close to an ounce break, weigh the finished piece.

Watch out for: a stiff piece (a board-backed certificate, a box) or a lumpy one is not flexible or uniformly thick, so it pays parcel prices instead (Notice 123, note 2). A clasp is fine as long as the piece stays flexible. Folding your pages to fit a 6 × 9 or a #10 can get them back down to letter postage, from $0.82.

Quick picks

• A few Letter or A4 pages you don't want creased: 9 × 12.
• A thick report, a catalog, or a stack too fat to slide into a 9 × 12: 10 × 13.
• Sending a 9 × 12 return envelope inside: 10 × 13 outer.
• Bigger than 9½ × 12½ in: look at 12 × 15 (the largest flat size). Compare every size in the envelope size chart.

FAQ

Will a 9 × 12 mail with one stamp? No. It's a flat, which starts at $1.69 for 1 oz, more than two Forever stamps ($0.82 × 2 = $1.64). Check any size with the USPS letter size checker.

Is a 10 × 13 more expensive to mail than a 9 × 12? Not by size. Flats are priced by weight, so the same weight costs the same in either.