Envelope Size Chart: US Business, A-Series & Catalog Sizes

Commercial envelopes — the business sizes

Commercial envelopes are the ones businesses use for letters, invoices, and statements. The number is a size code, not a measurement — a #10 is 4⅛ × 9½ in, the standard US business envelope, and a #9 is 3⅞ × 8⅞ in, the reply envelope that fits inside a #10. Every commercial size on this chart mails at the USPS letter rate, including the #14 at 5 × 11½ in, which sits exactly at the letter-rate maximum of 6⅛ × 11½ in.

A-series envelopes — the card sizes

A-series envelopes are sized for cards, not letters: the number roughly tracks the card it holds, and each one is about a quarter inch larger than the card inside it. An A7 at 5¼ × 7¼ in holds a 5 × 7 in greeting card, an A6 at 4¾ × 6½ in holds a 4 × 6 in photo card, and an A2 at 4⅛ × 5¾ in holds a 4.25 × 5.5 in reply card. All six A-series sizes mail at the USPS letter rate.

Catalog envelopes — the flat sizes

Catalog envelopes hold sheets flat and unfolded, so they are bigger — and that is what changes the postage. A 6 × 9 in catalog envelope is still narrow enough to mail at the letter rate (6 in is under the 6⅛ in limit), but a 9 × 12 in is 9 in wide, past the limit, so it mails as a flat — and a 10 × 13 in is a flat too. Flats cost more than letters and have their own size limits: up to 12 × 15 in and no more than ¾ in thick.

Find the envelope by what you are mailing

Start from the paper and the envelope follows. A US Letter sheet (8.5 × 11 in) folded in thirds fits a #10; a 5 × 7 in card fits an A7; a 4 × 6 in card fits an A6; and an 8.5 × 11 in sheet sent flat and unfolded needs a 9 × 12 in catalog envelope. The rule of thumb is a quarter inch of clearance on each side: the envelope should be at least half an inch larger than the card in both directions, so the card slides in without catching the flap.

Letter rate, flat rate, and the square-envelope surcharge

The USPS letter rate is the cheapest way to mail a paper envelope: at least 3½ × 5 in, at most 6⅛ × 11½ in, and no more than ¼ in thick. Anything over those limits is a flat, which costs more — that is the 9 × 12 in and 10 × 13 in on this chart. Two things catch people out. First, thickness: a letter stuffed past ¼ in thick becomes a flat even if its size is right. Second, shape: square envelopes carry the USPS nonmachinable surcharge, because square envelopes jam the sorting machines — a square card needs a rectangular envelope, or the surcharge on top of the letter rate. None of the sizes on this chart is square, so the surcharge does not apply to any of them.

Output · formula · meaning

Who uses this

• The small-business mailer: sends invoices in #10 envelopes and checks the size is 4⅛ × 9½ in (104.8 × 241.3 mm) — and that a #14 at 5 × 11½ in still mails at the letter rate for legal-size documents.
• The wedding-invitation buyer: orders 5 × 7 in invitations and learns they need A7 envelopes at 5¼ × 7¼ in (133.4 × 184.2 mm) — and that square invitations would carry the nonmachinable surcharge.
• The stationery seller: lists a 4.25 × 5.5 in reply card and learns it fits an A2 at 4⅛ × 5¾ in (111.1 × 146.1 mm), letter rate, so the listing can promise standard postage.
• The office manager: mails 8.5 × 11 in reports flat and learns they need a 9 × 12 in catalog envelope at 228.6 × 304.8 mm — and that it mails as a flat, not a letter, so the postage budget needs the higher rate.

Accuracy & limitations

The sizes on this chart are the standard US envelope sizes: commercial #6¼ through #14, A-series A2 through A10, and catalog 6 × 9, 9 × 12, and 10 × 13. Millimetres are the inches times 25.4, rounded to one decimal — so 4⅛ in is 104.8 mm and 9½ in is 241.3 mm. Envelope makers round their own dimensions a little: a #10 from one maker can be a sixteenth of an inch off another maker's, and the flap style (commercial, wallet, or booklet) does not change the size. The USPS rates are the letter and flat categories, not the prices, which change — check the current postage at usps.com before mailing in bulk. The letter-rate limits are 3½ × 5 in minimum and 6⅛ × 11½ in maximum at ¼ in thick; flats go up to 12 × 15 in at ¾ in thick. Square envelopes carry the nonmachinable surcharge even at letter size. This chart covers US sizes only — European C-series envelopes (C6, DL, C5) follow a different system and are not listed here.