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Envelope sizes, matched to what you are actually mailing

A2 (4-3/8 x 5-3/4 inches) fits a thank-you note or RSVP card. A7 (5-1/4 x 7-1/4 inches) is the standard for a 5x7 wedding invitation or announcement. A number 10 (4-1/8 x 9-1/2 inches) is the standard folded business letter size. Anything larger than a number 10, including a 9x12, is priced by USPS as a Flat rather than a Letter, which costs more.

Checked July 30, 2026/7 min read

Claire Post

Written by Claire PostReviewed against USPS Publication 28

What is on this page6 sections
  1. 01Step by step
  2. 02Quick reference
  3. 03Special cases
  4. 04Key terms
  5. 05Check before you mail
  6. 06Questions people ask

Envelope size looks like a simple matching problem, pick one big enough for what is going inside, until postage enters the picture. Cross a specific size or thickness threshold, even slightly, and USPS stops treating the piece as a Letter and starts treating it as a Flat, at a noticeably higher rate.

The sizes themselves are simple enough to memorize for the handful of situations most people actually run into: a small card, a standard invitation, a folded letter. The part worth double-checking, every time, is whether the size you picked quietly crossed into Flat pricing without you realizing it.

Step by step

  1. Match the size to what is actually going inside

    A card needs an envelope close to its own size, not one built for a folded letter. A folded letter needs room for the fold, not a snug fit.

  2. Use A2 for a small card

    4-3/8 by 5-3/4 inches. The standard for a thank-you note, an RSVP card, or a small informal note.

  3. Use A7 for a standard invitation

    5-1/4 by 7-1/4 inches. The standard for a 5x7 wedding invitation, birthday card, or formal announcement.

  4. Use a number 10 for a folded business letter

    4-1/8 by 9-1/2 inches. The everyday standard for a single sheet folded in thirds.

  5. Know the point where size stops being a Letter and becomes a Flat

    Once an envelope exceeds roughly 6-1/8 by 11-1/2 inches, or gets too thick, USPS reclassifies it from a Letter to a Flat, which costs more to mail. A 9x12 envelope, for example, is always priced as a Flat.

Quick reference

Pin this next to your printer
OccasionSize to useDimensions
Thank-you note, RSVP cardA24-3/8 x 5-3/4 in
Photo, gift cardA64-3/4 x 6-1/2 in
Wedding invitation, formal announcementA75-1/4 x 7-1/4 in
Folded business letterNumber 104-1/8 x 9-1/2 in
Unfolded resume or document9x12Flat rate, not Letter rate
UK/European standard letterDL110 x 220 mm
UK/European standard cardC6114 x 162 mm

Special cases

The rule above covers most people. Here is what changes for the rest.

How do international envelope sizes compare to US sizes?

They use an entirely different system, and the names do not translate directly. DL, the common European standard letter size, measures 110 by 220 millimeters, close in spirit to a number 10 but not identical. C6, a common European card and small-letter size, measures 114 by 162 millimeters, roughly comparable in role to an A6. If you are mailing internationally, match to the destination country own size convention rather than assuming a US size will look or fit the same way, and see the international addressing guide for the format side of that same question.

Does envelope thickness affect postage the same way size does?

Yes. A Letter also has a thickness limit, about a quarter inch, before it gets reclassified as a Flat regardless of its length and width. A padded or heavily stuffed number 10 envelope can trigger Flat pricing even though its footprint is still Letter-sized.

What size envelope should I use for a greeting card that is not a standard size?

Round up to the next standard size that comfortably fits the card with a small margin, rather than trying to match exactly. A card that is slightly too large for A2 but well within A6 should go in an A6, since a snug or overstuffed envelope is more likely to tear or jam in sorting equipment.

Key terms on this page

A7
A standard US envelope size, 5-1/4 by 7-1/4 inches, most commonly used for wedding invitations and formal announcements.
Number 10
The standard US business envelope size, 4-1/8 by 9-1/2 inches, sized for a single sheet folded in thirds.
Flat
A USPS mail category for larger or thicker pieces that exceed Letter-size limits, priced higher than a standard Letter.
DL
A common international (ISO) envelope size, 110 by 220 millimeters, roughly comparable in role to a US number 10.

Check the address before you mail

Questions people ask

What size envelope do I need for a standard 5x7 invitation?
A7, at 5-1/4 by 7-1/4 inches, is the standard match for a 5x7 card or invitation, giving a small comfortable margin on every side.
Why did my envelope cost more to mail than I expected?
It most likely crossed the Letter-to-Flat size or thickness threshold. Anything larger than roughly 6-1/8 by 11-1/2 inches, or thicker than about a quarter inch, is priced as a Flat, which costs noticeably more than standard Letter postage.
Is a 9x12 envelope ever priced as a regular Letter?
No. A 9x12 always exceeds the Letter-size dimensions, so it is always priced as a Flat regardless of how light or thin its contents are.
Do square envelopes cost more to mail even if they are small?
Often yes. USPS applies a surcharge to square envelopes regardless of their overall size, since automated sorting equipment is not built to handle a square shape the way it handles a standard rectangular one.

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