A2 envelope size and what fits

An A2 envelope is 4⅜ × 5¾ inches (111 × 146 mm). It's made for 4¼ × 5½ note cards, thank-you cards and RSVP cards, which is exactly an 8½ × 11 sheet folded into quarters, and it mails as a regular machinable letter.

What fits in an A2?

The fit for a 4¼ × 5½ card is deliberately close, so a card that is thick or has a ribbon or layered paper may need the next size up. A 5½ Bar envelope is the same 4⅜ × 5¾ size with a pointed (baronial) flap instead of a square one.

Does an A2 mail with one stamp?

Yes, by size, but only just on shape. USPS letters must have a length-to-height ratio between 1.3 and 2.5. For an A2, 5.75 ÷ 4.375 = 1.31, so it passes by a small margin (at 4⅜″ high, the length must be at least 1.3 × 4.375 = 5.69″). It is above the 3½ × 5 inch minimum and below the 6⅛ × 11½ inch letter maximum. Because it is more than 4¼″ high, the piece must be at least 0.009″ thick, which a card in an envelope easily is.

As of October 23, 2026 (Notice 123, effective October 4, 2026), a stamped First-Class letter is $0.82 up to 1 oz and $1.11 up to 2 oz. Add the $0.49 nonmachinable surcharge if the envelope is rigid, lumpy (a wax seal, button or charm) or addressed along the short side. Envelopes that are square, like a 5 × 5, always pay it (why). Check yours with the USPS letter size checker.

Can a 4¼ × 5½ card go without an envelope?

Yes, as a postcard, if it's uniform cardstock 0.007″ to 0.016″ thick. USPS postcards can be up to 4¼ × 6 in, so a 4¼ × 5½ card qualifies, and the postcard price is $0.65 as of October 23, 2026. A folded note card is not a postcard; it needs an envelope.

Nearby A-style sizes

• A1 (3⅝ × 5⅛): 3½ × 5 RSVP and reply cards.
• A6 (4¾ × 6½): 4 × 6 photos and 4½ × 6¼ cards. See the 4×6 guide.
• A7 (5¼ × 7¼): 5 × 7 cards and invitations. See the 5×7 guide.
• A9 (5¾ × 8¾): 5½ × 8½ cards, which is Letter folded in half.

Every standard size is side by side in the envelope size chart.

FAQ

Is A2 the same as ISO A2 paper? No. The A2 envelope is a US announcement size. ISO A2 is a 420 × 594 mm paper sheet; see the paper sizes page.

Is "A2" the same as "5½ Bar"? Same dimensions; the names differ by flap style.