What size envelope for a 4×6 photo or card?

An A6 envelope – 4¾ × 6½ inches (121 × 165 mm). A 4×6 print slides in with room to spare, and the envelope mails as a regular machinable letter in the US.

How much room does it leave?

That's more slack than a card-specific fit: A6 is also sold for 4½ × 6¼ cards, which leave only ⅛″ per side. If you want a 4×6 photo to sit snugly, a little extra room is normal; add a piece of card behind it if it slides around.

Does it mail with one stamp?

By size, yes. 6.5 ÷ 4.75 = 1.37, inside the USPS 1.3–2.5 aspect-ratio range, and 4¾ × 6½ is under the letter maximum of 11½ × 6⅛ inches. Because it is over 6″ long and over 4¼″ high, the piece must be at least 0.009″ thick, which any envelope with a photo inside easily is. As of October 21, 2026, a 1 oz stamped First-Class letter is $0.82 (Notice 123, effective October 4, 2026). If the photo is mounted on board or otherwise rigid, the letter becomes nonmachinable and adds the $0.49 surcharge. Check any envelope with the USPS letter size checker.

Can I skip the envelope and mail it as a postcard?

A 4×6 card can qualify for the cheaper postcard price ($0.65 as of October 21, 2026) if it meets USPS card rules: at least 3½ × 5 in and 0.007″ thick, and no more than 4¼ × 6 in and 0.016″ thick, made of uniform cardstock. Anything thicker or larger pays the letter price. A photo you care about is safer in an envelope.

Alternatives

• A7 (5¼ × 7¼): more room for a photo plus a note, or a 4×6 in a folded card. The 5×7 card guide covers A7 postage.
• C6 (114 × 162 mm, about 4.49 × 6.38 in): the ISO size used outside North America; a 4×6 (102 × 152 mm) fits with about 12 mm and 10 mm of total room.
• Avoid square envelopes for photos; they pay a surcharge (why).

See every standard size side by side in the envelope size chart.