#10 envelope dimensions and what fits

A #10 envelope is 4⅛ × 9½ inches (105 × 241 mm). It's the standard US business envelope, made to hold an 8½ × 11 sheet folded in thirds, and it mails as a regular machinable letter.

What fits in a #10?

Several letter sheets folded together fit fine; the practical limit is weight and the ¼″ thickness cap, not the envelope's width. A4 in thirds is a tight fit in height (6 mm total), so outside North America a DL envelope (110 × 220 mm) is the usual choice. A 4×6 photo does not fit flat (6 in is taller than 4⅛ in); use an A6 instead.

Does a #10 mail with one stamp?

Yes, by size. 9.5 ÷ 4.125 = 2.30, inside the USPS 1.3–2.5 aspect-ratio range, and 4⅛ × 9½ is under the letter maximum of 6⅛ × 11½ inches. Because it's over 6″ long, the piece must be at least 0.009″ thick, which an envelope with a folded sheet inside easily is.

As of October 22, 2026 (Notice 123, effective October 4, 2026), stamped First-Class letters cost $0.82 up to 1 oz, $1.11 up to 2 oz, $1.40 up to 3 oz and $1.69 up to 3.5 oz. Over 3.5 oz, it pays large-envelope (flat) prices. If you add something rigid or lumpy (a key, a coin, a pen), the letter becomes nonmachinable and adds a $0.49 surcharge. Not sure about yours? Try the USPS letter size checker.

Related business sizes

• #9 (3⅞ × 8⅞): the reply envelope that slips inside a #10.
• #11 (4½ × 10⅜) and #12 (4¾ × 11): a step up when a #10 is too tight; both are still letter-size.
• 6 × 9 or 9 × 12: for Letter paper folded once or not at all. See the envelope size chart.

FAQ

Is "#10" the same as "Commercial 10" or "business envelope"? Yes, those are all names for the 4⅛ × 9½ size.

Is a #10 the same as DL? No. DL is the ISO size (110 × 220 mm, about 4.33 × 8.66 in): slightly taller and about ¾ in shorter. Compare paper sizes on the paper sizes page.