What size envelope for US Letter?
US Letter fits a No.
Smallest envelope for each fold
US Letter is 8.5 × 11 inches (215.9 × 279.4 mm).
| Fold | Size of folded sheet | Smallest envelope | Envelope size | Spare (width, length) | Next sizes up |
|---|---|---|---|---|---|
| flat | 215.9 × 279.4 mm | C4 | 229 × 324 mm | 13.1 × 44.6 mm | - |
| folded in half | 215.9 × 139.7 mm | US A9 (Diplomat) | 146 × 222.2 mm | 6.3 × 6.3 mm | US A10 (Willow), C5, C4 |
| folded in three (letter fold) | 215.9 × 93.1 mm | US No. 9 | 98.4 × 225.4 mm | 5.3 × 9.5 mm | DL, US No. 10 (Business), C6/C5 |
| folded in quarters | 108 × 139.7 mm | US A2 (Lady Grey) | 111.1 × 146 mm | 3.2 × 6.3 mm | C6, US A6 (Thompson), US A7 (Besselheim) |
At least 3 mm spare each way is required. Computed from the dimensions of ISO C and DL envelopes and the US sizes on the envelope sizes page.
Every envelope tested
Spare room in mm (width × length) for each fold; a dash means it does not fit.
| Envelope | Size (mm) | Flat | Half | Thirds | Quarters |
|---|---|---|---|---|---|
| DL | 110 × 220 | – | – | 17 × 4 | – |
| C6 | 114 × 162 | – | – | – | 6 × 22 |
| C6/C5 | 114 × 229 | – | – | 21 × 13 | 6 × 89 |
| C5 | 162 × 229 | – | 22 × 13 | 69 × 13 | 54 × 89 |
| C4 | 229 × 324 | 13 × 45 | 89 × 108 | 136 × 108 | 121 × 184 |
| US No. 7 3/4 (Monarch) | 98 × 190 | – | – | – | – |
| US No. 9 | 98 × 225 | – | – | 5 × 10 | – |
| US No. 10 (Business) | 105 × 241 | – | – | 12 × 25 | – |
| US No. 11 | 114 × 264 | – | – | 21 × 48 | 6 × 124 |
| US No. 12 | 121 × 279 | – | – | 28 × 63 | 13 × 140 |
| US No. 14 | 127 × 292 | – | – | 34 × 76 | 19 × 152 |
| US A2 (Lady Grey) | 111 × 146 | – | – | – | 3 × 6 |
| US A6 (Thompson) | 121 × 165 | – | – | – | 13 × 25 |
| US A7 (Besselheim) | 133 × 184 | – | – | – | 25 × 44 |
| US A8 (Carrs) | 140 × 206 | – | – | – | 32 × 67 |
| US A9 (Diplomat) | 146 × 222 | – | 6 × 6 | 53 × 6 | 38 × 83 |
| US A10 (Willow) | 152 × 241 | – | 13 × 25 | 59 × 25 | 44 × 102 |
Which one to pick
For standard business mail, fold the Letter sheet in thirds and use a No. 10 envelope. Folded once, a Letter sheet is 5.5 × 8.5 in and fits an A9 envelope (5.75 × 8.75 in) or a 6 × 9 in booklet envelope. To keep it flat, use a 9 × 12 in catalog envelope. ISO envelopes also work: flat Letter fits a C4 (229 × 324 mm) with 13.1 mm and 44.6 mm to spare, and a Letter folded in three fits a DL with 16.9 mm across but only 4.1 mm along. See the No. 10 envelope page for the postal size limits.
Frequently asked questions
What envelope fits 8.5 x 11 paper?
A No. 10 (4 1/8 x 9 1/2 in) for paper folded in three, or a 9 x 12 in envelope for flat paper.
Does Letter fit a DL envelope?
Folded in three, yes, with about 4 mm to spare in length. It is a tight fit.
What size envelope is used for A4 in the US?
A4 is not a US size, but A4 folded in three fits a No. 10 envelope, with 5.8 mm to spare in height.