Envelope Sizes
ISO (DL, C4, C5, C6, B4) and US (#10, #9, A2, A7, 6×9) envelope sizes in mm and inches, which paper and fold fits each, and the best envelope for a document.
| Envelope | Size | Standard |
|---|---|---|
| DL | 110 × 220 mm (4.33 × 8.66 in) | ISO 269 / DIN 678 |
| C6 | 114 × 162 mm (4.49 × 6.38 in) | ISO 269 / DIN 678 |
| C6/5 | 114 × 229 mm (4.49 × 9.02 in) | ISO 269 / DIN 678 |
| C5 | 162 × 229 mm (6.38 × 9.02 in) | ISO 269 / DIN 678 |
| C4 | 229 × 324 mm (9.02 × 12.76 in) | ISO 269 / DIN 678 |
| C3 | 324 × 458 mm (12.76 × 18.03 in) | ISO 269 / DIN 678 |
| B6 | 125 × 176 mm (4.92 × 6.93 in) | ISO 269 / DIN 678 |
| B5 | 176 × 250 mm (6.93 × 9.84 in) | ISO 269 / DIN 678 |
| B4 | 250 × 353 mm (9.84 × 13.9 in) | ISO 269 / DIN 678 |
| #6¾ | 3.625 × 6.5 in (92.1 × 165.1 mm) | North American trade size |
| #9 | 3.875 × 8.875 in (98.4 × 225.4 mm) | North American trade size |
| #10 | 4.125 × 9.5 in (104.8 × 241.3 mm) | North American trade size |
| A2 | 4.375 × 5.75 in (111.1 × 146 mm) | North American trade size |
| A6 | 4.75 × 6.5 in (120.6 × 165.1 mm) | North American trade size |
| A7 | 5.25 × 7.25 in (133.3 × 184.1 mm) | North American trade size |
| 6 × 9 | 6 × 9 in (152.4 × 228.6 mm) | North American trade size |
| 9 × 12 | 9 × 12 in (228.6 × 304.8 mm) | North American trade size |
| 10 × 13 | 10 × 13 in (254 × 330.2 mm) | North American trade size |
Envelope sizes are designed around paper: the ISO C series fits A sizes (C4 holds A4 flat, C5 holds A4 folded once, C6 folded twice), DL takes A4 folded in thirds, and the US #10 is made for Letter folded in thirds. This chart gives every common ISO and US envelope in millimetres and inches, lists what fits inside, and finds the smallest envelope for your paper and fold.
How to use this tool
- Choose "What fits in this envelope" and pick an envelope, or
- choose "Which envelope fits my paper" and pick the paper size and how it is folded.
- Read the folded size and the best (smallest) envelope with room to spare.
- Check the table for alternatives and how much space each one leaves.
Formula
folded size: half = (H ÷ 2) × W · thirds = (H ÷ 3) × W · quarters = (W ÷ 2) × (H ÷ 2). An envelope fits when both its sides are at least 3 mm longer than the document's.
W and H are the paper's short and long sides. The 3 mm allowance leaves room for the paper thickness; thick inserts need more.
Example
An A4 letter folded in thirds measures 297 ÷ 3 = 99 × 210 mm. DL (110 × 220 mm) leaves 11 mm and 10 mm to spare, which makes it the best fit. A US Letter folded in thirds is 93.1 × 215.9 mm and fits a #9 (98.4 × 225.4 mm) — the classic reply envelope — as well as a #10.
Frequently asked questions
What size envelope do I need for A4 without folding?
C4, 229 × 324 mm. For A4 folded once use C5; folded twice, C6.
What is a #10 envelope?
The standard US business envelope, 4⅛ × 9½ in (104.8 × 241.3 mm), sized for Letter paper folded in thirds.
Which envelope for a 5 × 7 invitation?
An A7 envelope (5¼ × 7¼ in) leaves about 6 mm each way, the usual fit for 5 × 7 cards.
Sources
Last updated:
Page address with these inputs: https://kakao.mobile/tools/envelope-size-tool/
Cite or embed this tool
kakaoCalc. (2026, November 8). Envelope Sizes. https://kakao.mobile/tools/envelope-size-tool/
“Envelope Sizes.” kakaoCalc, 8 Nov. 2026, kakao.mobile/tools/envelope-size-tool/.
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