Envelopes are the unsung heroes of the paper world. The wrong envelope can crumple your carefully printed letter or make your invitation look amateur. Understanding the relationship between paper sizes and envelope sizes saves time, money, and embarrassment.
The ISO C Series: Designed for A Series
The ISO 269 standard defines the C series envelopes specifically to fit A series paper. The rule is simple: a Cn envelope holds an An sheet unfolded, or an An+1 sheet folded once, or an An+2 sheet folded twice. So:
- C4 — 229 × 324 mm — holds A4 flat
- C5 — 162 × 229 mm — holds A4 folded once, or A5 flat
- C6 — 114 × 162 mm — holds A4 folded twice, or A6 flat
- C7 — 81 × 114 mm — small cards and notes
- C3 — 324 × 458 mm — holds A3 flat (for posters)
DL: The Business Envelope
DL (Dimension Lengthwise, sometimes "DIN Lang") is the standard business envelope in Europe. It measures 110 × 220 mm and holds an A4 sheet folded into three equal panels. Its 1:2 aspect ratio is not part of the C series — which is why you will never see a "C6.5". DL is the workhorse of direct mail, invoices, and pay slips.
US Envelopes: The Numbering System
American envelopes use a different, older system based on numbers. The most common are:
- #10 — 4.125 × 9.5 in — the standard US business envelope, holds a Letter sheet folded in thirds
- #9 — 3.875 × 8.875 in — for personal correspondence
- #6¾ — 3.625 × 6.5 in — for personal letters and greeting cards
- #11 — 4.5 × 10.375 in — for larger business mailings
- #12 — 4.75 × 11 in — for correspondence where a bit more room is needed
- #14 — 5 × 11.5 in — for heavy documents and legal-size mail
Other Envelope Types
Beyond the standard rectangular envelopes, there are several specialised formats:
- Invitation envelopes — often square (e.g., 155 × 155 mm for European, A7 for US)
- Booklet envelopes — with the flap on the long side, for catalogues and magazines
- Window envelopes — with a transparent window that reveals the address printed on the enclosed document; saves printing on envelopes
- Padded envelopes — for shipping small items
- Wallet envelopes — with the flap on the short side
How to Choose the Right Envelope
- Identify your contents — an A4 letter, a card, a booklet?
- Decide on folding — flat, half-fold, or tri-fold?
- Add a small margin — the envelope should be 3–5 mm larger than the folded paper on each side so it slides in easily
- Match the postal service requirements — some countries require specific envelope sizes for reduced postage
Every envelope size is listed in our database with its dimensions in mm, cm, inches, points, picas, and pixels.