Envelopes are the unsung heroes of the paper world. The wrong envelope can crumple your letter or make your invitation look amateur. Understanding the relationship between paper sizes and envelope sizes saves time and money.
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 folded once, or an An+2 folded twice.
- 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:
- #10 — 4.125 × 9.5 in — standard US business envelope
- #9 — 3.875 × 8.875 in — personal correspondence
- #6¾ — 3.625 × 6.5 in — greeting cards
- #11 — 4.5 × 10.375 in — larger business mailings
- #14 — 5 × 11.5 in — heavy documents
Other Envelope Types
- Invitation envelopes — often square (e.g. 155 × 155 mm)
- Booklet envelopes — flap on long side, for catalogues
- Window envelopes — show the address on the enclosed document
- Padded envelopes — for shipping fragile items
- Wallet envelopes — flap on 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.
- Match the postal service requirements