C4 Envelope
C-Series Envelopes (ISO 269) · 7 specs
Direct Answer
The C4 envelope is an ISO 269 standard postal packet measuring 229 × 324 mm (9.02 × 12.76 inches). Sized specifically to enclose a full unfolded sheet of A4 paper (210 × 297 mm) with comfortable margin, it is the worldwide standard for mailing legal contracts, university diplomas, annual corporate reports, and magazines.
Source: ISO 269:1985 (DIN 678) · Last reviewed Aug 28, 2026
Specs
| Width × Height (Metric) | 229 × 324 mm |
| Width × Height (Imperial) | 9.02 × 12.76 in |
| Geometric Mean Origin | Geometric mean of A4 (210×297 mm) and B4 (250×353 mm) |
| Target Paper Pairing | Unfolded A4 paper (210 × 297 mm) or flat-folded A3 sheet |
| Paper Insertion Clearance | +19 mm width / +27 mm height allowance |
| Common Variants | Pocket (short-edge flap opening) or Banker/Wallet (long-edge opening) / Board-backed 'Do Not Bend' |
| Primary Applications | Legal contracts, commercial catalogs, official certificates, passport mailings |
The Unfolded A4 Standard
When legal contracts, original birth certificates, or presentation brochures must not be creased, the C4 envelope (229 × 324 mm) is mandated. It provides 19 mm of lateral clearance, enabling mailrooms to insert multi-page A4 documents or cardboard stiffeners with automated stuffing machines.
C4 vs North American 9×12 Catalog
The US equivalent to C4 is the 9×12 inch catalog envelope (228.6 × 304.8 mm). While nearly identical in width, the US 9×12 envelope is 19.2 mm shorter than C4 and will not close over a standard A4 document without crumpling the top edge.