C9 Envelope Size
C9 measures 40 × 57 mm (1.57 × 2.24 in). The C series is used for envelopes.
| Millimetres | 40 × 57 mm |
|---|---|
| Centimetres | 4 × 5.7 cm |
| Inches | 1.57 × 2.24 in |
| Aspect ratio (long ÷ short) | 1.425 |
| Series | ISO 269 C series (envelopes) |
| Fits | An A9 sheet laid flat fits inside a C9 envelope |
| Fits inside | A C9 envelope fits inside a B9 envelope |
Where you will see C9
C9 is a very small envelope used for tiny cards and labels.
How C9 relates to A and B sizes
In the ISO 269 C series, each C size is slightly larger than the A size with the same number (37 × 52 mm for A9), so a flat A9 sheet fits inside a C9 envelope. The C9 envelope in turn fits inside a B9 envelope (44 × 62 mm).
Frequently asked questions
What size is a C9 envelope?
A C9 envelope is 40 × 57 mm (1.57 × 2.24 inches).
What fits in a C9 envelope?
A flat A9 sheet (37 × 52 mm) fits in a C9 envelope with a small margin of 3 mm in width and 5 mm in height.
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Last updated: 2026-10-03