C10 Envelope Size
C10 measures 28 × 40 mm (1.10 × 1.57 in). The C series is used for envelopes.
| Millimetres | 28 × 40 mm |
|---|---|
| Centimetres | 2.8 × 4 cm |
| Inches | 1.10 × 1.57 in |
| Aspect ratio (long ÷ short) | 1.429 |
| Series | ISO 269 C series (envelopes) |
| Fits | An A10 sheet laid flat fits inside a C10 envelope |
| Fits inside | A C10 envelope fits inside a B10 envelope |
Where you will see C10
C10 is the smallest envelope size in the series and is rarely used.
How C10 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 (26 × 37 mm for A10), so a flat A10 sheet fits inside a C10 envelope. The C10 envelope in turn fits inside a B10 envelope (31 × 44 mm).
Frequently asked questions
What size is a C10 envelope?
A C10 envelope is 28 × 40 mm (1.10 × 1.57 inches).
What fits in a C10 envelope?
A flat A10 sheet (26 × 37 mm) fits in a C10 envelope with a small margin of 2 mm in width and 3 mm in height.
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Last updated: 2026-10-03