Back to Reference Table
C Envelope Series
ISO 269 Standard and the C Envelope Series
The ISO 269 standard defines the C series, primarily used for envelope sizes worldwide. The C series is designed to complement the A and B paper series, providing perfectly matched envelopes for standard paper formats.
Main Characteristic
The C series shares the same 1:√2 aspect ratio as the A series. Its dimensions are defined as the geometric mean of the A and B series sizes with the same number — for example, C4 is larger than A4 but smaller than B4. This precise relationship ensures that a sheet of A paper fits neatly into a C envelope of the same number. Furthermore, because of the 1:√2 ratio, an A4 sheet folded in half parallel to its shorter side fits into a C5 envelope, and folded twice it fits into a C6 envelope.
The Measurement System
Starting Point: The series begins with C0, whose dimensions are the geometric mean of A0 and B0, producing an envelope large enough to hold an unfolded A0 sheet.
Scaling: Each subsequent size follows the same geometric-mean rule: C1 fits A1, C2 fits A2, and so on. The tolerances match those of the A and B series — ±1.5 mm for dimensions up to 150 mm, ±2 mm for 150–600 mm, and ±3 mm above 600 mm.
Common Usage: The most frequently used sizes are C4 (324 × 229 mm) for unfolded A4 letters, C5 (229 × 162 mm) for A4 folded once, and C6 (162 × 114 mm) for A4 folded twice — which is why they are often referred to as A4, A5, and A6 envelope sizes respectively.
C1
C3
C5
C7
...
C8
C6
C4
C2