International reference
C3, C4, C5, C6 and C7 form a familiar size progression around A-series paper. DL is a long format commonly used for A4 folded into thirds.
Nominal size reference
Every table uses nominal exterior width × height. International sizes lead with millimetres; US sizes lead with conventional fractional inches.
| Format | Native size (width × height) | Converted size (width × height) | Source checked | Context |
|---|---|---|---|---|
| C3International | 324 × 458 mm | 12.756 × 18.031 in | Poczta Polska ↗27 August 2026 | A3 paper flat |
| C4International | 229 × 324 mm | 9.016 × 12.756 in | Envelope Manufacturers Association ↗27 August 2026 | A4 paper flat |
| C5International | 162 × 229 mm | 6.378 × 9.016 in | Envelope Manufacturers Association ↗27 August 2026 | A5 paper flat |
| C6International | 114 × 162 mm | 4.488 × 6.378 in | Envelope Manufacturers Association ↗27 August 2026 | A6 paper flat |
| C7International | 81 × 114 mm | 3.189 × 4.488 in | Envelope Manufacturers Association ↗27 August 2026 | A7 paper flat |
| DLInternational | 110 × 220 mm | 4.331 × 8.661 in | Envelope Manufacturers Association ↗27 August 2026 | A4 paper folded into thirds |
C-series sizes are widely used in current markets, but EnvelopeLogic does not call them a current ISO envelope standard. ISO 269:1985, which concerned correspondence-envelope designations and sizes, was withdrawn on 27 April 2009.
Nominal envelope dimensions describe the exterior format, not guaranteed usable internal space. Seams, lining, stock thickness, multiple pieces and manufacturing tolerances reduce practical room; sample before a bulk order or print run.
Measure after folding, keep width and height in the same unit, and allow clearance on every edge. The matcher tests both orientations.
ISO 216:2007 is published and specifies trimmed A- and B-series paper sizes. That does not make the withdrawn ISO 269 a current envelope standard.
EnvelopeLogic independently records the minimum factual dimensions and creates its own wording, calculations and diagrams. A source link is evidence for a stated fact, not permission to reproduce the source presentation.