Choosing a C-Series Envelope

How the C series maps to A sheets: which C size takes which A sheet, flat or folded.

C sizes are cut slightly larger than the A sheet they carry, so the sheet slides in without binding at the edges. C4 at 229 × 324 mm takes a flat A4 sheet; C5 at 162 × 229 mm takes an A5 sheet, or an A4 folded once across its long side.

The mapping follows the same halving logic as the sheets themselves. Each C size is named for the A size it wraps, and stepping down one C size corresponds to stepping down one A size or one fold.

Because envelopes share the √2 : 1 proportion with the sheets, the fit stays consistent when you move up or down a size: a C envelope is always one small step larger than the sheet inside it.

Some envelope formats fall outside the core C ladder — ISO 269 defines additional related sizes — but the C series remains the reference point for matching an A sheet to an envelope.

A practical check before choosing: identify whether the sheet goes in flat or folded, then pick the C size one step larger than the resulting A format.

• Flat A4 → C4, 229 × 324 mm
• A5, or A4 folded once → C5, 162 × 229 mm
• A6 → C6
• A4 folded twice → a C size two steps down from C4
• Additional envelope sizes are defined in ISO 269

Further reading

• The A, B and C Dimension Table
• The Root-2 Halving Rule