Paper & card guide

Envelope for A4 paper

Use C4 for A4 flat, C5 for A4 folded once to A5, or DL for a letter fold into thirds. C6 can take A4 folded twice to A6.

Nominal size reference

Compare the recorded formats

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
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
DLInternational	110 × 220 mm	4.331 × 8.661 in	Envelope Manufacturers Association ↗27 August 2026	A4 paper folded into thirds

Four useful A4 routes

A4 is 210 × 297 mm. The finished folded size, not the starting sheet, is the matcher input.

Flat: 210 × 297 mm → C4 is the familiar relationship.
Half-fold: 148.5 × 210 mm → C5.
Letter fold / thirds: approximately 99 × 210 mm → DL.
Two folds to A6: approximately 105 × 148.5 mm → C6.

Folds are not thickness-free

Each fold adds layers and can create spring-back. Several sheets, heavy stock and hand folds need more clearance than a single accurately folded sheet.

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.

Which should you choose?

Choose C4 when the sheet must stay flat, C5 when one clean fold is acceptable, DL for familiar correspondence proportions, and C6 when compactness matters more than minimizing folds.