Envelope · ISO C-series
C5 envelope
The C5 envelope is 229 × 162 mm. It holds an A4 sheet folded once or an A5 sheet flat, which makes it a natural choice for unfolded statements, single-fold business letters, and small booklets built on A4 folded once.
Exact dimensions
C5 is the third step in the ISO C-series, designed to hold an A5 sheet flat. Like all C-series envelopes, it shares the 1 : √2 ratio with the A-series paper sizes, which is what allows folded A4 to fit so cleanly.
| Unit | Width | Height |
|---|---|---|
| Millimetres (mm) | 229 | 162 |
| Centimetres (cm) | 22.9 | 16.2 |
| Inches (in) | 9.016 | 6.378 |
C5 — 229 × 162 mm
Dashed outline: a standard credit card (ISO/IEC 7810, 85.6 × 54 mm) at the same scale.
When C5 is the right choice
- · Letters that benefit from a single fold (less crease, easier to read)
- · A5 brochures or programmes sent flat
- · Wedding invitations and formal correspondence
- · Small magazines and quarterly newsletters
- · Statements that are too thick for a DL envelope
C5 vs DL — which to pick
DL is more compact and reads as more transactional (invoices, mailers, mass campaigns). C5 reads as more formal and considered (a single-fold A4 letter has a noticeably better feel than a tri-folded one). For high-end direct mail, C5 with a quality 100–120 gsm paper is the usual upgrade from DL.
Printing & mailing notes
Keep the bottom ~15 mm of the front clear for postal franking and routing marks, and place the address block in the lower-right two-thirds of the face. Home printers take C5 through the manual feed with a custom 162 × 229 mm size — feed one at a time and mind the flap orientation.
The fit maths: A4 folded once is 210 × 148.5 mm, which drops into 229 × 162 mm with about 9 mm of side clearance — and an unfolded A5 sheet is the same footprint, which is why C5 doubles as the flat-A5 envelope for cards and small certificates.
Frequently asked questions
What does C5 fit?
A4 folded once (along the short edge, giving ~210 × 148.5 mm) fits comfortably. A5 unfolded (148 × 210 mm) also fits with room to spare. C5 is a useful "between" size when you want an unfolded A5 letter or a single-fold A4.
How does C5 relate to C-series envelopes?
C5 is part of the C-series of the now-withdrawn ISO 269, which is mathematically derived from the A-series. Each C envelope is sized to hold the same-numbered A sheet flat. So C5 holds A5 unfolded, C4 holds A4 unfolded, C6 holds A6 unfolded.
Is C5 the right envelope for a small booklet?
Yes, if the booklet is built on folded A4 sheets. A 4-page A5 booklet (one A4 sheet folded once) fits cleanly. Thicker booklets may need C5/6 or C4 instead, depending on spine thickness.
Can I print address labels for C5?
Yes. The address-labels preset (21 per A4 sheet at 63.5 × 38.1 mm) works well — each label fits comfortably on the front of a C5 envelope without crowding.
What paper weight is typical for C5?
80–100 gsm for office C5. Premium business correspondence sometimes uses 120 gsm or laid paper for a more substantial feel.
Does C5 cost more to post than DL?
Often, yes. Several postal services tier by envelope dimensions, and C5 commonly falls into a “large letter” band while DL and C6 ride the standard letter rate — Royal Mail is the best-known example. If the content folds cleanly into thirds, DL is usually the cheaper ticket; check your local tariff before committing a bulk mailing to C5.
Does an A5 booklet fit in a C5 envelope?
Yes — an A5 booklet (210 × 148 mm landscape) sits in the 229 × 162 mm pocket with roughly 19 × 14 mm of clearance. Watch thickness rather than footprint: letter-rate bands typically cap at a few millimetres and “large letter” bands around 20–25 mm, depending on the service.
Can I nest a reply envelope inside a C5?
Comfortably — both a DL (220 × 110) and a C6 (162 × 114) reply envelope fit inside a C5 alongside a folded letter. This nesting is the reason invitation and statement suites standardise on the C-series.
When should I use C4 instead of C5?
When the A4 content must arrive unfolded — certificates, signed contracts, photo prints, stapled reports. C4 (324 × 229 mm) takes A4 flat; C5 always implies one fold. Between the two, C5 usually posts cheaper and fits standard letterboxes more reliably.