C-Series Envelope Guide

C-Series Envelope Guide

The C-series is the envelope-focused part of ISO 216. Every C-size is specifically designed to fit the corresponding A-size unfolded — making it easy to match envelopes to documents without any guesswork.

📏 Complete C-Series Dimensions

Size	Millimetres	Inches	Fits Paper
C0	917 × 1297 mm	36.1 × 51.1 in	A0 unfolded
C1	648 × 917 mm	25.5 × 36.1 in	A1 unfolded
C2	458 × 648 mm	18.0 × 25.5 in	A2 unfolded
C3	324 × 458 mm	12.8 × 18.0 in	A3 unfolded
C4	229 × 324 mm	9.0 × 12.8 in	A4 unfolded ⭐
C5	162 × 229 mm	6.4 × 9.0 in	A5 unfolded / A4 half
C6	114 × 162 mm	4.5 × 6.4 in	A6 unfolded / A4 quarter
C7	81 × 114 mm	3.2 × 4.5 in	A7 unfolded
C8	57 × 81 mm	2.2 × 3.2 in	A8 unfolded
C9	40 × 57 mm	1.6 × 2.2 in	A9 unfolded
C10	28 × 40 mm	1.1 × 1.6 in	A10 unfolded
DL	110 × 220 mm	4.3 × 8.7 in	A4 folded in thirds ⭐

📮 Which Envelope Fits Which Paper?

The rule is simple: C + number = A + number (envelope fits paper unfolded).

Paper	Fold	Best Envelope
A4	Unfolded	C4 (229 × 324 mm)
A4	Folded in half	C5 (162 × 229 mm)
A4	Folded in thirds	DL (110 × 220 mm)
A4	Folded in quarters	C6 (114 × 162 mm)
A5	Unfolded	C5 (162 × 229 mm)
A6	Unfolded	C6 (114 × 162 mm)
A3	Unfolded	C3 (324 × 458 mm)
A2	Unfolded	C2 (458 × 648 mm)

🎨 Common C-Series Uses

C3–C4: Business Documents

• Full A4 letters and documents
• Contracts and certificates
• Invoices and statements (unfolded)
• Marketing materials

C5: Cards and Smaller Documents

• Greeting cards (A5)
• Invitations
• Folded A4 letters
• Small brochures

C6: Small Cards

• Postcards (A6)
• Thank-you cards
• Small photos

DL: The UK's Business Standard

DL (110 × 220 mm) is the single most common business envelope size in the UK. It fits an A4 sheet folded into thirds — the classic mail format for:

• Invoices and statements
• Business letters
• Direct mail campaigns
• Pay slips
• Charity mailings

🖨️ Envelope Printing Tips

• Check envelope material — Windows, coloured interiors and textured stock may affect printing
• Add 3mm bleed — Print to bleed for a professional finish
• Use window envelopes — Show the address through a transparent panel for speed
• Weight matters — 100-120gsm is standard; premium envelopes are 120-150gsm
• Check Royal Mail sizing — DL is a standard letter size (fits a first-class stamp)

🇬🇧 UK Envelope Sizes

In the UK, the most commonly stocked envelope sizes are:

• DL — Business letters, invoices, statements
• C4 — Unfolded A4 documents
• C5 — Greeting cards, invitations
• C6 — Postcards, thank-you notes
• B4 — Larger documents

🔗 Related Pages

• C-Series Hub
• DL to C4
• C4 to C5
• A-Series Guide
• Envelope Size Calculator