Envelope Sizes UK: C4, C5, C6, DL & A4 Size Chart

UK envelope sizes follow the international ISO standard, so the name tells you what fits: a C4 envelope takes A4 paper flat, a C5 takes A5, and so on. This guide gives every common size in millimetres and inches, shows which paper fits which envelope, and explains how each size is treated by Royal Mail.

Envelope sizes chart

Sizes are the ISO 269 standard dimensions. C sizes are made to hold A-size paper, and B sizes are a little larger again, for bulky contents or for holding a C-size envelope.

What size envelope for A4?

It depends how you fold it:

• A4 flat: C4 (229 × 324 mm). Best for certificates, contracts and anything that must not be creased.
• A4 folded once: C5 (162 × 229 mm).
• A4 folded in thirds: DL (110 × 220 mm), the standard business letter envelope.
• A4 folded in quarters: C6 (114 × 162 mm), common for cards and invitations.
• A thick stack of A4: B4 (250 × 353 mm), which leaves room for the depth.

Paper sizes and matching envelopes

C5, C4, C6 and DL envelope sizes explained

C5 envelope size

162 × 229 mm (6.4 × 9 in). Holds A5 paper flat or A4 folded once. The most searched envelope size in the UK, used for statements, booklets and greeting cards.

C4 envelope size

229 × 324 mm (9 × 12.8 in). Holds A4 flat, so it is the standard choice for documents that must stay uncreased.

C6 envelope size

114 × 162 mm (4.5 × 6.4 in). Holds A6 flat or A4 folded in quarters. Common for greeting cards and postcards.

DL envelope size

110 × 220 mm (4.3 × 8.7 in). Holds A4 folded in thirds, which makes it the usual envelope for business letters and invoices.

Envelope sizes and Royal Mail postage

Royal Mail prices by format. A Letter can be up to 24 × 16.5 cm, 0.5 cm thick and 100 g, so C5, C6, DL and C7 envelopes are Letter size when thin and light enough. A C4 envelope is too large for Letter and goes as a Large Letter (up to 35.3 × 25 cm, 2.5 cm thick, 750 g). Our Royal Mail large letter size guide has the full limits.

When a paper envelope is not enough

Paper envelopes protect documents, not objects. For anything that could bend, scratch or break, use a padded or plastic mailer:

• Padded mailing envelopes (bubble-lined), including A5-size 150 × 215 mm and A4-size 240 × 335 mm, for small items and documents that need protection.
• Jiffy padded bags for books and heavier or hard-edged items.
• Grey mailing bags for clothing and soft goods: waterproof, light and opaque.
• Bubble bags as a protective inner layer for small items.

Frequently asked questions

What size is a C4 envelope?

229 × 324 mm (9 × 12.8 in). It holds A4 paper flat.

What size is a C5 envelope?

162 × 229 mm (6.4 × 9 in). It holds A5 flat or A4 folded once.

What size is a DL envelope?

110 × 220 mm (4.3 × 8.7 in). It holds A4 folded in thirds.

What size envelope do I need for A4?

C4 for A4 flat, C5 for A4 folded once, DL for A4 folded in thirds, and C6 for A4 folded in quarters.

What size envelope do I need for A5?

C5 for A5 flat, or C6 for A5 folded once.