C6 envelopes are a popular choice for everyday post, especially when you need something compact but still functional. Whether you’re sending invitations, greetings cards or folded documents, the C6 envelope is a reliable format that works well with standard paper sizes. It's one of the most commonly used envelopes across personal, business and creative settings.
Part of the ISO envelope sizing system, the C6 envelope is specifically designed to fit A-size paper formats without the need for trimming or complicated folding. It’s widely available and easy to match with card and paper stock, making it a go-to option for both personal correspondence and commercial mailings.
Standard C6 envelope size in centimetres and inches
A standard C6 envelope measures 11.4 x 16.2 cm, which is approximately 4.5 x 6.4 inches. These dimensions are ideal for holding:
- An A6 sheet (10.5 x 14.8 cm) flat
- An A4 sheet folded twice into quarters (A6)
- An A5 sheet folded once in half
This size is perfect for posting greeting cards, RSVP cards, small flyers or invitations. It offers a neat and tidy look, and its proportions are well-suited to mail that’s meant to be more personal or visually appealing.
What fits into a C6 envelope
C6 envelopes are designed to fit A6 paper, so they’re commonly paired with items like:
- Greeting cards and thank you notes
- Wedding RSVPs or event invites
- Flyers and handouts
- Folded A5 or A4 documents
If you fold an A4 sheet in half, then in half again, you’ll have a quarter-sized piece of paper — and that’s exactly what fits into a C6 envelope. It's compact without feeling cramped, and perfect when space, postage costs or design presentation matter.
C6 envelope postage and mailing info
In most cases, a C6 envelope meets the criteria for standard letter postage under Royal Mail. As long as the thickness doesn’t exceed 5 mm, and the weight is under 100g, you can post it at the standard letter rate. This makes C6 a cost-efficient choice for both personal and professional use.
There are also board-backed or padded C6 envelopes available for sending small, delicate items that need extra protection. Just bear in mind that added thickness or bulk may push the envelope into the next postage category.
Summary
A C6 envelope measures 11.4 x 16.2 cm, or 4.5 x 6.4 inches. It’s ideal for A6-sized cards and documents, or folded A4 sheets. Compact, neat, and postal-friendly, it’s one of the most versatile envelope sizes used across the UK for invitations, cards and light letters.