C6 envelopes are among the most commonly used envelope sizes for everyday post, particularly in the UK and Europe. Whether you're sending out invitations, cards or folded documents, this compact envelope offers the right fit without adding bulk. It's a practical choice for both personal and business use.
The C6 size falls under the international ISO envelope standard, meaning it’s specifically designed to work with A-size paper formats — making it easy to pair with standard printing sizes like A4, A5 and A6.
Standard C6 envelope size in centimetres and inches
The exact dimensions of a C6 envelope are 11.4 cm x 16.2 cm, which is roughly 4.5 inches x 6.4 inches. This size is designed to fit the following:
- A6 paper (10.5 x 14.8 cm) unfolded
- A5 paper (14.8 x 21 cm) folded once
- A4 paper (21 x 29.7 cm) folded twice
These dimensions make it an ideal envelope for sending greeting cards, RSVP cards, invitations and quarter-folded documents. The proportions are compact, easy to handle, and still large enough to make an impression.
What fits into a C6 envelope
The C6 envelope is best suited for A6-sized contents or any standard sheet of paper folded to A6. Typical contents include:
- Personal or business greeting cards
- Event invitations or RSVP cards
- Flyers, leaflets, or promotional inserts
- Folded A5 or A4 letters for a smaller, neat format
It’s especially popular for personal stationery, thank-you cards and compact direct mail pieces where presentation matters and space is limited.
C6 envelope postage and usage tips
As long as the thickness stays under 5 mm and the weight remains below 100g, a C6 envelope qualifies for standard letter postage with Royal Mail. This makes it a cost-effective option for mass mailings or casual correspondence.
The envelope’s size also makes it ideal for use with higher quality materials — such as textured card, metallic finishes or custom-printed inserts. You can also find C6 envelopes with protective or decorative features, including board-backed options or coloured paper for branding or occasions.
Summary
The dimensions of a C6 envelope are 11.4 cm x 16.2 cm, or 4.5 inches x 6.4 inches. It fits A6 paper perfectly and accommodates A4 or A5 sheets when folded. Compact and efficient, C6 envelopes are a practical, professional choice for cards, invitations and letters sent with standard postage.