C5 envelopes are one of the most commonly used sizes for mailing letters and documents in the UK. They offer just the right amount of space for standard paper sizes without being too bulky or requiring higher postage. If you’re printing materials, preparing documents, or planning a direct mail campaign, knowing the exact dimensions of a C5 envelope will help you get the fit and presentation right.
The C5 size is part of the ISO 216 standard, which aligns perfectly with the A-series paper sizes used throughout the UK and Europe. This means it’s designed to match up with A4 and A5 paper formats without the need for trimming or folding into awkward shapes.
Standard C5 envelope size in centimetres and inches
The standard dimensions of a C5 envelope are 16.2 x 22.9 cm, or 6.4 x 9 inches. This size is designed to hold:
- An A5 sheet (14.8 x 21 cm) flat
- An A4 sheet (21 x 29.7 cm) folded once across its width
The envelope itself has a slim rectangular profile and is often available in various finishes — from basic white office envelopes to premium textured or coloured options used for invitations and business communications.
What fits into a C5 envelope
Because of its carefully designed dimensions, the C5 envelope is ideal for a range of uses. It fits:
- A5 paper perfectly without folding
- A4 paper folded once across its longest side
- Certificates, event invites, forms and letters
- Small brochures or thin promotional materials
C5 envelopes can typically hold a few sheets of standard paper without exceeding mailing limits, making them practical and cost-effective.
C5 envelope postage and usage
When sealed, a C5 envelope will usually fall under Royal Mail’s standard letter size, as long as the contents stay under 5 mm in thickness and 100g in weight. That means you can mail standard letters, folded documents and slim booklets without paying for a large letter or parcel rate.
Heavier contents or thicker inserts might push it into the large letter category, so it’s always worth weighing before posting. You can also get board-backed C5 envelopes if your contents need extra protection during shipping.
C5 is a go-to size in offices, schools, and mailing campaigns because of its neat presentation, predictable fit and efficient use of space.
Summary
The dimensions of a C5 envelope are 16.2 x 22.9 cm or 6.4 x 9 inches. It's made to fit A5 paper without folding, or an A4 sheet folded in half. This envelope size is widely used in the UK for sending letters, documents and small brochures while keeping postage costs low and presentation tidy.