C5 is one of the most widely used envelope sizes in the UK and Europe. It strikes a practical balance between being compact and large enough to hold common paper sizes without folding awkwardly. If you're posting documents, invitations or leaflets, understanding C5 sizing helps ensure your contents fit perfectly and your postage stays within standard limits.
The C series of envelopes is based on the ISO 216 standard, which is the international system for paper sizing. This means C5 is designed to work seamlessly with A-size paper formats, especially A4 and A5.
Standard C5 envelope size in centimetres and inches
A C5 envelope measures 16.2 x 22.9 cm, or roughly 6.4 x 9 inches. This size is ideal for holding:
- An A5 sheet (14.8 x 21 cm) unfolded
- An A4 sheet (21 x 29.7 cm) folded once in half
This makes C5 especially versatile for mailing letters, forms, brochures and flyers. It's often used in business settings as well as for invitations, marketing materials and everyday correspondence.
The shape of a C5 envelope is rectangular, and it’s commonly available with different flap styles, including wallet flap, pocket flap and peel-and-seal closures, depending on use and preference.
What fits in a C5 envelope
One of the reasons the C5 size is so popular is because of how well it fits standard paper sizes without the need for trimming or excessive folding. Here’s what typically fits:
- A5 paper slides in perfectly without folding
- A4 paper fits neatly when folded once along the longest edge
- Booklets or thin brochures up to A5 size
- Certificates and formal documents printed on A5 or folded A4
The envelope has enough internal space to allow for a few sheets of paper or a lightweight booklet without bulging or requiring oversized postage.
C5 envelope postage and mailing considerations
C5 envelopes generally fall within Royal Mail’s standard letter size, as long as the thickness is under 5 mm and the weight is under 100g. This keeps postage costs economical. However, if you include more pages or thicker materials inside, it may be classed as a large letter, which costs more to post.
For official documents, using board-backed C5 envelopes helps prevent bending. For invitations or mailers, coloured or textured C5 envelopes are often chosen to add a touch of quality and personality.
Summary
A C5 envelope measures 16.2 x 22.9 cm, and is ideal for sending A5 paper or folded A4 sheets. It’s one of the most commonly used envelope sizes in the UK, combining practicality with a clean, professional look. Whether for business or personal use, C5 offers a smart fit and efficient postage for a wide range of content.