C4 envelopes are a popular choice when you need to send documents without folding them. They are part of the international ISO paper and envelope sizing system and are designed specifically to fit A4 paper. Whether used for formal letters, contracts, certificates or marketing materials, the C4 envelope offers a clean and professional way to present documents as they were intended to be seen.
This envelope size is commonly used across offices, schools and businesses that require official paperwork to remain uncreased during postage. Unlike smaller formats that require folding, the C4 size preserves the integrity of the sheet and provides a neat, flat presentation.
Standard C4 envelope size in centimetres and inches
A standard C4 envelope measures 32.4 by 22.9 centimetres, which is approximately 12.8 by 9 inches. This is the exact size needed to hold a sheet of A4 paper, which measures 29.7 by 21 centimetres, without folding. The envelope allows a slight margin around the paper to ensure it slides in and out smoothly, even if a few sheets are included.
Because of its size, a C4 envelope is often classed as a large letter by postal services, which means it may incur higher postage than smaller envelopes. However, the benefit is that the contents remain intact and neatly presented on arrival.
What fits into a C4 envelope
The C4 envelope is designed to hold A4 paper in its original, flat form. This makes it ideal for sending documents such as legal papers, contracts, reports, printed forms and certificates. It is also frequently used for presentation folders, magazines, promotional brochures and slim product catalogues.
When extra protection is needed, C4 envelopes are often available in board-backed or padded versions to keep the contents from being damaged during transit. These are particularly useful when mailing sensitive or valuable items.
C4 envelope usage and postage
Due to their size, C4 envelopes usually fall under the large letter category for Royal Mail, provided the thickness does not exceed 25 millimetres and the weight stays within the set limits. As a result, it’s important to consider both the weight and bulk of your contents when using this envelope size, especially if you're mailing multiple pages or enclosed booklets.
Despite the higher postage cost compared to standard letter sizes, the C4 format remains a preferred option when presentation matters. It gives a polished, professional look to your correspondence and ensures that nothing arrives bent, crumpled or misaligned.
Summary
A C4 envelope measures 32.4 by 22.9 centimetres and is designed to hold an A4 sheet without folding. It is a widely used envelope size for formal documents and presentation materials, offering a clean, flat format that protects the appearance of what’s inside. Although it may cost more to post than smaller envelopes, the result is a more professional and presentable delivery.
