C4 boxes

C4 boxes are postal boxes sized around the C4 paper format, which measures 324 mm by 229 mm and takes an A4 sheet without folding. UK businesses use C4 boxes to post documents, certificates, artwork, calendars and slim catalogues with more protection than an envelope gives, while keeping the parcel flat enough for efficient posting.

What C4 boxes are designed to hold

The C4 format exists so that A4 paper fits inside unfolded. That matters for certificates, contracts, exam papers, photographs and printed proofs that lose value if they are creased. A box adds rigid sides around that paper, so the contents are far less likely to bend in a post bag or under other parcels. Most C4 boxes are shallow, which keeps the contents flat and stops sheets sliding around and curling at the corners.

C4, C5 and A4 explained in plain terms

Paper and envelope sizes follow a simple pattern. An A4 sheet measures 210 mm by 297 mm. A C4 envelope or box is slightly larger than A4 so the sheet slides in flat. A C5 format holds A5 paper, or A4 folded in half, and a DL format holds A4 folded into thirds. Choosing the C size that matches your paper avoids both a tight squeeze and wasted space.

Why internal measurements matter most

Always check the internal measurements of a box rather than the external ones. Board thickness takes up a few millimetres on every side, so a box that looks right from the outside can be too small inside. Measure your document or product, allow a small amount of room for any inner sleeve or wrapping, and then match that figure to the internal size.

C4 boxes and Royal Mail sizing

Royal Mail prices letters and parcels by size as well as weight, a system often called pricing in proportion. A large letter can measure up to 353 mm by 250 mm by 25 mm and weigh up to 750 g. A shallow C4 box can fall within those large letter limits when it is packed carefully and kept under the thickness and weight limits, which can cost less than sending the same item as a small parcel. Anything thicker or heavier moves into the small parcel band, which allows up to 450 mm by 350 mm by 160 mm and 2 kg. The Royal Mail small parcel guidance sets out the current bands, and our C4 postal boxes are made with these proportions in mind.

Keeping a C4 box within large letter limits

Thickness is the measurement that catches people out. Tape, labels and a bulging document wallet all add depth, so measure the finished parcel at its thickest point rather than the empty box. If your contents are naturally deeper, look at large letter postal boxes for other flat formats or letterbox postal boxes for boxes designed to pass through a standard letterbox. Custom Postal Boxes manufactures in house in Wolverhampton, offers low minimum order quantities and free design support, and delivers free within the UK.

Choosing and packing C4 boxes well

Corrugated cardboard gives the best crush protection for posted documents, while thinner solid board suits hand delivery and retail presentation. Printed C4 boxes can carry your logo, handling messages and brand colours, turning a routine document mailing into a professional touch. When packing, place documents in a sleeve or wallet first, centre them in the box, and seal all openings with tape so nothing can work loose. Request a quote through our contact page with your internal size, quantity and print requirements, and pricing will be worked out from those details.

FAQs about C4 boxes

What size is a C4 box?

A C4 box is built around the C4 format of 324 mm by 229 mm, which holds an A4 sheet unfolded. Postal versions are usually shallow, often around 20 mm to 25 mm deep, although exact internal measurements vary, so always check the internal size before ordering.

Do C4 boxes fit through a letterbox?

A shallow C4 box can pass through many letterboxes because it is flat, but letterbox apertures vary between properties. If letterbox delivery is essential, measure the aperture you are targeting and choose a box with some room to spare on every dimension.

Can C4 boxes be sent as a large letter?

A C4 box can qualify as a Royal Mail large letter if the finished parcel stays within 353 mm by 250 mm by 25 mm and under 750 g. Measure the packed parcel, including tape and labels, because extra thickness moves it into the small parcel band.

What can I post in C4 boxes?

C4 boxes suit A4 documents, certificates, contracts, artwork, photographs, calendars, magazines and slim catalogues. The rigid board protects flat items from bending and creasing far better than an envelope, which is why businesses use them for important paperwork and printed material.