C5 mailing boxes

C5 mailing boxes are shallow postal boxes built around the C5 paper size, which takes an A5 sheet or an A4 sheet folded once. Their slim profile protects flat contents far better than an envelope while staying shallow enough for large letter post when the finished pack meets the size and weight limits. Custom Postal Boxes supplies C5 mailing boxes made to order for UK sellers who post documents and slim goods every day.

The appeal is practical. A rigid shallow box stops corners folding in transit, presents paperwork and products neatly on arrival, and stores flat on a shelf until the moment it is needed at the packing bench.

Why C5 mailing boxes suit large letter post

Royal Mail large letter post allows a finished item up to 353 mm long, 250 mm wide and 25 mm thick, weighing no more than 750 g. A well made C5 mailing box keeps its contents inside those boundaries with room to spare, which is the whole point of the format. You can confirm the current bands on the Royal Mail service and size information page before you settle a design. Staying within large letter limits avoids paying parcel rates for a flat item, and the saving repeats across every single order you send.

What fits inside a C5 box

Typical contents include A5 catalogues and brochures, certificates and welcome packs, greeting card bundles, slim paperback books, art prints, phone cases and flat accessories such as cables or straps. If the contents are close to A5 in footprint and no more than about 15 mm thick once wrapped, a C5 box is usually the natural home for them. Anything thicker or heavier belongs in a small parcel box instead, and forcing it into a shallow box only risks a sprung lid in the sorting system.

Checking the finished pack against the limit

Measure the outside of the packed box, not the empty box, and weigh it with the label and any paperwork included. Tape, a return card or a thick insert can add the millimetres that push a pack over the 25 mm thickness limit. A simple template gauge at the packing bench, cut to the large letter slot size, turns the check into a habit that takes seconds.

Board choices for C5 mailing boxes

Single wall corrugated board is the usual choice for this format, because the fluting cushions knocks while keeping the pack light and shallow. Kraft board gives a natural look that suits stationery brands and handmade goods, while white board takes colour print sharply and looks at home in corporate mailings. Our large letter postal boxes and A5 postal boxes sit alongside this format for sellers who post the same contents in more than one size. Board should be chosen by the journey the pack will take, with longer routes and heavier contents calling for the sturdier grade.

Closures that keep a shallow box shut

A shallow box has little depth for its lid to grip, so the closure matters. Self seal strips give a fast clean close at a busy bench and open neatly at the other end. Tape over the centre seam suits heavier contents, while a tear strip adds easy opening without scissors. Whichever closure you choose, the lid should sit flat and not bow, because a bowed lid can catch in automated sorting and can push the pack over the thickness limit.

Buying C5 mailing boxes for regular despatch

Consistency is what regular sellers are really buying. The same internal size every time means packing staff can work quickly, postage stays predictable and returns arrive in reusable condition more often. If some of your range outgrows this format, our C4 postal boxes cover the next paper size up while keeping the same shallow logic. Pricing depends on size, material and quantity, so request a quote through our contact page with your internal measurements and an idea of monthly volume, and use the free design support if you want the boxes printed rather than plain.

FAQs about C5 mailing boxes

What size is a C5 mailing box?

A C5 mailing box is built around the C5 paper size of 229 mm by 162 mm, so internal dimensions are usually a little smaller than that area once the board thickness is allowed for, with a shallow depth of around 20 mm. Always confirm the internal size against your largest item before ordering, because board and closure design vary between styles.

Will a C5 mailing box go as a large letter?

A C5 mailing box can go as a large letter when the finished packed item is no larger than 353 mm by 250 mm by 25 mm and weighs no more than 750 g. Measure and weigh the complete pack including contents, tape and label, because the packed result is what Royal Mail measures, not the empty box on its own.

What can I send in C5 mailing boxes?

You can send A5 documents, catalogues, certificates, greeting cards, slim books, art prints and flat accessories in C5 mailing boxes. The format suits anything wide and flat rather than bulky, and contents should sit still inside the box, with a sheet of board or tissue used to stop light items sliding against the lid.

Can C5 mailing boxes be printed with branding?

Yes. C5 mailing boxes can be printed with a logo, return address details and brand artwork, which turns a functional postal pack into a recognisable delivery. Outside print is read first on arrival, while a short message inside the lid is a simple way to thank the customer or explain returns without adding a separate card.