large letter mailing boxes

large letter mailing boxes are flat cardboard boxes sized so that a packed item stays within the Royal Mail large letter limits of 353mm in length, 250mm in width, 25mm in depth and 750g in weight. Custom Postal Boxes, a project of Vertex Packaging based in Wolverhampton, manufactures printed large letter mailing boxes in house for UK businesses that post flat products every day. A correct fit protects the contents, keeps postage in the large letter band and presents your brand neatly.

What large letter mailing boxes do best

The product must lie flat, the board must stay rigid in transit and the finished parcel must still measure no more than 25mm deep at its thickest point once tape is added. A purpose made box does that work far better than a loose envelope, because the walls hold their shape and the contents cannot slide into a bulky fold.

Typical contents include paperback books, magazines, art prints, A4 documents, greetings cards, slim clothing and flat packed craft kits. If an item is flat, light and worth protecting from bending, this is the first format to consider. Our large letter postal boxes show the core product this page supports, while letterbox postal boxes suit items that need a little more depth while still passing through a standard letterbox.

Why a rigid box beats a soft mailer for flat goods

Paper and printed matter fail in predictable ways. Corners dog ear, covers crease and glossy surfaces scuff when parcels rub in a mail sack. A shallow board box spreads pressure across the whole face of the item instead of concentrating it at one fold, so a book or print arrives flat rather than curled, and is far less likely to come back as a return.

Checking thickness before you order

Measure the product at its thickest point, then add the wrap, any insert and the tape. Many parcels leave the large letter band on depth alone, even when length and width look comfortable. Pack one item exactly as you would post it and measure the finished parcel rather than the empty box. If the packed depth sits close to 25mm, choose the next format up.

Sizes, weight and Royal Mail pricing in proportion

Royal Mail prices letters by format as well as weight, a system often called pricing in proportion. The large letter ceiling is 353mm long, 250mm wide and 25mm thick, with a maximum weight of 750g for standard services. Some online tracked services allow a higher weight for the same footprint, so check the rules of the service you buy. The Royal Mail Tracked 24 service publishes its own size and weight table for large letters bought online.

Work from internal dimensions when you choose a size, because board thickness and folds take up room. Leave a small margin on length and width for easy packing, but guard the depth budget closely. Browsing boxes by size in our postal boxes by size range helps you compare footprints before you commit to a bespoke size.

Materials that keep a shallow box strong

Most large letter boxes use single wall corrugated cardboard with a fine flute, or solid board where a smoother print surface matters more. Fine flute gives good rigidity for very little depth. Kraft board offers a natural look, while white board takes colour print cleanly.

Print, packing and ordering advice

A flat box gives you two large print faces: prime branding space. Many sellers print a logo on the lid, then use the inside face for care guidance or a thank you note. Keep artwork clear of the folds and sealing edge. Free design support at Custom Postal Boxes checks position and legibility before production begins.

Place the item centrally, add a stiffener for flexible contents and tape along the seam rather than around the whole parcel, which adds depth. Businesses also have duties around packaging they place on the market, and the GOV.UK guidance on packaging producer responsibilities explains who is affected. When you are ready to compare sizes and print options, request a quote through our contact page and tell us the product dimensions and quantity. Pricing depends on size, material and quantity, minimum order quantities are low and UK delivery is free.

FAQs about large letter mailing boxes

These answers cover the questions buyers ask most often.

What is the maximum size for a Royal Mail large letter?

The maximum size is 353mm in length, 250mm in width and 25mm in depth, with a maximum weight of 750g for standard services. Measurements apply to the finished parcel, so measure after packing.

What can I send in large letter mailing boxes?

Popular contents include paperback books, magazines, A4 documents, art prints, calendars, greetings cards, slim folded clothing and flat craft kits. Anything flat and light enough to stay under the weight limit is a natural fit.

Will a large letter box fit through a letterbox?

Most will, because the footprint and depth suit a standard UK letterbox. Fit depends on the individual letterbox, so keeping depth at or below 25mm gives the best chance of delivery first time.

Can large letter mailing boxes be printed with my branding?

Yes. Custom Postal Boxes prints logos, brand colours and messages inside and outside the box. Free design support is included, so artwork is checked for position and clarity before production.

What happens if my parcel is slightly too thick?

A parcel over 25mm in depth or 750g in weight moves into the small parcel format, which costs more to post. Choosing a larger format from the start is usually cheaper than facing surcharges.