How to Choose the Right Letterbox Box Size for Your Products

Size limits

Royal Mail large letter max: 353mm x 250mm x 25mm thick, up to 750g

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Standard UK letterbox opening: approximately 250mm wide x 38mm tall

Your box must pass both tests: Royal Mail classification AND physical letterbox fit

Quick answer

Measure your product, add 2-3mm for the box walls, and ensure total thickness stays under 25mm for Royal Mail and under 38mm for letterbox fit. Common letterbox box sizes include 200x150x20mm, 250x180x20mm, and 340x240x20mm. Always test-pack and test-post a sample. The most popular size for small e-commerce items is 250x180x20mm, which fits most products while staying comfortably within all limits.

How to measure

Measuring steps0 / 5 checked

Measure product length, width, and thickness at widest points

Add 2-3mm per side for box wall thickness

Add 1-2mm for any internal padding or tissue paper

Verify total thickness is under 25mm (Royal Mail) and under 38mm (letterbox)

Check total weight with product and packaging stays under 750g

Common sizes

Jewellery, earrings, phone cases, small accessories.

Cosmetics, larger accessories, thin electronics, stationery.

A4 documents, books, clothing items, larger flat products.

If standard sizes do not fit your product, custom dimensions can be manufactured.

Postage rules: Royal Mail letterbox rules or shipping cost savings.

Sizing tips

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Snug is better than loose - a tight fit prevents movement and feels premium

Account for packaging - tissue paper, cards, and inserts add to thickness

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Never assume it fits - always test through an actual letterbox before ordering bulk

From ProcuraPack

Letterbox boxes in every size

Stock and custom letterbox box sizes from ProcuraPack.

Browse letterbox boxes

Frequently asked questions

353mm x 250mm x 25mm thick for Royal Mail large letter, but also must fit through a 250mm x 38mm letterbox opening.

250 x 180 x 20mm suits most small e-commerce items.

Yes, add 1-2mm for tissue paper or thin padding inside.

Yes, most suppliers offer custom dimensions for letterbox boxes.

Pack a sample with your product and physically test it through a standard UK letterbox.