Royal Mail Letterbox Size Rules Explained for UK Businesses

The size rules

Maximum dimensions: 353mm long x 250mm wide x 25mm thick

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Maximum weight: 750g across four weight bands

Exceeding any dimension reclassifies as small parcel at significantly higher postage

Thickness is the critical measurement for letterbox packaging compliance

Quick answer

Royal Mail large letter: maximum 353mm x 250mm x 25mm, up to 750g. Exceeding any of these dimensions by even 1mm means paying small parcel rates (from £3.35 vs 93p for large letter). Thickness is the measurement most likely to cause problems with letterbox boxes. Always measure the packed box, not just the empty box, as the product inside adds to the overall thickness.

Exact dimensions

Length and width together must not exceed these limits.

This is where most letterbox packaging compliance failures occur.

Product, padding, inserts, and any bulging all count toward total thickness.

Royal Mail uses sizing templates. Even 1mm over results in small parcel classification.

Weight bands

Large letter pricing increases with weight: 0-100g is cheapest, then 101-250g, 251-500g, and 501-750g. Above 750g, even if dimensions fit, the item becomes a small parcel. Keep packaging lightweight to stay in the cheapest weight band for your product.

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0-100g: 93p 2nd class / £1.19 1st class (cheapest band)

101-250g: slightly more, good for most small products with packaging

501-750g: highest large letter rate, still much cheaper than small parcel

Optimise costs: shipping savings guide or sizing guide.

Compliance tips

Stay within limits0 / 4 checked

Measure packed boxes with callipers, not just a ruler

Account for product, padding, inserts, and any box wall flex

Test-post samples to confirm Royal Mail accepts them as large letter

Weigh packed boxes to verify you are in the correct weight band

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Frequently asked questions

Maximum 353mm x 250mm x 25mm thick, up to 750g.

The item is reclassified as a small parcel, increasing postage from 93p to £3.35+.

Packed. The total thickness including product, padding, and inserts must stay under 25mm.

0-100g, 101-250g, 251-500g, 501-750g. Each band has a different postage price.

No, Royal Mail uses precise sizing templates. Any dimension over the limit triggers reclassification.