Glossary
Royal Mail Large Letter
Definition
A Royal Mail PiP format for flat items up to 353 x 250 x 25mm and 750g. Staying within these limits keeps postage cheaper than Small Parcel rates.
Also known as: Large Letter format, LL
Large Letter is one of the Royal Mail Pricing in Proportion bands and is the sweet spot for many small e-commerce items. To qualify, an item must be no larger than 353mm long, 250mm wide and 25mm thick, and weigh no more than 750g. Anything exceeding even one of these limits is priced as a Small Parcel or above.
The 25mm thickness limit is usually the hardest to meet, because packed products, padding and tape all add depth. This is why slim die-cut postal boxes, board-backed envelopes and padded mailers are designed to hold contents flat and firm within the Large Letter profile.
Getting a product to post as a Large Letter rather than a Small Parcel can meaningfully cut per-order postage, which adds up across high volumes. Retailers often redesign packaging specifically to stay inside the Large Letter envelope, then test finished items with a PiP slot gauge before committing to a run.
What to check
- The finished, packed item must be within 353 x 250 x 25mm and 750g.
- Thickness is measured at the deepest point, including tape and seams.
- Choose packaging that compresses or holds contents flat to stay under 25mm.
- Test with a Large Letter slot gauge, not just a ruler.
- Verify current limits directly with Royal Mail before printing or ordering.
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Royal Mail Large Letter — FAQs
What is the maximum size for a Royal Mail Large Letter?
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