Skip to content

Letter Size Guide

Use our helpful letter size guide to help you decide if your postable letter is a small letter or a large letter so you always pay the correct postage. We also have our full Sending a Letter guide with more information about postage costs and sending options as well as our Envelope Guide which explains all the types and sizes of envelopes. 

What postage stamp do I need for my Letter?

Postable letters for Royal Mail come in two sizes, a small Letter (just called a ‘Letter’) and a Large Letter. There are two types of stamps your can use for a letter weighing less than 100 grams – a Large Letter stamp and a ‘normal’ Letter stamp. Both come in 1st and 2nd class options. Please see our Stamp purchasing section of our Letter guide for more information on purchasing stamps.

What size is a Letter and a Large Letter?

If you just want to put a single stamp on a Letter it has to weigh less than 100 grams and be the following size:

Letter / Small Letter

  • Max length: 24cm
  • Max width: 16.5cm
  • Max thickness: 0.5cm

Large Letter

  • Max length: 35.3cm
  • Max width: 25cm
  • Max thickness: 2.5cm

If you have a Large Letter and it weigh’s more than 100 grams but less than 750 grams, it is still postable but needs a more bespoke price or combination of stamps. Please see the Royal Mail Postage finder to find out the postage cost for a Large Letter up to 750 grams.

See our envelope guide for more information on purchasing envelopes for letters.

Royal Mail Sizing Guide

Amazon sell Perspex Acrylic Plastic Royal Mail sizing guides which can be helpful if you do a lot of letter posting and want to get it right everytime!