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Confused about which envelope you need to stay within the Royal Mail postage restrictions? Perhaps you are planning a large scale mailing and want to keep postage costs down by staying under the letter / parcel limits, or working out a cost analysis for sending your products through the post. It can be tricky to work out which envelope sizes are letters and which are parcel rates.

Our handy envelope postage guide below will help you work out the postage costs for different envelope sizes. We’ve answered your ‘most asked questions’ about envelopes and postage costs below too!

Letter & Parcel Size Guide for Royal Mail Postage

Max Dimensions

Max Thickness

Max Weight

Example of Contents

Envelope Suggestions


 240mm x 165mm



 Greeting card, postcards, letters

 C6, DL & C5 Envelopes


 353mm x 250mm



 A4 documents, magazines, CD's/DVD's

 C4 Envelopes


 450mm x 350mm



 Clothing items, speakers, desk lamp

 Padded Envelopes, Book Wraps


 610mm x 460mm


 Gifts, shoes, heavy/bulky items

 Large Book Wraps, Boxes

Suggested Envelope Sizes for Royal Mail Postage Rates




C5 Envelopes for Letter rate C4 Envelopes for Large Letter rate Envelopes for parcels

C5 Envelope Size @ 162mm x 229mm

C4 Envelope Size @ 229mm x 324mm

Padded Envelopes or Book Wraps


FAQ'S About Envelopes & Postage Rates

What is the largest size envelope that can be sent as "letter"?

The maximum size dimensions for a letter to be mailed as Royal Mail 'letter' is 240mm x 165mm. The largest standard envelope size to fit within this range is a C5, which measures 229mm x 162mm, giving a few mm extra allowance to fit nicely under the limit. C5 envelopes are designed to fit A5 papers/cards or A4 folded in half. Weight of the envelope filled with contents must be less than 100g and thickness must be less than 5mm. View our range of C5 envelopes

What is the largest envelope size for standard post?

Many people refer to the Royal Mail 'letter' format as standard post. The maximum envelope size you can use for standard post is 240mm x 165mm. The largest standard envelope size for standard post would be a C5, with dimensions of 229mm x 162mm. The weight must be less than 100g when filled and sealed and the depth/thickness must be less than 5mm. Click here to see our range of C5 Envelopes . 

What is the largest size envelope that can be sent as "Large Letter"?

The maximum size dimensions for a letter to be mailed as Royal Mail large letter is 353mm x 250mm. The largest standard envelope size to fit within this range is a C4, which measures 324mm x 229mm, fitting nicely under the size limits. C4 envelopes are traditionally designed to fit A4 papers / documents without having to fold the papers. Take a look at our range of C4 envelopes here. Don't forget that the weight of the envelope filled with contents must be less than 750g and the thickness must be less than 25mm.

If you are looking to get the maximum size possible within the large letter range we have a few specially sized envelopes with dimensions 249mm x 352mm - just 1mm under the height and width restrictions! 

Large Letter Envelopes (249mm x 352mm)

How much does standard letter postage cost?

The current Royal Mail standard letter postage costs 70p for 1st Class and 61p for 2nd Class.

How much does Large Letter postage cost?

If your letter falls within the large letter size and weight range, the cost of postage can still vary dependent on the actual weight. See the cart below for current Royal Mail Large Letter prices:

Weight 1st Class 2nd Class
up to 100g £1.06 83p
 101g to 250g £1.50 £1.32
251g to 500g £1.97 £1.72
501g to 750g £2.72 £2.33

*prices above checked on 31st October 2019

What is the maximum size envelope for a First Class stamp? 

The maximum dimensions of an envelope you can send with a standard first class stamp is 240mm x 165mm. Larger than this and postage rate will increase to large letter or parcel rate. See our Royal Mail Letter & Parcel Size Guide above for more info.

Which size envelopes require extra postage?

To be eligible for standard mailing rate the envelope must be less than 240mm x 165mm and weigh less than 100g and be less than 5mm thick. Mail over this size or weight will require extra postage. Scroll up to see the costs and weight ranges above standard rate, or head to the Royal Mail website for the most up to date information.

What is the cheapest UK postage rate and which envelope can I use?

The cheapest rate would be a standard 2nd Class postage. Your envelope will need to be less than 240mm x 165mm and weigh less than 100g, plus the thickness under 5mm, to qualify for this postage rate, so you could use a C6DL or C5 envelope to fit within this size range. The cost of a standard 2nd class stamp at letter rate is 61p. 

Information correct as of 31st October 2019. Please use this page as a guideline. We always recommend measuring/weighing your postage and checking the Royal Mail website for the most up to date rates. We cannot be held responsible for any costs incurred based on the above information.