Envelope sizes in the UK follow the ISO 216 system, so the most common formats are easy to remember once you know the pattern. The four you will use most are C4 (229 x 324mm) for an unfolded A4 sheet, C5 (162 x 229mm) for A4 folded once, C6 (114 x 162mm) for A4 folded twice, and DL (110 x 220mm) for A4 folded into thirds. Choosing the right size keeps your documents looking professional and, just as importantly, helps control your postage costs.
Below we explain each standard size, what it is designed to hold, and how the dimensions link to Royal Mail pricing so you can pick with confidence.
How UK envelope sizes work
The C series is part of the same ISO 216 standard that gives us A4 and A5 paper. The clever bit is how they relate: a C-size envelope is built to hold the matching A-size sheet without folding. So a C4 envelope fits a full A4 page, a C5 fits A5 (or A4 folded in half), and a C6 fits A6 (or A4 folded into quarters). The DL size sits slightly outside the pure C series but is the workhorse of British business post, sized to take an A4 letter folded neatly into three.
Getting the pairing right matters. An envelope that is too tight creases the contents, while one that is too large lets documents slide around and can push you into a more expensive postage band. If you are ordering custom printed envelopes, matching the size to your typical mailing is the single easiest way to keep everything tidy and cost-effective.
Standard envelope sizes at a glance
Here are the everyday formats and their dimensions in millimetres:
- DL - 110 x 220mm: The classic business envelope. Holds an A4 sheet folded into thirds and suits letters, invoices and compliment slips.
- C6 - 114 x 162mm: A compact envelope for A6 cards, greeting cards and short notes, or an A4 sheet folded twice.
- C5 - 162 x 229mm: Takes an A5 sheet flat or an A4 sheet folded once. Popular for booklets, brochures and multi-page letters you would rather not fold heavily.
- C4 - 229 x 324mm: Holds a full, unfolded A4 document. Ideal for contracts, certificates, reports and anything that must arrive flat.
Two hybrid sizes are also worth knowing. The C5/C6 and the C6/C5 give you options between the standard steps when your contents do not fit neatly into one band. For most businesses, though, DL, C5 and C4 cover the vast majority of everyday mailings.
Which envelope for which job?
As a quick rule of thumb: use DL for standard letters and statements, C5 for lightly folded documents or small booklets, and C4 when you need to send A4 flat and uncreased. If you post cards or invitations, a C6 keeps them protected without excess bulk. Adding a printed design to your envelopes also lifts each of these formats from plain stationery into a branded touchpoint.
How envelope size affects Royal Mail postage
Size is not just about fit. It directly determines which Royal Mail pricing format your mail falls into, and that decides the stamp price. The two formats most relevant to envelopes are Letter and Large Letter.
A Letter must be no larger than 240 x 165mm, up to 5mm thick, and weigh no more than 100g. A DL or C6 envelope carrying a few sheets will usually qualify as a Letter, the cheapest tier. A Large Letter can be up to 353 x 250mm, 25mm thick and 750g in weight, according to the current Royal Mail size and weight guide. C5 and C4 envelopes, and anything thicker or heavier, typically fall into this band.
The practical takeaway is that a C4 envelope will almost always cost more to post than a DL, so it pays to send documents flat only when they genuinely need to be. For a fuller breakdown of the price differences, see our guide on whether to use a Letter or Large Letter.
Weight and thickness matter too
Even a correctly sized envelope can be bumped into a pricier format if it is overfilled. A C5 stuffed past 5mm becomes a Large Letter, and a Large Letter over 25mm thick becomes a small parcel. If you regularly send bulkier items, a board-backed or padded envelope sized to stay within the Large Letter limits can protect contents while keeping postage predictable.
Choosing the right envelope for your business
When you settle on a house envelope, think about your most common enclosure first, then your brand. A consistent DL for correspondence and a C4 or C5 for documents covers most needs, and printing your logo, return address or a marketing message turns each mailing into free advertising. Ordering in the right size from the outset also reduces waste, since you avoid oversized envelopes that cost more to buy and to send.
Frequently asked questions
What is the most common envelope size in the UK?
The DL envelope, measuring 110 x 220mm, is the most widely used size in British business. It is designed to hold a standard A4 sheet folded into three, which makes it ideal for letters, invoices and statements.
What size envelope fits an A4 sheet without folding?
A C4 envelope, at 229 x 324mm, is designed to hold a full A4 document flat and unfolded. It is the best choice for certificates, contracts and reports that need to arrive without creases.
What is the difference between C5 and C4 envelopes?
A C5 envelope (162 x 229mm) holds an A5 sheet flat or an A4 sheet folded once, while a C4 envelope (229 x 324mm) holds a full A4 sheet unfolded. C4 is exactly double the size of C5, so C5 is effectively a C4 folded in half.
Do bigger envelopes cost more to post?
Usually, yes. Larger envelopes such as C5 and C4 tend to fall into Royal Mail's Large Letter format, which costs more than the standard Letter rate that smaller DL and C6 envelopes often qualify for. Weight and thickness also affect the final price.
What is the maximum size for a Royal Mail Large Letter?
The maximum Large Letter dimensions are 353mm long, 250mm wide and 25mm thick, with a weight limit of 750g. Anything larger, thicker or heavier is classed as a parcel.
Order custom printed envelopes in any size
Whether you need crisp DL envelopes for daily correspondence or C4 envelopes to send documents flat, choosing the right size keeps your post looking sharp and your postage under control. Browse the full range and add your own branding at Fast Printed Envelopes, where every size can be printed to match your business.