Standard Envelope Sizes UK

The UK’s standard envelope sizes follow the ISO 216 C series: C4 (229 x 324 mm), C5 (162 x 229 mm), C6 (114 x 162 mm), and C7 (81 x 114 mm), plus the DL envelope (110 x 220 mm), the size most commonly used for business letters. Each is built to hold A-series paper flat or folded, and Royal Mail’s postage bands, Letter, Large Letter, and Parcel, are set around these same dimensions, so choosing the right envelope size directly affects what you pay to post it.

What Counts as a “Standard” Envelope Size?

A standard envelope size in the UK isn’t an arbitrary convention; it’s defined by ISO 216, the same international standard that sets A4, A5, and A3 paper dimensions. The “C series” of envelopes (C0 through C10, plus DL) is calculated as the geometric mean between the A-series and B-series paper sizes of the same number. In practical terms, that means every C-size envelope is built to hold its matching A-size sheet with a small margin for the paper to move freely, whether flat or folded.

DL sits slightly outside the C series numbering but is defined in the same ISO standard. It’s the size most UK businesses reach for by default, because it’s built specifically to hold an A4 sheet folded into three, the standard fold for letters, invoices, and statements.

DL Envelopes (110 x 220 mm)

DL is the workhorse of UK business mail. It holds an A4 sheet folded into three equal panels, which is why it’s the default choice for letters, invoices, remittance advice, and billing statements. Its narrow profile also keeps it comfortably within Royal Mail’s letter format, provided the contents stay under 5 mm thick, which is useful to know if you’re stuffing more than a page or two inside.

C4 Envelopes (229 x 324 mm)

C4 is the largest envelope in everyday business use and the only one on this list built to take an A4 sheet completely unfolded. That makes it the right choice for contracts, certificates, tender documents, and any mailing where you don’t want the recipient to see fold creases. Larger A3 documents can also go in a C4 envelope if folded in half. Because of its size, a C4 mailing will usually fall into Royal Mail’s Large Letter format rather than the cheaper Letter rate, so it’s worth weighing whether the presentation benefit is worth the postage difference for high-volume sends.

C5 Envelopes (162 x 229 mm)

C5 takes an A4 sheet folded once in half or an A5 sheet completely flat. It strikes a middle ground: less bulky than C4, but roomier than DL, which makes it a common choice for welcome packs, onboarding kits, and larger greeting cards where a single fold is acceptable.

C6 Envelopes (114 x 162 mm)

C6 holds an A4 sheet folded into quarters or an A5 sheet folded once. It’s the standard size for invitations, thank-you notes, and small promotional mailers where a compact, personal format matters more than fitting a full page.

C7 Envelopes (81 x 114 mm)

C7 is the smallest envelope in regular commercial use, built for an A7 card or an A5 sheet folded into quarters. It’s most often seen carrying gift cards, vouchers, loyalty cards, and RSVP replies rather than full letters.

Choosing an envelope isn’t only about fitting the paper; it’s also about which Royal Mail postage format your mail item falls into. Royal Mail prices are posted by format rather than by exact size, and your item is classed by whichever format its largest dimension and weight both still fit inside.

Thickness is the detail most senders overlook. A DL or C5 envelope stuffed with more than a few sheets can easily creep past the 5 mm letter limit or the 25 mm large letter limit, pushing the item into a more expensive band even though the length and width haven’t changed. Both Letter and Large Letter items must also be flexible enough to bend; rigid inserts such as thick card or plastic can force a reclassification regardless of size. Because postage rates change periodically, always confirm current pricing at royalmail.com before finalising a large mailing.

Work through these questions in order rather than picking a size on appearance alone:

• Does the document need to stay uncreased? If yes, use C4 and accept the large letter rate.
• Can the document be folded once or more? DL (three folds), C5 (one fold), or C6 (two folds) will all cost less to post than C4.
• How many pages and what thickness? Check the total against the Letter (5mm) or Large Letter (25mm) thickness limit before finalising the envelope, not after.
• Does it need a return envelope inside? Factor that extra thickness into your total before choosing a size.
• Is this a high-volume mailing? At scale, a few millimetres of unnecessary thickness or an oversized envelope can add a meaningful amount to your total postage bill.

The right envelope size often depends less on the document and more on the industry sending it. A few common patterns:

• Insurance: DL and C6 envelopes are standard for renewal notices, billing statements, and cancellation letters, high-volume, low-page-count mail where postage efficiency matters most.
• Healthcare: C5 envelopes suit patient welcome packs and multi-page compliance notices, while DL covers routine appointment reminders and statements. Given the sensitivity of health correspondence, address accuracy matters as much as envelope choice; a misdelivered patient letter is a data-protection concern, not just a wasted stamp.
• Financial services: DL remains the default for statements and correspondence, while C4 is common for signed agreements, mortgage documents, and contracts that need to arrive uncreased.
• SaaS and B2B: C5 and C6 envelopes are popular for onboarding kits and account-based marketing mailers, often paired with a QR code or personalised URL so response can be tracked digitally.
• Retail and e-commerce: C6 and C5 sizes work well for loyalty mailers and personalised offers, while C4 is the natural fit for full-size catalogues and lookbooks.

Choosing an envelope by thickness alone, then overfilling it just past the letter or large letter limit.

• Using a rigid or padded envelope for a mailing that would otherwise qualify for the cheaper Letter or Large Letter rate.
• Placing the address block off-centre or too close to an edge, which can interfere with Royal Mail’s automated sorting and cause delays.
• Defaulting to C4 for documents that would fold perfectly well into a cheaper DL or C5 envelope.

Getting the envelope size right only pays off if the address on it is correct. PostGrid’s address verification API validates, standardises, and autocompletes UK addresses in real time, with batch processing available for larger contact lists, so mail isn’t returned or delayed over formatting errors. For businesses producing letters, statements, or campaigns at volume, our Print & Mail API and template editor let you manage layout, merge variables, and QR or PURL tracking from a single workflow, while Virtual Mailboxes give teams a way to receive and manage incoming physical mail digitally. None of this replaces choosing the right envelope for the job, but it does remove the address accuracy and production overhead that often cost more than the postage itself.