A letter and a large letter are two different Royal Mail postage formats, and the one your item falls into decides what you pay. In short: a letter must be no bigger than 24cm x 16.5cm x 0.5cm and weigh up to 100g, while a large letter can be up to 35.3cm x 25cm x 2.5cm and weigh up to 750g. Anything thicker or heavier becomes a small parcel, which costs a lot more. Getting the format right is one of the simplest ways for a UK business to stop overpaying on post in 2026.
This guide breaks down the exact dimensions and weights, shows the 2026 franked prices for each format, and explains the small measuring habits that keep your mail in the cheaper band. If you frank your post with a Mailmark machine, the savings stack up further, as we will see below.
Royal Mail letter and large letter sizes at a glance
Royal Mail sorts most everyday business post into three format bands based on size, thickness and weight. Your item has to satisfy every limit for a format, not just one, so a wafer-thin envelope that is too long still counts as a large letter. Here are the current maximums.
Letter
A standard letter measures a maximum of 24cm long, 16.5cm wide and 0.5cm thick, with a weight limit of 100g. Think of a typical DL or C5 envelope holding a few folded sheets. This is the cheapest format Royal Mail offers, so it is worth keeping single-page invoices, statements and cards inside these limits wherever you can.
Large letter
A large letter measures up to 35.3cm long, 25cm wide and 2.5cm thick, and can weigh up to 750g. A C4 (A4) envelope, an A5 booklet or a slim padded mailer usually sits here. The 2.5cm thickness limit is the one that catches people out, because a well-stuffed A4 envelope frequently tips over 25mm and quietly becomes a small parcel.
Small parcel and beyond
Once your item is thicker than 2.5cm or heavier than 750g, it is a small parcel, and the price jumps sharply. This is the single biggest cost step in the Royal Mail ladder, which is exactly why the letter and large letter formats matter so much for day-to-day mailing. You can read Royal Mail's own UK sending formats and prices for the full detail on parcels.
2026 franked prices for letters and large letters
The 2026 Royal Mail prices took effect on 7 April 2026. Franking with a Mailmark machine remains cheaper than stamps on letters and large letters, which is where the biggest wins are. Here are the current First Class Mailmark franked prices, with the stamp price in brackets so you can see the gap.
First Class (Mailmark franking): Letter up to 100g is £1.77 (£1.80 by stamp). Large letter up to 100g is £2.86 (£3.30). Large letter 101 to 250g is £3.28. Large letter 251 to 500g is £3.30. Large letter 501 to 750g is £3.30. A First Class small parcel up to 2kg is £5.45.
Second Class (Mailmark franking): Letter up to 100g is 88p (91p by stamp). Large letter up to 100g is £1.52 (£1.55). Large letter 101 to 250g is £1.81. Large letter 251 to 500g is £2.30. Large letter 501 to 750g is £2.50. A Second Class small parcel up to 2kg is £4.25.
Two things stand out. First, a Mailmark machine saves you up to 44p on every First Class large letter compared with a stamp, and 3p on a standard letter. Second, look at the cliff edge: a First Class large letter costs at most £3.30, but the moment it becomes a small parcel it is £5.45. That is more than £2 of extra postage for a couple of millimetres of thickness. You can see your own numbers with the Mailcoms franking savings calculator. For a fuller stamp-versus-franking breakdown, see our guide to whether a franking machine is cheaper than stamps in 2026.
How to avoid costly surcharges
Royal Mail surcharges mail that has been underpaid, which usually happens when an item is measured or weighed incorrectly at the point of posting. The recipient can be asked to pay a surcharge plus a fee, which is not a great look for a business. The good news is that surcharges are almost always avoidable with a few simple habits, as Mailcoms sets out in its practical guide to avoiding surcharges.
Measure thickness, not just length
Thickness is the limit most businesses trip over. A handy trick is to cut a 25mm slot in a piece of card and post each item through it, the same way Royal Mail's own format gauges work. If it will not pass, it is a small parcel and should be priced as one. Folding contents flat or using a slightly larger, thinner envelope can keep a bulky A4 mailing in the large letter band.
Weigh every item on accurate scales
Guessing the weight is a false economy. A connected weighing platform lets your franking machine select the correct format and price automatically, so you never under or overpay. Green Franking supplies inks and labels rather than the machines themselves, so for the hardware and servicing side our sister company Mailcoms can help you buy, lease or service a franking machine.
Choose the right envelope
The envelope you pick decides your format before you have added a single sheet. Slim, well-sized business envelopes keep letters within the 0.5cm limit and stop large letters drifting into parcel territory. Right-sizing your packaging is also one of the easiest ways to cut both cost and carbon, as we explain in our guide to sustainable mailing for small businesses.
Keep your franking supplies in good order
An underinked or smudged impression can be rejected and treated as unpaid, so genuine, well-stored ink matters. Using the correct Mailmark ink for your machine keeps every frank crisp and scannable. When you post through a franking label rather than directly onto the envelope, the same rules apply, and our guide to franking machine labels explains when to reach for one.
Why franking beats stamps for format-heavy mail
Because Mailmark franking prices are lower than stamp prices on both letters and large letters, businesses that send a steady mix of formats save the most. Every large letter alone can be up to 44p cheaper, and across hundreds of items a month that adds up quickly. Franking machines also weigh and price each item precisely, which removes the guesswork that leads to surcharges in the first place. Non-Mailmark meters have now been decertified by Royal Mail, so a current Mailmark machine is the only way to access these franked rates. If yours is older, Mailcoms can advise on the decertification of non-Mailmark machines and the best replacement.
Put simply, knowing your formats protects your margins, and franking your post locks in the lowest legitimate price for each one. Keep a format gauge and a set of scales by your outgoing tray, stock the right envelopes and ink, and underpaid mail becomes a thing of the past.
Frequently asked questions
What is the maximum size for a Royal Mail letter?
A standard letter can be up to 24cm long, 16.5cm wide and 0.5cm thick, and must weigh no more than 100g. If it exceeds any one of these limits it moves up to the large letter format.
What are the dimensions of a large letter?
A large letter can be up to 35.3cm long, 25cm wide and 2.5cm thick, with a maximum weight of 750g. The 2.5cm thickness limit is the one most businesses exceed, so always measure the thickest point of your item.
How much does a large letter cost to frank in 2026?
From 7 April 2026, a First Class large letter up to 100g costs £2.86 by Mailmark franking, compared with £3.30 by stamp, saving up to 44p. A Second Class large letter up to 100g costs £1.52 by franking. Heavier large letters cost more, up to £3.30 First Class and £2.50 Second Class for 501 to 750g.
Why did my letter get surcharged?
Surcharges usually happen when an item is underpaid because it was measured or weighed incorrectly, most often when a thick envelope is priced as a large letter but is actually a small parcel. Weighing on accurate scales and checking thickness against a 25mm gauge prevents this.
Is franking cheaper than stamps for large letters?
Yes. Mailmark franking prices are lower than stamp prices on both letters and large letters, with savings of up to 44p on a First Class large letter. Because non-Mailmark meters have been decertified, a current Mailmark machine is the only way to access these franked rates.
What happens if my large letter is too thick?
If your item is thicker than 2.5cm or heavier than 750g it becomes a small parcel, which is significantly more expensive. A First Class small parcel is £5.45 against a maximum of £3.30 for a large letter, so keeping items flat and within the thickness limit protects your postage budget.