Letter, Large Letter or Parcel? A Franking Format Guide
Letter, Large Letter or Parcel? Royal Mail sorts every UK item into one of four formats, and your franking machine prices the item against whichever format you select. A Letter must fit within 24cm x 16.5cm x 0.5cm and weigh no more than 100g. A Large Letter must fit within 35.3cm x 25cm x 2.5cm. A Small Parcel must fit within 45cm x 35cm x 16cm and weigh up to 2kg, and a Medium Parcel must fit within 61cm x 46cm x 46cm and weigh up to 20kg. If an item breaks any one of those limits, it moves up to the next format, and the thing that catches most mailrooms out is not the weight. It is the depth.
The four UK formats at a glance
Royal Mail publishes the full specification in its size and weight guide, but here is the short version that matters at the franking machine.
- Letter - maximum 24cm long, 16.5cm wide, 0.5cm deep, up to 100g. Think invoices, statements, single-sheet correspondence and greetings cards.
- Large Letter - maximum 35.3cm long, 25cm wide, 2.5cm deep. The weight ceiling depends on the service, at 750g on 1st and 2nd Class and 1kg on Tracked 24 and Tracked 48. Think A4 documents, brochures, catalogues and thin card-backed items.
- Small Parcel - maximum 45cm x 35cm x 16cm, up to 2kg. Think sample packs, boxed spares and small retail orders.
- Medium Parcel - maximum 61cm x 46cm x 46cm, up to 20kg. Anything heavier than 20kg needs Parcelforce Worldwide rather than Royal Mail.
An item has to satisfy every dimension and the weight limit to qualify for a format. One oversized measurement is enough to push it up a band, and the price with it.
Why format matters more when you frank than when you stamp
With stamps, an underpaid item usually gets surcharged at the far end and the recipient is asked to pay before it is released. With franking, the impression printed on the envelope is a declaration. Royal Mail accepts it on trust, and if a sampling check finds you have been declaring A4 brochures as Letters, the shortfall is recovered against your account rather than quietly absorbed. Repeated under-declaration is the sort of thing that turns into an awkward conversation with your provider.
The reverse is just as costly and far more common. Plenty of businesses frank everything as a Large Letter because it is the safe option, and pay a premium on thousands of items a year that would comfortably have gone as Letters. Our comparison of franking versus stamps in 2026 covers the underlying savings, and the current bands are set out in our Royal Mail postage rates update.
Depth is the measurement that catches people out
Weight is easy. You have scales, and most modern Mailmark machines weigh the item for you. Depth is the one nobody checks, and it is where the money leaks.
A Letter is allowed just 5mm of thickness. That is roughly five sheets of 80gsm paper in a standard DL envelope, so a six-page contract with a compliment slip is already a Large Letter even though it weighs well under 100g. A Large Letter gets 25mm, which sounds generous until you fold a 40-page brochure and add a covering letter. Once the envelope will not slide through a 25mm slot without being squashed, it is a Small Parcel.
The cheapest fix is nearly always the envelope rather than the machine. Switching a bulky fold into a larger, flatter envelope can drop an item back a band. Our range of franking machine envelopes includes self seal envelopes for high volume runs and meter posting envelopes designed to feed cleanly through a franking machine deck. If you are unsure which stock suits your model, our guide to what envelopes you can use with a franking machine goes through the practical constraints.
A ten second format check for the mailroom
Stick this sequence on the wall above the franking machine and most format errors disappear.
- Weigh it. Let the machine do this rather than estimating. Royal Mail's advice on how to weigh and measure an item is worth reading if you are working without integrated scales.
- Check the depth with a gauge. A piece of card with 5mm and 25mm slots cut into it costs nothing and settles the Letter or Large Letter question instantly.
- Check length and width. Only after depth, because depth rules out far more items.
- Select the format on the machine before you frank. Not after. Reversing a wrongly franked item means a refund claim and a wasted envelope.
Rigid items are not Large Letters
An item that fits the Large Letter dimensions on paper can still be treated as a parcel if it is rigid or has an unusual shape. Royal Mail's automated sorting equipment needs Large Letters to have some flexibility, so a boxed item 20mm deep will usually be handled as a Small Parcel even though a tape measure says otherwise. Packets with prominent lumps, cylinders and anything in a rigid card mailer fall into the same category.
The rule of thumb is simple. If it will not bend at all, price it as a parcel and you will not be caught out.
What to do when the item cannot go through the machine
Parcels raise a second problem. Most franking machines cannot feed a box, so the impression has to go onto a franking machine label which is then applied to the packaging. This is normal, accepted practice, but only if the label is the correct type for your machine and is applied flat in the top right corner of the largest face.
Labels that curl, lift at the edges or sit across a seam are the most common cause of rejected parcels, and we covered the detail in our guide to when to use franking machine labels. Buying the right label for your specific model matters as much as buying the right ink cartridge, and we stock both by manufacturer for Pitney Bowes, Quadient and FP Mailing.
Business mail formats have tighter tolerances
If you post in volume on a business mail contract rather than at standard franking rates, be aware that the machine-readable formats used for bulk mail carry tighter limits than the retail formats above. Envelope colour, window position and address block placement all come into play, and the specification is published separately by Royal Mail for business mail services. Do not assume a retail Large Letter automatically qualifies for a machine-readable business mail rate.
Frequently asked questions
What size is a Letter with Royal Mail?
A Letter must be no larger than 24cm long, 16.5cm wide and 0.5cm deep, and must weigh 100g or less. Any item that exceeds even one of those limits is classed as a Large Letter or larger.
How thick can a Large Letter be?
A Large Letter can be up to 2.5cm, or 25mm, deep. It must also stay within 35.3cm x 25cm and meet the weight limit for the service you are using, which is 750g on 1st and 2nd Class.
Does my franking machine work out the format automatically?
Most modern Mailmark machines weigh the item and offer the matching price bands, but the operator still selects the format. The machine cannot measure depth or judge rigidity, so the final decision is always yours.
What happens if I frank an item as the wrong format?
Under-declared items can be surcharged or returned, and repeated shortfalls are recovered from your postage account. Over-declared items are simply overpaid, which is why a quick depth check before franking usually pays for itself.
Can I frank a parcel directly onto the box?
No. Franking machines feed flat items only, so parcels need a franking label applied to the packaging instead. Use the label type specified for your machine and apply it flat on the largest face.
Getting formats right is the cheapest postage saving available to any mailroom, because it costs nothing but a moment of attention. If your next job is making sure the impression itself is crisp enough to be read, browse our full range of franking supplies and envelopes, or get in touch with our team and we will match the right ink and labels to your machine.