Letter, Large Letter or Parcel? Franking the Right Format

Every item you put through a franking machine sits in one of Royal Mail's format bands, and that band is what decides the price. Choose correctly and you pay the cheapest legitimate rate. Choose wrongly and Royal Mail will usually still deliver the item, then recover the shortfall from you along with an administration charge. Format errors are one of the quietest ways a UK mailroom leaks money, and they are also one of the easiest problems to fix.

The short answer

Royal Mail sorts UK mail into four main formats: Letter, Large Letter, Small Parcel and Medium Parcel. A Standard Letter must be no larger than 240mm x 165mm x 5mm and weigh no more than 100g. A Large Letter must be no larger than 353mm x 250mm x 25mm and weigh no more than 750g. A Small Parcel must fit within 45cm x 35cm x 16cm and weigh up to 2kg, and a Medium Parcel within 61cm x 46cm x 46cm and up to 20kg. An item has to satisfy every limit in its band. If it breaks just one, it moves up to the next format and the next price.

Why format beats price when you are budgeting

Businesses tend to focus on the headline postage rate and then wonder why the actual spend does not match the forecast. The rate is only half the equation. The format is the multiplier. A single envelope design that sits 1mm over the Large Letter thickness limit will be charged as a parcel on every item in the run, so a modest mailing can cost far more than planned.

That is also why format discipline is where most of the real savings live. Our breakdown of how much a franking machine can save your business assumes you are franking each item at the correct format. If you would like the current figures for each band, see our summary of the latest Royal Mail postage rates.

The four formats in detail

Letter

Maximum 240mm x 165mm x 5mm, up to 100g. In practice this covers DL and C5 envelopes holding a handful of folded A4 sheets. The 5mm depth limit is much tighter than most people expect, and a C5 envelope with six or seven sheets plus a reply envelope inside can quietly exceed it.

Large Letter

Maximum 353mm x 250mm x 25mm, up to 750g. This is the workhorse band for UK business post: C4 envelopes, unfolded A4 documents, certificates, brochures and slim booklets. Note that a C4 envelope is already too big to be a Standard Letter on dimensions alone, regardless of how little it weighs.

Small Parcel

Maximum 45cm x 35cm x 16cm, up to 2kg. Anything padded, boxed or bulkier than 25mm thick lands here even if it is very light. A padded mailer holding a single small item is a classic example.

Medium Parcel

Maximum 61cm x 46cm x 46cm, up to 20kg. This is the top of the standard Royal Mail size range for parcels sent through the retail and business services most franking users touch.

You can always check the current limits on Royal Mail's own size and weight guide before committing to a new envelope or packaging format.

How to measure an item properly

Format checks fail because people measure one dimension and assume the rest. Work through all four criteria every time you introduce a new mailing:

• Length and width: measure the finished, sealed envelope, not the flat sheet size.
• Thickness: measure the fullest point, usually where a folded insert or a staple sits, not a thin corner.
• Weight: weigh a made-up sample with every insert included, then weigh a second sample to confirm.
• Rigidity: if the item cannot pass through a slot without force, treat it as a parcel and check rather than hope.

Do this once per mailing type and record the result. A laminated card taped inside the mailroom cupboard saves far more money than it costs.

Thickness is the trap, not weight

Most surcharges we hear about are not caused by heavy mail. They are caused by thick mail. A 40g item at 7mm thick is a Large Letter, not a Letter, and the scales on your franking machine cannot see that. Integrated weighing platforms measure mass, and on many machines the format still has to be selected by the operator. If your machine offers automatic format detection, it is still worth spot checking new mailings by hand.

Choosing the right stationery removes most of the risk. Our guidance on the best envelopes for franking machines covers feed reliability, and the envelope range includes self seal envelopes and approved meter posting envelopes designed for franked post.

What your franking machine can actually print on

Format also determines how the postage mark gets onto the item. Most desktop franking machines can only feed flat mail through the print path, typically letters and large letters within the machine's own thickness tolerance. Parcels, padded mailers and anything rigid need a self adhesive label instead.

That is what franking labels are for. Keeping a roll or pack alongside your franking machine supplies means an unexpected parcel never stops a mail run. If you are unsure which consumables your model takes, our guide on identifying the right ink cartridge will narrow it down in a couple of minutes.

What happens if you frank the wrong format

Royal Mail applies surcharges and correction charges to underpaid mail, and the details for letter products are published in the adjustment charges section of its Retail Letters Price Guide. The current position is set out on the Royal Mail surcharges and correction charges page.

For users on a Mailmark franking machine, the practical effect is that Royal Mail has item level data about what you sent. Underpayments are identified and recovered through your postage account, itemised on your invoice, rather than being argued about weeks later. That is a good thing for record keeping, but it does mean sloppy format selection shows up quickly.

Five habits that prevent format errors

1. Standardise your envelopes. Fewer sizes means fewer judgement calls at the machine.
2. Keep a measuring gauge next to the franking machine. A simple slot template settles thickness disputes instantly.
3. Weigh every non standard item. Guessing on one off mail is where most surcharges begin.
4. Brief every operator, not just the main one. Holiday cover is a common source of mis franked runs.
5. Keep the impression clean and legible. A faint or streaky mark can cause handling problems of its own, so replace cartridges before they run dry and read our fix for faint or streaky printing if quality drops.

Frequently asked questions

What are the Royal Mail Letter and Large Letter size limits?

A Standard Letter must be no larger than 240mm x 165mm x 5mm and weigh no more than 100g. A Large Letter must be no larger than 353mm x 250mm x 25mm and weigh no more than 750g. An item must meet every limit to qualify for that format.

Does thickness or weight decide the format?

Both do, along with length and width. Whichever measurement breaks its limit first pushes the item into the next format up, so a light but bulky envelope can still be charged as a Large Letter or a parcel.

What happens if I frank an item as the wrong format?

Royal Mail will normally still deliver the item, but it can apply a surcharge or correction charge for the underpayment plus an administration charge. On a Mailmark meter these are recovered through your postage account and itemised on your invoice.

Can a franking machine handle parcels?

Most desktop franking machines can only feed flat letters and large letters through the print path. For parcels, padded mailers and thicker items you print a self adhesive franking label and apply it to the packaging instead.

How do I stop format mistakes happening in my mailroom?

Standardise on a small number of envelope sizes, keep a measuring gauge and scales beside the machine, weigh every non standard item, and make sure anyone covering the mail run knows how to check all four measurements.

Getting the format right costs nothing and protects your postage budget every single day. Once your process is sound, keep the hardware side simple too: browse our best selling franking ink cartridges for fast dispatch, or get in touch with our team if you need help matching supplies to your machine.