Letter, Large Letter or Parcel? Sizing Franked Mail Right
Getting the format right is the single biggest lever you have over your franking costs. Royal Mail prices UK mail by format first and weight second, so an item that tips from Letter into Large Letter, or from Large Letter into Small Parcel, jumps a price band even if it only gained a few millimetres of thickness. A Letter must fit within 24cm x 16.5cm x 0.5cm and weigh up to 100g; a Large Letter must be no bigger than 35.3cm x 25cm x 2.5cm and up to 750g; a Small Parcel is up to 45cm x 35cm x 16cm and 2kg. Measure before you frank, not after.
If your franking machine is set to the wrong format, one of two things happens. You overpay quietly on every item, month after month, or you underpay and Royal Mail recovers the shortfall plus an administration charge. Neither shows up on an invoice with a label saying "you measured this wrong", which is exactly why format errors are so persistent in UK mailrooms.
How Royal Mail decides which format your item is
Royal Mail uses four measurements for every UK item: length, width, depth and weight. The item must satisfy all four limits for a band to apply. Fail any one of them and the whole item moves up to the next band, whatever the other three say. A slim A4 envelope weighing 60g is still a Small Parcel if a ring binder inside makes it 3cm deep.
The current UK format limits, as published in the Royal Mail size and weight guide, are:
Letter
Maximum 24cm long, 16.5cm wide, 0.5cm deep and 100g. This covers ordinary business correspondence in DL or C5 envelopes, invoices, statements, remittance advices and greetings cards. The depth limit is the one that catches people out. Five or six folded A4 sheets are fine, but add a plastic card, a sachet or a stapled booklet and you are over 0.5cm.
Large Letter
Maximum 35.3cm long, 25cm wide, 2.5cm deep and 750g. This is the workhorse band for anything flat and unfolded: A4 contracts, certificates, brochures, catalogues, annual reports and thicker multi page mailings. A C4 envelope fits comfortably inside the length and width limits, so weight and depth are what you need to watch.
Small Parcel
Maximum 45cm long, 35cm wide, 16cm deep and 2kg. Once an item is bulky rather than flat, this is where it lands. Samples, small hardware, boxed items and anything rigid usually belong here.
Medium Parcel
Maximum 61cm long, 46cm wide, 46cm deep and 20kg. Above this you are into larger parcel services rather than standard franked mail.
The three format mistakes that cost UK businesses the most
1. Franking everything as a Letter by habit
Many machines default to Letter on power up. If an operator runs a batch of A4 brochures without changing the setting, every item is underpaid. Royal Mail's sortation equipment measures items automatically, so these are picked up reliably rather than occasionally.
2. Treating a padded envelope as flat mail
Padded and bubble lined envelopes are frequently franked as Large Letters because they look like envelopes. What matters is the finished depth with contents inside. A padded envelope holding a boxed item is almost always a Small Parcel.
3. Measuring an empty envelope
Depth must be measured when the item is filled and sealed, on a flat surface, at its thickest point. An envelope that measures 2mm empty can easily finish at 8mm once stuffed. If you are choosing stationery for a recurring mailing, our range of business envelopes makes it easier to standardise on a size you have already tested.
What happens if you get the format wrong
Underpaid franked mail is handled through Royal Mail's surcharge process. Items identified during sortation are still delivered, but the shortfall and an administration fee are recovered from the sender, typically collected through your franking machine supplier's postage account rather than left as a fee to pay for your recipient. The rules and current charges are set out on the Royal Mail surcharges page, and we cover the practical side in our guide to Royal Mail surcharges on franked mail.
Overpayment is the quieter problem. Nobody writes to tell you about it. If you frank 500 items a month as Small Parcels when they qualify as Large Letters, the difference compounds across a year into a meaningful sum, and it never appears as an error anywhere in your accounts.
A simple mailroom routine that prevents format errors
You do not need new equipment to fix this. You need a repeatable check.
- Keep a format gauge on the bench. A slotted template that an item either passes through or does not removes all argument about depth.
- Weigh the heaviest item in the batch, not the first one. Batches are rarely uniform. Price to the worst case and you will never underpay.
- Sort by format before you frank, not as you go. Group Letters, Large Letters and parcels into three physical piles, then run each pile with the machine set once.
- Re-check after any content change. Adding a single insert to a standing mailing is the most common cause of a format shift.
- Frank parcels with labels rather than trying to feed them. Rigid items should not go through the transport deck. Use franking machine labels and apply them by hand.
If you are new to running a machine, our walkthrough on how to prepare and post franked mail correctly covers presentation, bundling and handover alongside format selection.
Does the format affect your franking discount?
Franked prices sit below the equivalent stamp prices across the common UK formats, which is the core reason businesses run a machine at all. The saving is applied within each format band, so choosing the right band is what determines whether you actually realise it. Franking a Large Letter correctly beats franking it as a parcel, regardless of how good your franked rate is. We compare current franked and stamped pricing in our analysis of franking machine savings versus stamps.
One practical note: a correctly sized item is only worth franking if the impression is legible. Faded or streaked franks can be rejected at sortation even when the postage is right, so keep a spare cartridge on hand from our franking machine ink guide.
Quick reference table
| Format | Max length | Max width | Max depth | Max weight |
|---|---|---|---|---|
| Letter | 24cm | 16.5cm | 0.5cm | 100g |
| Large Letter | 35.3cm | 25cm | 2.5cm | 750g |
| Small Parcel | 45cm | 35cm | 16cm | 2kg |
| Medium Parcel | 61cm | 46cm | 46cm | 20kg |
Always confirm against Royal Mail's published guide before making a pricing decision, as limits and prices are reviewed periodically.
Frequently Asked Questions
What size is a Large Letter for franking?
A Large Letter must be no more than 35.3cm long, 25cm wide and 2.5cm deep, and must weigh no more than 750g. All four limits must be met. If the item exceeds any single one of them, it is priced as a Small Parcel instead.
How do I measure the depth of an envelope correctly?
Measure the sealed item at its thickest point, lying flat, with all contents inside. Do not measure an empty envelope and do not compress the item to make it fit. A slotted format gauge is the most reliable check because the item either passes through the slot or it does not.
What happens if I frank an item as the wrong format?
If you underpay, Royal Mail identifies the item during sortation, delivers it, and recovers the shortfall plus an administration charge from your postage account through your franking supplier. If you overpay, nothing is refunded automatically, so the extra cost is simply absorbed.
Can I frank a parcel directly through the machine?
Only very thin items should pass through the transport deck. Anything rigid or bulky should be franked onto a self adhesive label and applied by hand. This protects the feed mechanism and produces a cleaner impression on uneven surfaces.
Does a padded envelope count as a Large Letter?
It depends entirely on the finished dimensions once filled. A thin padded envelope holding flat contents can qualify as a Large Letter if it stays within 2.5cm of depth and 750g. A padded envelope containing a boxed or rigid item almost always becomes a Small Parcel.
Do format limits differ for franked mail and stamped mail?
The format definitions themselves are the same for franked and stamped UK mail. What differs is the price attached to each band, with franked rates generally sitting below the equivalent stamp prices. Choosing the correct band matters equally either way.
Need help with supplies?
If you are unsure which labels, ink or envelopes suit your machine and your typical mail profile, get in touch with our team and we will point you to the right items for your model.