The 5mm Mistake: How Packaging Choices Increase Business Postage Costs

Five millimetres is the difference between a Letter and a Large Letter. Since 7 April 2026 that gap has been worth £1.50 on every first class item, because a first class letter costs £1.80 and a first class large letter £3.30. On second class it is 64p an item. A business sending 500 second class mailings a quarter is paying roughly £320 extra each time for a packaging decision nobody made deliberately.

The contents are rarely the real problem. Envelope choice, the number of folds, one extra insert or a card-backed wallet can each decide which format applies. None of it appears on the invoice as a packaging decision. It appears as postage.

Why does the 5mm thickness limit matter?

Royal Mail sorts post by the packed dimensions and weight of the finished item. The current limits are:

An item only needs to break one limit to move up a format. A 4mm thick item that is 250mm long is still a Letter. The same item at 6mm becomes a Large Letter, even though nothing else has changed.

There is one more condition that catches people out: a Letter has to be flexible enough to bend, which rules out anything with rigid internal packaging.

These limits apply to the finished item, not the empty envelope. A mailing can look like a comfortable fit when everything is laid flat and still fail once it has been folded, packed and sealed. Royal Mail provides a full size and weight guide for businesses that need to check how their post will be classified.

The difference between 4mm and 6mm is very hard to judge by eye. Measure a completed sample instead of trusting the dimensions printed on the envelope box.

Understanding common UK business envelope sizes

Many UK envelope sizes follow the ISO 216 paper size system, which lets an A4 sheet be folded into predictable formats without being cut.

Three sizes turn up most often in business post:

DL envelopes are usually 110 × 220mm and take an A4 sheet folded into thirds. They are the standard choice for letters, invoices, statements and customer correspondence.

C5 envelopes are usually 162 × 229mm and take an A4 sheet folded once, or an unfolded A5 document.

C4 envelopes are usually 229 × 324mm and hold an unfolded A4 sheet.

DL and C5 envelopes can both sit inside the Letter dimensions, as long as the sealed item stays under 5mm and 100g. A C4 envelope is too large to qualify as a Letter at all, so it will normally be charged as a Large Letter whatever is inside it. That matters when comparing business envelope sizes in the UK. Reaching for the bigger envelope because it is easier to pack raises the postage before weight or thickness has even been considered.

How a Letter quietly becomes a Large Letter

The envelope is only part of the calculation. A handful of small choices push items over the 5mm line.

Picture a quarterly customer mailing packed by four members of staff. One person folds the brochure neatly. Another leaves the inserts slightly out of line. Same envelope, same contents, but only one of those finished items passes through the 5mm slot.

Adding one insert too many. A single business letter is unlikely to cause a problem. Combined thickness builds quickly once a mailing includes a brochure, a return form, a promotional card and a covering letter. An A4 brochure on heavier stock adds far more bulk than the same number of standard office sheets, and folding several inserts together creates a raised central section even when the outer edges of the envelope still look flat.

Using folders, clips or document wallets. Plastic wallets, card-backed documents and paper clips create concentrated points of thickness. An item does not have to be over 5mm across its whole surface to fail. One raised clip is enough. Stapling pages usually produces a flatter result than a large paper clip, though the finished mailing still needs checking.

Selecting padded packaging unnecessarily. Padded envelopes protect keys, small components and storage media, and that protection adds thickness before anything goes inside. Compare office envelopes and padded bags against what you are actually sending, the internal dimensions you need and the likely packed thickness. A standard mailing wallet is often enough for paper documents. Save padding for things that genuinely need cushioning.

Allowing contents to bunch up. Loose contents shift during packing and create an uneven bulge. Poor folds, trapped air and overfilled envelopes do the same. A consistent folding method keeps letters flat, and businesses running higher volumes may find automated folding and inserting machines worth the outlay when every item in a mailing has to meet the same dimensions.

Measuring the empty envelope. Envelope sizing tells you length and width. It tells you nothing about how thick the mailing becomes once it is filled. Seal the finished item and measure it at its thickest point. Where contents vary between recipients, test the bulkiest version rather than assuming the first sample represents the batch.

The cost is not limited to postage

Paying the Large Letter rate is the obvious consequence. It is not the only one.

Staff have to reopen and repack anything that fails an internal size check. Incorrectly prepared post can be delayed, returned or surcharged, and if the recipient is asked to make up insufficient postage, a routine business letter turns into a customer service problem.

Then there is the wasted material. Oversized envelopes, excess padding and inserts nobody needed all add to purchasing and storage. Individually, these are trivial. Repeated across every mailing for a year, they become a standing operating expense.

How to reduce avoidable business postage costs

Before the next batch leaves the office, check one finished example.

1. Standardise common mailings. List the post you send regularly: invoices, contracts, welcome packs, marketing material. Assign each one a set envelope size and folding method. When teams use the same envelopes, labels and other office stationery, outgoing post is quicker to prepare and purchasing staff can see what needs reordering.

2. Create a finished sample. Pack and seal one item exactly as it will be sent, including every insert, return envelope and promotional extra. Measure length, width and thickest point, then weigh it.

3. Use a postal size guide. A rigid Letter and Large Letter measuring guide is more reliable than a ruler. If the item will not pass through the 5mm slot without being squashed or forced, it is not a Letter.

4. Review the inserts before you accept the higher format. Where a mailing only just exceeds the limit, ask whether an insert can be dropped, printed on lighter paper or moved online. Removing a fold or swapping a bulky document wallet is often enough to bring the item back under 5mm. The aim is not to force unsuitable contents into an undersized envelope. Documents still have to arrive in good condition.

5. Recheck the limits and the prices. Royal Mail usually raises stamp prices once a year, in late March or early April, and formats and services change too. Review the current rates before a large campaign, or whenever you introduce new packaging. Skip the Stamp

Five millimetres adds up

One oversized envelope costs £1.50. That is not worth anyone’s attention. The expense only becomes visible when the same mistake is repeated across hundreds or thousands of items, quarter after quarter, and by then it looks like a rising postage bill rather than a packaging fault.

Checking dimensions, packed thickness and weight before dispatch takes a minute or two. Judge the completed mailing, not the empty envelope.

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