Royal Mail prices your post by its format, not just its weight, and the line between a cheaper Letter and a pricier Large Letter comes down to a few centimetres. A Letter must be no bigger than 240mm x 165mm x 5mm and weigh up to 100g; a Large Letter can be up to 353mm x 250mm x 25mm and weigh up to 750g. Anything thicker than 25mm tips into Small Parcel rates. Choosing the right envelope for what you are sending is therefore one of the simplest ways to keep your postage costs predictable.
Letter vs Large Letter: the rules that decide your price
Every item you post is measured against three things at once: its length, its width and its thickness, plus its weight. To qualify for a band, your item has to satisfy all of those limits. Slip over a single one - usually thickness - and Royal Mail charges you at the next format up, regardless of how light the contents are.
Here are the current thresholds for the two formats that matter most to letters and flat mailings:
Letter
- Maximum size: 240mm long x 165mm wide x 5mm thick
- Maximum weight: 100g
- Typical envelopes: DL, C6 and C5 (when only lightly filled)
Large Letter
- Maximum size: 353mm long x 250mm wide x 25mm thick
- Maximum weight: 750g
- Typical envelopes: C4 (A4), C5 and bulkier C5/C6 contents
The jump in price between the two is significant. As of the April 2026 tariff, a First Class Letter costs £1.80 and a First Class Large Letter costs £3.30. On Second Class the gap is similar: 91p for a Letter against £1.55 for a Large Letter. Multiply that difference across a mailing run of a few hundred items and the envelope you choose stops being a detail and starts being a budget line.
Why thickness catches so many businesses out
Most people instinctively think about the length and width of an envelope, because that is what they can see. Thickness is the limit that quietly does the damage. A C5 envelope is comfortably within Letter dimensions on length and width, but the moment you fold in a few sheets, a brochure or a small promotional item, it can breach the 5mm depth limit and become a Large Letter - even though it still weighs only a few grams.
The same trap sits at the top of the Large Letter band. A well-stuffed A4 envelope, or a document wallet with a rigid backing, can easily push past 25mm. At that point it is no longer a Large Letter at all; it is a Small Parcel, which is charged at a higher rate again. The practical lesson is to leave yourself a margin: aim to pack a couple of millimetres under each thickness limit rather than testing it.
Matching envelope sizes to the format you want
The cleanest way to control your postage is to start from the format you want to hit and work backwards to the envelope. Below is how the common envelope sizes tend to fall.
If you want to stay in the Letter band
A DL envelope (110mm x 220mm) is the classic business choice. It takes an A4 sheet folded into three and, with one to three sheets inside, sits well within both the size and weight limits for a Letter. C6 (114mm x 162mm) suits a folded A5 sheet or a compliments slip. C5 envelope (162mm x 229mm) holds an A4 sheet folded once or an A5 sheet flat, and stays a Letter as long as you keep the contents thin and under 5mm.
If you are happy with the Large Letter band
A C4 envelope (229mm x 324mm) takes an unfolded A4 document and is the natural home for contracts, certificates, brochures and reports you would rather not crease. C4 almost always travels as a Large Letter, so price it on that basis from the outset. A C5 that has become too thick for a Letter also lands here, which is why C5 is such a flexible everyday size.
If you regularly send items that hover near a threshold, it is worth keeping a simple postage gauge or a small set of scales on the desk. A five-second check before sealing is far cheaper than discovering an underpaid-postage surcharge after the fact.
Weight matters too, not just dimensions
It is easy to fixate on size and forget that weight has its own ceilings. A Letter is capped at 100g, which sounds generous until you add a heavier card insert, a thicker gsm paper stock or several enclosures. The paper weight of the envelope itself contributes here: a 120gsm envelope plus a multi-page insert can creep towards that 100g line more quickly than you would expect. If your mailing is content-heavy, a Large Letter envelope with its 750g allowance gives you far more headroom and removes the risk of an over-weight Letter being surcharged.
How printed and windowed envelopes fit in
Printing your branding on an envelope does not change its format - a printed DL is still a Letter, and a printed C4 is still a Large Letter - so you get the marketing benefit without altering your postage band. Window envelopes behave the same way: the window is part of the face of the envelope and does not affect how Royal Mail measures or prices it. What does matter is keeping the address clearly visible and correctly positioned within the window, and making sure any printing does not interfere with the clear address area or, if you use it, the Mailmark barcode zone. Choosing the right size first, then adding print or a window, keeps both your costs and your presentation under control. Browse our range of custom printed envelopes to find the right fit for your mailing.
A quick checklist before you buy envelopes in bulk
- Decide the format you want to post at (Letter or Large Letter) before choosing a size.
- Check all three dimensions and the weight - you must meet every limit, not just one.
- Leave a thickness margin of 2–3mm so a full envelope does not slip into the next band.
- Account for the weight of inserts and the envelope itself, not just the paper count.
- Order a sensible gsm - heavy enough to protect contents and look professional, light enough to stay in band.
Need envelopes that fit your format and carry your brand? See our full range of custom printed business envelopes or explore sizes including DL, C5 and C4.
Frequently Asked Questions
What is the difference between a Letter and a Large Letter?
A Letter must be no larger than 240mm x 165mm x 5mm and weigh up to 100g. A Large Letter can be up to 353mm x 250mm x 25mm and weigh up to 750g. The Large Letter band exists for flatter, slightly bulkier items such as unfolded A4 documents, and it is charged at a higher rate than a standard Letter.
Which envelope size is cheapest to post?
Smaller, thinner envelopes such as DL, C6 and lightly filled C5 are the cheapest to post because they fall within the Letter format. As of April 2026, a First Class Letter costs £1.80 compared with £3.30 for a First Class Large Letter, so keeping an item within Letter limits roughly halves the cost.
Does a C5 envelope count as a Letter or a Large Letter?
It depends on how thick it is. A C5 (162mm x 229mm) is within Letter dimensions on length and width, so a thin C5 with one or two folded sheets can post as a Letter. Once the contents push it past 5mm thick, it becomes a Large Letter, even if it is still very light.
Why has my flat envelope been charged as a parcel?
Almost always because it exceeded 25mm in thickness. Once a flat item is thicker than the Large Letter limit, Royal Mail classes it as a Small Parcel and prices it accordingly. Bulky enclosures, rigid backing boards and over-stuffed A4 envelopes are the usual culprits, so pack a little under the limit.
Do printed or window envelopes cost more to post?
No. Printing and windows do not change an envelope's format, so a printed or windowed envelope posts at exactly the same Letter or Large Letter rate as a plain one of the same size. Just keep the address clearly visible through the window and avoid printing over the clear address area or barcode zone.
How do I check the format before I post?
Measure the length, width and thickness, then weigh the item, and confirm it meets every limit for the band you want. A simple postage gauge or a small set of scales on your desk makes this a quick routine check - far cheaper than an underpaid-postage surcharge later.