If you sell anything small and flat, cards, stickers, prints, thin accessories, the Letter versus Large Letter boundary is the single most profitable piece of Royal Mail knowledge you can have. The two bands are priced very differently, and the line between them is thinner than most people expect: a single rigid card in a stiff envelope can push you over it without you realising.
The part that catches sellers out isn't the length or width, it's the thickness. Let's walk through the exact numbers and the practical test you can do at the packing bench.
The Letter limits, exactly
A Royal Mail Letter must be no more than 240mm long, 165mm wide and 5mm thick, and weigh no more than 100g. Those are hard limits. Exceed any one of them and the item is no longer a Letter, it's a Large Letter, and the price steps up. The thickness limit of 5mm is the one that gets ignored, because most people don't carry a caliper and it's genuinely easy to underestimate.
The Large Letter limits
A Large Letter can be up to 353mm long, 250mm wide and 25mm thick, up to 750g. That's a much roomier envelope, but it's still a letter format. Once you pass 25mm thick, the item becomes a Small Parcel, which is a different price again and a whole separate conversation covered in our Large Letter vs Small Parcel guide.
The 5mm thickness trap
Here's the scenario that costs sellers money every single day: a greeting card in a stiff envelope, or a few stickers in a cardboard mailer, that's maybe 6 or 7mm thick once sealed. It looks like a letter, it feels like a letter, but it's over 5mm, so Royal Mail's machinery classes it as a Large Letter. The sender pays the Letter rate, the difference is surcharged back to them or to the recipient. Our surcharges guide explains exactly what happens when you get it wrong.
The practical thickness test
You don't need a caliper. Royal Mail's own advice is that a Letter should pass through a slot 5mm high, and a Large Letter through one 25mm high. A cheap cardboard or plastic slot gauge on the bench takes the guesswork out entirely, and it's the kind of one-off purchase that pays for itself the first time it stops you sending a 6mm item at the Letter rate.
Why it matters beyond the immediate surcharge
A surcharge isn't just a fee, it's a customer experience problem. When the recipient has to pay the underpaid postage before the item is released, you get an annoyed buyer and often a bad review to go with it. The cost of getting the band right is a two-second measurement; the cost of getting it wrong is a fee plus a damaged relationship.
Which service to use once you know the band
The band determines the base price, but the service you choose on top of it is a separate decision. A Letter or Large Letter can go 1st Class, 2nd Class or Signed For, and the same size logic applies regardless. For anything with value, the tracked or signed upgrade is worth considering even on a flat item, because the base letter services carry very little compensation.
Weighing it as well as measuring
Weight matters within each band too. A Letter is fine up to 100g, but every gram over pushes you into Large Letter territory on weight alone, even if the thickness is fine. The habit to build is simple: measure the dimensions, weigh the finished item, then let Click & Drop quote from those real numbers rather than a guess. It removes both the size and weight mistakes in one routine.
A quick band checklist
Before you buy postage, run this in order. Is it under 240mm × 165mm × 5mm and 100g? Letter. Over any of those but under 353mm × 250mm × 25mm and 750g? Large Letter. Over 25mm thick? Small Parcel. Once you've done it a few times it becomes automatic, and you'll stop leaving money on the table at the counter.
Frequently asked questions
What is the thickness limit for a Royal Mail Letter? 5mm. Anything over 5mm thick is a Large Letter, even if the length, width and weight are all within the Letter limits.
Does a greeting card count as a Letter? Only if the finished envelope is 5mm thick or less. A rigid card in a stiff envelope often pushes past 5mm and becomes a Large Letter.
What happens if I send a Large Letter as a Letter? Royal Mail applies a surcharge for the underpaid postage, which may be charged to the recipient before delivery, so it's cheaper to measure first.
Flat items still need a clean label
Even a letter-sized shipment prints its postage as a 100 × 150mm barcode, and our direct thermal labels keep it crisp and scannable at any size.
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