The boundary between a Large Letter and a Small Parcel is one of the most common places sellers get Royal Mail pricing wrong, because it's about physical dimensions, not weight. Step a few millimetres over the Large Letter limits and your item jumps into the Small Parcel band, where the price is materially higher. Knowing the exact thresholds is the difference between paying the right amount and being surcharged.
The exact Large Letter dimensions
A Large Letter must be no larger than 353mm long, 250mm wide and 25mm thick, and no heavier than 750g. The thickness limit is the one that catches people out most often: a padded envelope or a small box that's over 25mm thick fails the Large Letter test even if its length and width are fine.
What counts as a Small Parcel
Anything too big for a Large Letter becomes a Small Parcel, which allows up to 450mm long, 350mm wide and 160mm thick, with a maximum weight of 2kg. Small Parcels are priced as a band rather than precise weight steps within those limits, which is why most e-commerce orders sit comfortably here.
The 25mm thickness trap
Thickness is the dimension sellers misjudge most. A standard shipping box or a jiffy bag with any real contents usually exceeds 25mm, pushing an item out of Large Letter territory. If you're unsure, measure the thickest point, because that's what Royal Mail's sorting machinery checks, and a 26mm item will be surcharged as a Small Parcel.
Why the band matters for cost
The price difference between Large Letter and Small Parcel is significant, often a pound or more per item, because Small Parcels travel through a different, more expensive part of the network. At volume, that difference over hundreds of orders a month is real money, which is why accurate sizing is worth building into your dispatch routine.
Letter vs Large Letter: the first boundary
Before you even get to Large Letter versus Small Parcel, there's the Letter versus Large Letter line: a Letter must be under 5mm thick, 240mm long and 165mm wide, and under 100g. Anything over 5mm thick becomes a Large Letter. This first step catches thin but rigid items like a card in a stiff envelope.
How to measure reliably every time
A simple sizing guide or a set of cardboard templates for the common bands removes guesswork at the packing bench. Measure length, width and thickness at the widest points, weigh the finished parcel, and let Click & Drop price it from those real numbers. Consistency here is what keeps surcharges, covered in our surcharges guide, at zero.
What happens if you get the band wrong
If an item is sent in a band it doesn't qualify for, Royal Mail applies a surcharge for the difference rather than returning it. The recipient may be asked to pay before delivery, which is a poor experience, or the sender is billed. Either way, the cost of getting it wrong is higher than the cost of measuring it right the first time.
Frequently asked questions
What is the maximum thickness for a Royal Mail Large Letter? A Large Letter can be up to 25mm thick. Anything thicker becomes a Small Parcel, regardless of length, width or weight.
Is a Small Parcel priced by weight? No, within its limits a Small Parcel is priced as a band, so a 500g parcel and a 1.9kg parcel of the same dimensions cost the same.
What happens if I send a Small Parcel as a Large Letter? Royal Mail will apply a surcharge for the difference in postage, which the recipient may be asked to pay before delivery, or the sender is billed.
Right size, right label
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