Nappy Sizes by Weight: A Quick UK Guide

Part of our guide: Reusable vs Disposable Nappies: The Real Cost

By the BasketLab team · updated 27 July 2026 · how we rank

Nappies are sized by weight, not age, which catches a lot of new parents out. Two babies born the same week can easily be two sizes apart. The numbers on the packet are your guide, and the ranges overlap on purpose, so here's the rundown and how to know when you've got it wrong.

The size-by-weight rundown

These are the broad UK ranges most brands work to. Handy for knowing roughly where you are, but read the next bit before you take them as gospel.

• Size 1 (newborn): roughly 2 to 5kg, the size most babies come home in.
• Size 2: around 3 to 6kg, usually a few weeks in.
• Size 3: about 6 to 10kg, often the longest-lived size as they start crawling.
• Size 4: roughly 9 to 14kg, proper toddler territory.
• Size 5: around 11 to 16kg.
• Size 6 and up: 13kg and over, into potty-training age.

A quick caveat on the numbers

There's also a micro or size 0 below all this, for premature and very small newborns, usually under 2.5kg. And here's the important bit: those ranges above are ballpark. Pampers, by Amazon and the rest all set their bands slightly differently, so the weight printed on the actual pack always wins over any chart, including this one.

It's worth knowing that fit varies by brand shape, not just by the numbers. A baby with chunky thighs might sit better in one brand's cut than another's at the exact same weight, which is why it pays to try a small pack before you buy in bulk.

How to tell the size is wrong

Weight gets you in the right area, but the fit tells you the truth. Here's what the wrong size actually looks like.

• Red marks around the thighs or tummy after a change mean it's digging in, so you've likely outgrown it.
• Regular leaks up the back or out the legs, especially overnight, usually mean it's too small to hold a full load.
• Gaps around the legs, or a nappy that sags and slides down, means it's too big.
• Tabs you can only just do up, pulling right towards the middle, is another too-small giveaway.

When one leak isn't a verdict

A single leak proves nothing. Sometimes the nappy just wasn't done up snugly, or it was a big night. A pattern is what counts. If you're getting leaks night after night despite doing everything right, size up before you commit to another big pack.

One more thing, and this is the plain health note: if your baby seems to be moving through the sizes very slowly, or you're worried about their weight or growth generally, that's a conversation for your health visitor rather than the nappy aisle. They weigh babies at the clinic and can tell you whether things are tracking as they should. Nappy fit is a useful day-to-day clue, not a substitute for the red book and the scales.