Baby Nappy Size Guide UK: Find the Right Nappy Size by Age and Weight
, by Sleepy UK, 2 min reading time
A complete UK nappy size guide by weight, from newborn to toddler. Includes the full Sleepy size chart and how to tell when it's time to size up.
Why getting the right nappy size matters
The right nappy size is the difference between a happy, dry baby and constant leaks, blowouts and red marks. Too small and the nappy digs in and overflows; too big and gaps let everything escape. The good news: nappy sizing is based on your baby's weight, not their age, so once you know the weight you can pick with confidence. Babies grow at very different rates, so two children of the same age may need different sizes.
This guide walks through every size in the Sleepy baby range, with the exact weight range for each, plus how to tell when it's time to size up.
Sleepy nappy size chart (by weight)
Sleepy Bio Natural nappies run from newborn all the way to toddler, so you can stick with one trusted brand as your baby grows:
- Size 1 – Newborn (2–5 kg): for the very first weeks. Try the Sleepy Size 1 Newborn nappies.
- Size 2 – Mini (3–6 kg): for growing newborns who aren't quite ready for midi.
- Size 3 – Midi (4–10 kg): the long-stay size most babies spend months in.
- Size 4 – Maxi (7–16 kg): for active, crawling babies. The Sleepy Size 4 Maxi comes in a large value pack.
- Size 5 – Junior (11–20 kg): for toddlers on the move.
- Size 6 – X Large (16–27 kg): for bigger toddlers.
- Size 7 – XX Large (20–30 kg): the largest size, ideal towards potty training.
You'll notice the weight ranges overlap. When your baby's weight falls into two sizes, other signs decide it — read on.
How to tell if the nappy size is right
Use these quick checks:
- The finger test: you should be able to slide a finger comfortably under the waistband. If it's tight, size up.
- Red marks: marks on the tummy or thighs mean the nappy is too small.
- Frequent leaks: regular leaks often mean the nappy can't hold enough — a sign to go up a size.
- Coverage: the nappy should sit just below the belly button and fully cover the bottom.
When to move up a size
Common signs it's time to size up include leaks (especially overnight), red marks, the tabs reaching further round than before, or the nappy looking visibly small. Many parents use a slightly larger size at night for extra capacity during longer stretches of sleep.
A gentle routine from top to bottom
Whatever size you're on, pair every change with a gentle wipe. For the early weeks, our guide to the best baby wipes for newborns is a helpful read, and for the wider range see our complete baby wipes guide for 2026. With the right size nappy and a soft wipe, changing time stays quick, clean and comfortable.