How to Choose the Right Nappy Size (And When to Size Up)

If you’re dealing with mystery leaks, midnight outfit changes, or angry red marks around tiny thighs, chances are the issue isn’t your baby. It’s the nappy size.

Choosing the right nappy size is one of the easiest ways to improve comfort, reduce leaks, and keep baby skin happy. And despite what the packet might suggest, it’s not about age, it’s all about fit.

Nappy Sizes: How They Actually Work

Nappy sizing is based on weight ranges, not age. Two babies the same age can be in completely different sizes depending on their shape, build and how fast they’re growing.

A handy rule of thumb?

If your baby is creeping towards the top of the weight range on the pack, it’s usually time to try the next size up.

Signs It’s Time to Size Up

If any of these sound familiar, sizing up the nappy is often the fix:

·         Leaks or blowouts happening more often

·         Red marks around the waist or legs

·         Tabs that barely reach across the front

·         A nappy that feels full very quickly

Many parents are surprised to find that going up a size solves leaks almost instantly, even if their baby technically still fits the smaller size on paper.

Fit Beats Absorbency Every Time

A well-fitting nappy should:

Sit comfortably around the waist without digging in

Seal neatly around the legs with no gaps

Stay in place while your baby wriggles, rolls and crawls

Even the most absorbent nappy can leak if the fit is off.

What About Night-Time Leaks?

If leaks are mostly happening overnight, sizing up can help…but so can switching to a dedicated overnight nappy. These are designed to hold more liquid over longer stretches of sleep (and fewer 3am pyjama changes is always a win).

Final Tip

If you’re unsure, grab one pack in the next size and see how it goes. Many families briefly use two sizes during growth spurts, totally normal and very practical.

Quick FAQs

Can a nappy be too big?

Yes. If it gaps around the legs, leaks are more likely.

Should I size up just for nights?

Often yes, especially for heavier wetters or older babies.