The Ultimate Parent’s Guide to Nappy Fit
If you’ve ever stood in the baby aisle holding two nappy packs and wondering “Is it time to move up a size?” — you’re not alone.
Babies grow fast, and nappies that fit perfectly one week can suddenly start leaking the next. One day everything feels snug and secure, the next you’re dealing with outfit changes, irritated skin, and disrupted sleep.
The good news? Your baby will usually give you clear signals when it’s time to size up. In this guide, we’ll walk you through the most common signs a nappy is too small, plus answer frequently asked questions from parents — so you can feel confident you’re making the right choice.
How Do You Know When It’s Time to Size Up a Nappy?
Nappy sizes are usually based on weight ranges, but every baby is built differently. Rather than relying on numbers alone, it’s far more helpful to look at fit, comfort, and performance.
Here are the key signs to watch for.
1. Leaks Are Happening More Often
An increase in leaks is often the first clue.
If your baby’s nappy feels very full after short periods, leaks during naps or overnight, or you’re changing clothes more often than nappies — the nappy may no longer be holding enough.
As babies grow, they produce more urine and move more. A bigger nappy usually means better absorbency and improved leg and waist coverage, helping to prevent leaks and keep skin dry.
2. Red Marks Around the Waist or Legs
Check your baby’s skin during changes. If you notice:
✔️ Red lines around the tummy or thighs
✔️ The nappy leaving marks that take time to fade
✔️ Skin looking tight, warm, or irritated
…it’s likely the nappy is too snug.
A well-fitting nappy should sit comfortably without digging in. Sizing up often improves airflow and reduces friction, which can make a big difference to your baby’s comfort.
3. The Fit Just Doesn’t Look Right
Sometimes, it’s simply visual.
A correctly fitting nappy should:
✔️ Sit just below the belly button
✔️ Be snug around the legs without gaps
✔️ Fully cover the bottom
✔️ Allow free movement for rolling, crawling, and play
If the nappy looks low at the back, tight at the front, or needs constant adjusting, it’s probably time to try the next size.
4. The Tabs Are Hard to Fasten
When fastening a nappy, the tabs should close comfortably near the centre of the waistband.
If you’re:
✔️ Pulling hard to make the tabs reach
✔️ Finding they sit right at the edges
✔️ Re-fastening frequently to keep it in place
…that’s a strong indicator the nappy is now too small.
Moving up a size usually results in a more secure fit and fewer leaks.
5. Your Baby Is More Active (or Showing Potty Interest)
As babies become more mobile — crawling, cruising, walking — their nappy needs change.
Many parents find that:
✔️ Bigger sizes fit moving bodies better
✔️ Nappy pants or pull-ups are easier for quick changes
✔️ Older babies enjoy practising independence
There’s no “right age” to change styles, but increased movement often goes hand-in-hand with needing more flexibility and room.
A Simple Rule: When in Doubt, Size Up
If your baby is nearing the top of the weight range for their current nappy — or showing several of the signs above — moving up a size is usually the best option.
A slightly bigger nappy is almost always:
✔️ More comfortable
✔️ More absorbent
✔️ Less likely to leak
Quick Checklist: Signs a Nappy Is Too Small
✔️ More frequent leaks
✔️ Red marks on tummy or thighs
✔️ Tight or awkward fit
✔️ Tabs don’t meet comfortably
✔️ Increased movement or crawling
If you’re ticking two or more, it’s time to size up.
Nappy Size FAQs
Aim for snug, not tight.
A helpful tip is the two-finger rule:
✔️ You should be able to slide two fingers comfortably under the waistband and leg cuffs
✔️ If you can’t, it’s too tight
✔️ If there are gaps, it’s too loose
Yes — ideally the tabs should fasten around the centre of the waistband.
If they’re stretched to the edges or difficult to secure, the fit is too small.
For most standard nappies, no. The waistband should sit just below the belly button, allowing room for movement while staying secure.
Most parents start with newborn or size 1 nappies, depending on expected birth weight.
Babies grow quickly in the early weeks, so it’s usually best not to over-stock one size.
There’s no fixed timeline. Some babies move up within weeks, others stay longer. Watch for fit and comfort rather than age — babies grow at their own pace.
Many children around 12 months are in size 4, but this varies widely depending on build and activity level.
Use the same signs:
✔️ Leaks
✔️ Red marks
✔️ Tab position
✔️ Comfort during movement
You Don’t Have to Figure It All Out Alone
From nappy changes and sleep routines to independence and toilet learning, early parenthood comes with a lot of questions — and that’s completely normal.
Experienced childcare professionals change nappies every day and are well-placed to notice when a child may need a different size or style. Open communication between parents and caregivers helps ensure children stay comfortable, confident, and happy through every stage of development.
If you ever feel unsure, trust your instincts — and remember, you’re doing a great job 💛
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