Diaper Sizes: When to Size Up
Size up when you see a pattern over a few days: the tabs no longer reach comfortably, the elastic leaves marks or chafing that keep coming back, the diaper doesn't cover her bottom, or leaks keep happening with a good fit and changes on time. The next size should still seal at the legs and waist, and if there's a gap, it's too soon.
Diaper sizes overlap, they don't match from one brand to the next, and often the only way to know is to try. A size 3 in one brand is a size 2 in another, and the weight on the packet is a best guess by somebody who has never met your baby.
Charles is one and has worn three brands. At the same age, Richard didn't suit any of them. Same parents, same flat, very different thighs.
The signals that matter
None of them is the number on the box. You can check all of them on her body in about ten seconds.
| Size | Weight range | Typical stage |
|---|---|---|
| Newborn | Up to ~5kg (11lb) | First 2-4 weeks; some babies skip it |
| S / Size 1 | ~4-8kg (9-18lb) | First month or two |
| M / Size 2 | ~6-11kg (13-24lb) | Longest-worn size of the first year |
| L / Size 3 | ~9-14kg (20-31lb) | Around the time they start moving with intent |
| XL and up | 12kg+ (26lb+) | Pants once she's standing, and no more tapes |
| Signal | Too small | Too big |
|---|---|---|
| Marks on the skin | Indents or chafing that keep coming back | None |
| Tabs | Fastening at the very edge | Crossing over toward the middle |
| Waistband | Digging in; no finger room | More than two fingers' slack; visible gap |
| Leaks | A flood; diaper heavy and saturated | A trickle; diaper still feels light |
| Rise | Sits below the belly button | Rides up toward the ribs |
| Leg cuffs | Cutting into the thigh crease | Loose enough to see a gap at the groin |
Leaks that keep coming with a heavy diaper point to too small, if the positioning is right and the changes are on time. Leaks with a diaper that still feels light point to too big. One leak on its own tells you very little.
Here are the checks for sizing up. No single one settles it, so look for a pattern over a few days.
- The tabs no longer reach comfortably.
- The waist or leg elastic leaves an indent or chafing that keeps coming back.
- The diaper doesn't cover her bottom properly.
- Leaks keep happening even with the cuffs pulled out, the back positioned right and changes on time.
- She's outside the maker's weight range for the size.
- The next size up gives a secure seal at the legs and waist, without gaps.
Sizing up isn't a cure for leaks on its own. If the leg openings end up too big for her thighs, a larger size can leak more, not less. Frequent blowouts up the back can also mean the size is wrong, though more often the cause is fit and technique, and there's a guide on that below. A diaper that's too big looks different. It rarely leaves a mark. It gaps.
Why the weight ranges overlap
Babies aren't cylinders. Two babies at identical weights can have entirely different proportions, a long torso with slim thighs, or the reverse. The fit that counts is around the thigh and across the rise, neither of which tracks neatly with mass.
The overlap is the makers allowing for this. When your baby is in the shared zone between two sizes, the right size is whichever one leaves no marks and no gaps. The weight ranges are the makers' own, and they don't predict how long a size will fit. If she's near the top of one and the checks above say it's snug, try the next size and see whether it seals without gaps.
What the wrong size looks like
How the sizes usually run
Use these as a rough guide, because every brand cuts its sizes a little differently. For reference, the maker's ranges for our Little Rei tape are Newborn 2 to 5kg, S 4 to 8kg and M 6 to 11kg, with pants in M and L up to 14kg. The fit at the thigh is what you judge on.
Newborn runs to roughly 5kg, and many babies spend two to four weeks there while some skip it altogether, so you buy one small pack and no carton. S, or size 1, covers roughly 4 to 8kg and usually the first month or two.
M, or size 2, covers roughly 6 to 11kg and is typically the longest-worn size of the whole first year. L, or size 3, runs roughly 9 to 14kg and usually arrives around the time she starts moving about on purpose. Charles moved into L at about ten months, though I may be a few weeks out.
XL and beyond starts around 12kg, and by that point you'll be buying pants, because fastening tapes on a toddler who's standing up is a battle I lost many times with Richard.
Should I buy the big box?
The per-piece price on a carton is better and it's the right choice for the size your baby is settled in the middle of. It's the wrong choice for newborn and size 1, where a growth spurt can strand you with three hundred diapers your baby outgrew on Tuesday.
A sensible approach is small packs until she's past newborn and clearly established in S or M, then a carton in the size she's in the middle of, and one small pack of the next size up kept in the cupboard. Growth spurts come without warning.
One more tip: buy the next size before you run out. Nobody enjoys a ten o'clock trip to a 24-hour pharmacy for a size you knew last week you'd need.
Mentioned above
Related decisions
Sources: NHS: How to change your baby's nappy · NHS: Nappy rash
This guide is general information, written to help you understand a topic and ask better questions. It is not medical advice and does not replace your own doctor, midwife or paediatrician. Anything that worries you about your baby belongs with them.
Read next: Why Does My Baby's Diaper Keep Leaking? · Tape Diapers vs Pull-Up Pants: When Should You Switch?
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