The Nightstand · runs in your browser

Diaper Size Calculator

Put in your baby's weight and this shows every size band she currently falls inside — usually two, sometimes three — drawn on one axis so you can see how far they overlap. Brands disagree with each other, so the answer is a shortlist and your own brand's chart is the tiebreaker.

No account, no email, and nothing you type is sent anywhere.

Sizes she is currently inside
Size 1 8–14 lb Size 2 12–18 lb
Usually two, sometimes three. That is not a failure of the estimate — the bands genuinely overlap, and where they do, the smaller size tends to fit better by day and the larger one holds more overnight.
Changes a day
6 to 12
The NHS puts young babies at as many as 10 or 12 and older babies at around 6 to 8, changed as soon as they are wet or dirty rather than on a schedule.
Wet ones that matter
4 to 6+
The NHS looks for around 6 wet nappies a day from a few days after birth; the AAP treats at least 4 a day as generally reassuring.
Every band on one weight axis
Preemie
Newborn
Size 1
Size 2
Size 3
Size 4
Size 5
Size 6

The pale line is your baby, drawn straight through every band at once. At 13.2 lb (6 kg) she is inside 2 of them. Nobody sets these ranges: there is no standard for infant nappy sizing and no health body publishes one, so every manufacturer picks weights to suit its own cut. Two sizes that overlap by six pounds are two sizes that will both more or less work, and the chart on the pack you are actually buying beats this one.

What actually decides it

None of this is arithmetic, and all of it outranks the chart above. Tick anything you are seeing.

Nothing ticked, so the chart above stands on its own. The NHS’s own instruction is to adjust the nappy to fit snugly round the waist and legs — snug, not tight, and a leak is far more often a size too small than a size too big.

Whose numbers these are. Not the NHS’s, not the American Academy of Pediatrics’, not the WHO’s and not the product-safety regulator’s — no allowed publisher states a figure for nappy sizing, because none of them sizes nappies. The bands above are compiled from the weights manufacturers print on their own packs and rounded, so they are an estimate of an industry convention rather than guidance. The range on the pack in your hand is the one that counts, and it will not be identical to this.

What this page refuses to do. It will not print one size, because at almost every weight the honest answer is two. It names no brand and recommends none: the differences between them are real, but nothing on this site takes a position on which cut suits your baby, and there is nothing here to buy. It does not ask for an age, because a weight-for-age chart is a spread rather than a line and any months-to-sizes table has quietly thrown that away.

The number that matters more than the size. How many wet nappies a day is a real signal and the size on the pack is not. The NHS looks for around six a day from a few days after birth, with clear or pale yellow urine, and at least one poo a day early on; the AAP treats at least four wet a day as generally reassuring and notes the count can halve in hot weather or illness and still be normal. If the count drops well below that, the urine is dark, weight gain stalls, or your baby is hard to rouse for feeds, call your pediatrician or health visitor. Separately and always: babies sleep on their back, on a firm flat surface, in a cot clear of blankets, pillows, bumpers and soft toys, with nothing weighted on or over them. None of this is medical advice.

How this is worked out

  1. You give it one thing: your baby's weight, in kilograms or pounds. It is converted and read against the size bands in the browser, and nothing you type is sent anywhere.
  2. Where the bands come from, plainly: not from the NHS, the American Academy of Pediatrics, the WHO or the product-safety regulator, because not one of them publishes a nappy size chart. The ranges here are compiled from the weights manufacturers themselves print on their packs, rounded to the nearest sensible pound and kilogram. They are a summary of an industry convention, not guidance, and this page says so in the drawing rather than in a footnote.
  3. The bands overlap on purpose and the chart shows it. A size 2 and a size 3 share several pounds of range in almost every brand, which is why a baby can be genuinely between sizes for a month. The answer you get is therefore every band your weight falls inside, in order, rather than one confident number.
  4. Age is deliberately not an input. The WHO publishes weight-for-age as percentile charts from birth to five years precisely because there is a spread of normal weights at every age, so a six-month-old can be in three different sizes depending on which six-month-old she is. Any chart that maps months to sizes has thrown that away.
  5. The fit checklist is the part that actually decides it, and none of it is arithmetic. Red marks, leaks up the back at night, tabs pulled to their last position and a waistband you can no longer get two fingers under all say size up regardless of where the weight lands on the chart above.
Questions

The short answers

Usually one of two, and the chart above will tell you which two. Weigh your baby, read across, and you will normally find her sitting inside two neighbouring bands with a few pounds of overlap between them — that overlap is real and deliberate, not a rounding error. When she is in the overlap, the smaller size is usually the better fit during the day and the larger one holds more overnight, which is why a lot of families run two sizes at once for a few weeks and stop feeling silly about it.
Because nobody sets it. There is no standard for infant nappy sizing and no health body publishes one, so each manufacturer picks its own weight ranges to suit its own cut. The practical effect is that a size 4 in one brand can fit like a 3 in another, and a baby who leaks in one brand at the top of a band may be perfectly dry in a different brand at the same weight. Always check the chart on the pack you are actually buying — this page is a starting point and your brand's own numbers beat it.
When the fit tells you, not when the weight does. Move up if you are getting leaks up the back or out of the legs at night, if the tabs are on their last position and the waistband is tight, if there are red marks around the thighs or waist after a change, or if the nappy no longer covers as much of the bottom as it did. Weight is a rough sorter; those five things are the evidence. Going up a size when a nappy is leaking is more often the fix than going down.
The NHS puts young babies at as many as 10 or 12 changes a day, dropping to around 6 to 8 for older babies, and the rule underneath it is to change as soon as they are wet or dirty rather than on a schedule, because sitting in it is what makes skin sore. The chart above shows the figure for your baby's weight band as a range, and a range is all it can honestly be — a newborn who feeds every two hours and a nine-month-old on solids do not produce the same day.
This is the one number on the page that is genuinely worth watching. The NHS looks for around six wet nappies a day from a few days after birth, with clear or pale yellow urine, and at least one poo a day in the early weeks. The AAP puts a newborn anywhere between peeing every one to three hours and four to six times a day, and treats at least four wet nappies a day as generally reassuring. If the count drops well below that, if the urine is dark, or if your baby is hard to rouse for feeds, call your pediatrician or health visitor. That is a judgement about your own baby, and nothing on this page is medical advice.

Where this comes from

  1. NHS (2024). How to change your baby's nappy. https://www.nhs.uk/baby/caring-for-a-newborn/how-to-change-your-babys-nappy/
  2. American Academy of Pediatrics (HealthyChildren.org) (2024). Baby's First Days: Bowel Movements and Urination. https://www.healthychildren.org/English/ages-stages/baby/Pages/babys-first-days-bowel-movements-and-urination.aspx
  3. NHS (2023). Formula milk: common questions. https://www.nhs.uk/baby/breastfeeding-and-bottle-feeding/bottle-feeding/formula-milk-questions/
  4. World Health Organization (2026). Child growth standards: weight-for-age. https://www.who.int/tools/child-growth-standards/standards/weight-for-age
  5. American Academy of Pediatrics (HealthyChildren.org) (2022). How to Keep Your Sleeping Baby Safe: AAP Policy Explained. https://www.healthychildren.org/English/ages-stages/baby/sleep/Pages/A-Parents-Guide-to-Safe-Sleep.aspx
  6. NHS (2025). Reduce the risk of sudden infant death syndrome (SIDS). https://www.nhs.uk/baby/caring-for-a-newborn/reduce-the-risk-of-sudden-infant-death-syndrome/

Every link above was opened and read when this page was last updated. Snuggle is not affiliated with any of these organisations and none of them has reviewed this page. Nothing here is medical advice.

Go deeper

The reading behind this page

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The app this site is for

Snuggle does this with your baby’s own pattern.

This page works from an age band, because an age band is all a web page knows about you. Snuggle starts from the same bands and then blends them with what your baby actually did today — how long she lasted, when she went down, how the last nap ran — and shows on the face of the ring how sure it is. Early estimates say they are early.

It logs sleep, feeds, nappies and growth in one tap on a dim red screen, and every caregiver in the household is on the same timeline in their own account. The arithmetic runs on the phone, on your baby’s own history. There are no AI features in it.

In development for iPhone and Android. Not released yet — when it is, it will be on this site.

What Snuggle is