Diapers are sized by weight, and the published ranges overlap so heavily that for most of the first two years there are genuinely two correct answers. Enter your baby's weight for the size to buy, the other size that also fits, and the signs it is time to move up.
Two sizes fit, and no chart tells you that
Look up a diaper size chart and you get a ladder of weight ranges. Look at it properly and you notice the ranges overlap. Size 2 runs about 12 to 18 lb and size 3 runs about 16 to 28 lb, so a 17 lb baby is squarely inside both, and the chart offers no opinion about which one you should actually buy.
That overlap is deliberate, not sloppy. Babies of the same weight are different shapes, and a manufacturer would rather two sizes work for you than none. But it means the honest answer to "what size does my baby wear" is usually two numbers, and every chart that prints one is quietly hiding the interesting part.
The rule this page uses
buy the LOWER size while your baby sits in the bottom three quarters of its range
move up once they reach the top quarter
then let leaks and red marks overrule both
A diaper at the very top of its range is about to be outgrown, and the tabs start landing wide before the weight actually crosses over. Going up too early costs you absorbency and buys leaks around the legs, which is the opposite of what you wanted.
Worked example
A baby at 17 lb is inside size 2 (12 to 18) and size 3 (16 to 28). At 17 they are in the top quarter of size 2, so the answer is size 3: the smaller one has about a pound left in it and you will be buying again in a fortnight. Now take 16 lb, which is also inside both. That is the very bottom of size 3 and the middle of size 2, so size 2 is the better buy and will fit more closely where it matters. One pound of difference, two different answers, and the chart alone cannot tell you either.
Age is the wrong input, and it is how everyone searches
Diapers are sized by weight and nothing else. There is no age printed on the pack, because a chunky four month old and a slim ten month old genuinely wear the same diaper. Buying for the birthday rather than the baby is the single most common way parents end up a size too small.
The age field above exists only to disagree with you. If the weight you entered sits outside the usual band for that age, the result says so and tells you to follow the weight. That is the useful thing an age chart can do: catch a mismatch. It is not the useful thing most of them try to do.
When to move up, in order of reliability
- Overnight leaks. The most reliable signal there is, and it usually arrives before the weight crosses into the next range. One bad night is a bad night; three in a week is a size.
- Tabs not reaching the middle. If you are fastening at the outer edge of the waistband to get it closed, the diaper is already too small.
- Red marks at the thighs or waist after a change. That is the elastic doing a job it was not cut for.
- Gapping at the legs, or sagging when dry. The opposite problem, and the reason to stay down rather than up. Leg gap is where leaks come from, so a bigger diaper is not automatically a drier one.
Do brands agree?
Close enough to plan with, not close enough to assume. Pampers and Huggies publish nearly identical ranges through the common sizes, which is why a general chart like this one is worth anything at all. Individual lines drift at the edges, store brands drift further, and the cut around the legs and waist varies more than the weight range suggests.
Every pack prints its own range. That number describes the diaper in your hand rather than the brand in general, and it beats this page or any other one.
Related sizing that also runs on measurement
The same principle runs through everything here: size by the measurement, not by the label or the age. For growing feet, kids shoe size conversion handles the US, UK and EU scales and the little-kid to big-kid jump that catches people out. For your own sizes from one set of measurements, What Size Am I covers dress, jeans, jacket, shirt and shoe together. And if you have ever wondered why a garment measurement never matches a body measurement, that gap has a name and a page: ease.
Frequently asked questions
What size diaper should my baby wear?
Whichever size their weight falls into, and often that is two sizes at once. Enter the weight above and you get the size to buy plus the other one that also fits. Do not size by age: a chunky four month old and a slim ten month old wear the same diaper, and buying for the birthday rather than the baby is the most common way parents end up a size too small.
Why does my baby's weight fit two diaper sizes?
Because the published ranges overlap on purpose. Size 2 runs about 12 to 18 lb and size 3 about 16 to 28 lb, so anything from 16 to 18 lb is genuinely inside both. That is not an error in the chart and you are not misreading it. Both sizes are real answers, and fit rather than weight is what separates them.
When should I move up a diaper size?
When the fit tells you, not when the scale does. Move up if you are getting overnight leaks, if the tabs no longer reach the middle of the waistband, or if there are red marks at the thighs or waist when you change. Leaks are the most reliable signal and they usually arrive before the weight crosses into the next range.
What weight is each diaper size?
Newborn is up to about 10 lb, size 1 is 8 to 14, size 2 is 12 to 18, size 3 is 16 to 28, size 4 is 22 to 37, size 5 is 27 lb and up, size 6 is 35 and up, and size 7 is 41 and up. Preemie sizes sit below newborn and are a clinical size rather than a step on the ladder. Pampers and Huggies agree on most of this, and individual lines drift at the edges.
Are Pampers and Huggies sizes the same?
Close enough to plan with, not close enough to assume. The two brands publish nearly identical weight ranges through the common sizes, but specific lines differ at the margins and cut differently around the legs and waist. The range is printed on every pack, and that number describes the diaper in your hand rather than the brand in general.
Should I size up for overnight?
Many parents do and it usually works, because the extra size buys absorbency for a twelve hour stretch. The trade is a looser fit at the legs, which is where leaks come from, so it helps some babies and makes things worse for others. If you are already at the top of your current size, an overnight-specific diaper in your normal size is the safer first thing to try.
How do I weigh my baby at home?
Stand on the bathroom scale, note the number, then weigh yourself holding the baby and subtract. It is accurate to within a pound or so, which sounds rough but sits comfortably inside the overlap between two diaper sizes. You do not need a baby scale for this decision.
Is my baby too big for their diaper size?
Check three things rather than the number. The tabs should reach the middle of the waistband with room to spare, there should be no red marks at the thighs or waist after a change, and the diaper should hold an overnight stretch without leaking. Fail any of those and go up, even if the weight says you have room left in the current size.