Diaper Size Calculator by Baby's Weight
Enter your baby's weight in pounds to see which diaper size fits now, the range that size covers, and roughly when you will move up to the next one.
Quick samples
Enter a weight to see the suggested size.
Add your baby’s weight in pounds, or try a quick sample, and the matching diaper size appears here.
Size weight ranges are general brand-agnostic guides. The right test is fit: two finger-tips under the waistband, no red leg marks, and no blowouts. Always check the weight range printed on the pack.
The formula
Diaper size = the size band whose weight range contains your baby's current weight
- Weight band — The span of pounds a given diaper size is designed to fit, printed on every pack.
- Overlap — The pounds where two adjacent sizes both fit, which is why brand charts differ at the edges.
- Size up — Moving to the next size when your baby nears the top of the current band or leaks.
- Fit check — The physical test — waistband comfort, leg marks, containment — that outranks the number.
Typical diaper size by weight
| Size | Common weight range | Typical ages |
|---|---|---|
| Newborn | Up to about 10 lb | 0–1 months |
| Size 1 | 8–14 lb | 0–4 months |
| Size 2 | 12–18 lb | 3–8 months |
| Size 3 | 16–24 lb | 5–24 months |
| Size 4 | 22–37 lb | 18 months–3 years |
| Size 5 | 27–44 lb | 2–5 years |
| Size 6 | 35 lb+ (or pull-ups) | 3+ years |
Ranges are general, brand-agnostic guides. The exact weight limits differ by brand and are printed on each pack, so always confirm against the pack you buy.
Choosing a diaper size is really a weighing exercise disguised as an age one. Age gives a rough clue, but the fit that stops leaks comes from matching your baby's weight to the range printed on the pack. This calculator does that matching for you: enter the pounds, and it returns the size band that fits right now.
It also shows the weight span for that size and where your baby sits inside it, so you can see whether you have room to go before the next size, and roughly when to expect to move up. Add a birth date and it cross-checks the size against age in months, because the two do not always agree.
Size charts run slightly differently at the edges between brands, so treat the answer as a well-informed starting point rather than a fixed rule. The deciding test is always fit on the baby: a comfortable waistband, no deep red marks, and no blowouts at the leg or back.
How diaper sizes work
Every diaper size covers a band of weight, not a single point, because a one-pound difference should not change what fits. Newborn typically runs to about ten pounds, size one roughly eight to fourteen, and the numbers climb from there with overlapping edges so a baby near a boundary can wear either size comfortably.
The overlap is deliberate and is also why two brands can disagree. A baby at fifteen pounds might be a snug size three in one brand and a roomy size two in another. Neither is wrong; the ranges are approximate and shaped by each brand's cut, waistband and leg elastic.
That is why weight leads and age follows. A bigger three-month-old can outgrow newborn before a smaller one ever wears it, so buying a heap of newborn sizes for an expected due date can leave them unused. The calculator keys off real weight, which is the number that actually determines fit.
The size chart, and where your baby fits
Broadly, newborn covers up to about ten pounds, size one eight to fourteen, size two twelve to eighteen, and size three sixteen to twenty-four. Sizes four, five and six then carry a baby from the twenties up into preschool, often as pull-up style training pants rather than taped diapers.
The calculator maps your baby's weight to that ladder and highlights the band they land in. Because the ranges overlap, it uses the first band whose upper edge is above your baby's weight, then tells you the span so you can see how much headroom is left.
It also shows where inside the band your baby sits. Near the bottom means the size will last months; near the top means you should expect to move up soon, which is useful before you commit to a bulk box of a size they are about to outgrow.
A worked example
A carer weighs a four-month-old at about fifteen pounds and enters fifteen. The tool suggests size three, whose common range is roughly sixteen to twenty-four, and notes the baby is right at its lower edge with a whole band of room ahead.
It reads that as: size three is the move now, and they will likely stay in it for months as they grow toward the twenties. If a snug size two still fits without leaking, that is also a valid in-between choice for a few more weeks.
She can then preview the next step. The tool shows size four begins around twenty-two pounds, so she knows roughly how much growing is left in size three before the next change, and buys accordingly rather than over-stock.
Fit signs that beat any chart
The number is a starting point; the body is the real test. You should be able to slide two finger-tips under the waistband once fastened, so it is snug but not digging in. If the elastic leaves a deep red line that stays after a few minutes, the size is too small.
Leaks are the other tell, and they point in two directions. Blowouts up the back or leaks at the leg often mean the diaper is too small to contain, or too big and gaping. A leg mark that is faint and even is normal; one that is red and indented is not.
Shape matters as much as scale. A long, thin baby and a stocky one of the same weight can need different sizes, and some brands suit chunkier thighs better than others. If fit seems off despite the weight matching, try a different brand before assuming you need a whole size up.
- Two finger-tips should fit under the fastened waistband.
- Faint even leg marks are fine; deep red indentations mean size up.
- Persistent blowouts usually mean too small, not too big.
Diapers versus pull-ups
As babies become mobile, taped diapers are often swapped for pull-up style training pants, usually somewhere in the size four to five range. Pull-ups come in the same weight bands, so this calculator applies to them too; the difference is how you put them on, not how they are sized.
Pull-ups trade a little absorbency capacity and adjustability for the ability to stand and step into them, which suits a wriggling toddler. Because they cannot be re-fastened at the waist, getting the size right matters even more, or gaps appear at the legs.
Many families use both: taped diapers overnight or for long stretches when maximum containment helps, and pull-ups during the day for ease. Weight decides the size in either format, so you can move between them without changing what you buy.
When to size up or down
Size up when your baby is at or above the top of the current range, when the waistband leaves lasting marks, or when leaks have become routine despite correct fastening. A fresh size usually solves all three at once because it adds both length and waistroom.
Size down, rarely, if a diaper gapes at the legs even when snug at the waist, which happens with slender babies in a wide-cut brand. That is usually more a brand-shape problem than a true size one, so comparing cuts across brands often helps before you change size.
As a rough running speed, most babies pass through newborn quickly, spend a few months each in the early sizes, and slow down in size four and five. Knowing a size is short-lived is the practical gift of the calculator: buy those early boxes small.
Key terms
- Weight band
- The span of pounds a diaper size is made to fit, printed on every pack.
- Overlap
- The weight range where two sizes both fit, which is why brand charts differ at the edges.
- Blowout
- A leak up the back, usually a sign the diaper is too small or wrongly fastened.
- Pull-ups
- Step-on training pants in the same weight sizes as diapers, common from around size four.
Pro tips
- Buy the newest, smallest sizes in small quantities; babies outgrow them fast.
- Read the weight range on each pack; brands cut sizes slightly differently.
- Sizing up fixes most leaks and red marks; check fit before blaming the brand.
Common mistakes
- Stocking up on newborn sizes before knowing baby's birth weight.
- Choosing a size by age when weight points to a different band.
- Ignoring lasting red marks or repeat leaks, which mean the size is wrong.
Diaper sizing — your questions
How do I know which diaper size my baby needs?
Match your baby's current weight to the range printed on the pack. Weight decides fit far more reliably than age, because babies of the same month can be several pounds apart. Enter the pounds in this calculator to see the matching size band, then confirm on your baby: a comfortable waistband, faint leg marks at worst, and no routine leaks mean you have chosen correctly.
Should I go by weight or by age?
Weight first, age as a loose cross-check. Diaper sizes are defined by pounds, so a chart that says size three from about five to twenty-four months really means once a baby weighs roughly sixteen to twenty-four pounds. Big babies move up early and small ones later. Add a birth date here and the tool shows both so you can see when weight and age disagree.
How do I tell if the size is too small?
The clearest signs are a waistband that leaves a deep red line that lingers, leaks or blowouts that have become routine, and tabs that no longer reach comfortably around the waist. A baby who is at or just above the top of a size's weight range is also due to move up. If two finger-tips will not slide under the fastened waistband, it is too snug and the next size up should help.
My baby is between two sizes — which should I pick?
Because adjacent sizes overlap, a baby near a boundary genuinely fits either, so choose on fit rather than the number. If leaks and marks appear in the smaller size, move up. If the larger size gaps at the legs, stay put a little longer. Brand cut matters here, so it is worth trying both brands before assuming you need a whole size change.
Do boys and girls need different sizes?
No — diaper sizing is unisex and based only on weight, so the calculator gives the same size for a boy and a girl at the same pounds. Average growth differs a little between sexes, but not enough to change which band fits. The sex field here is optional and does not alter the size; it exists only so you can think about build and where a boy or girl may sit within a band.
When do babies move to pull-ups?
Many families switch to pull-up style pants when a baby can stand and cooperate, often somewhere around size four or five, though there is no fixed age. Pull-ups come in the same weight bands, so this calculator applies to them too. Because they cannot be re-fastened at the waist, matching the size to your toddler's weight matters even more to prevent gaps and leaks during the day.