Baby Growth & Diaper Size Calculator
Enter your baby's birth date, weight, and sex to find their current diaper size and see how their weight compares on the WHO growth chart.
Last updated
Used to match the correct WHO growth reference. "Prefer not to say" averages both.
How it works
Diaper size here is based on published weight brackets: Newborn up to 10 lbs, Size 1 from about 8-14 lbs, Size 2 from about 12-18 lbs, Size 3 from about 16-28 lbs, Size 4 from about 22-37 lbs, Size 5 from about 27 lbs and up, and Size 6 from about 35 lbs and up. Because brands publish overlapping ranges like this, the calculator picks a single primary recommendation at each weight rather than showing every size that technically fits, but it's worth checking your specific diaper brand's own chart too since sizing varies slightly brand to brand.
The growth percentile uses the WHO Child Growth Standards, the same weight-for-age (and length-for-age, if you enter a length) reference data most pediatricians use for children under 2. Your baby's weight is compared against the full distribution of babies the same age and sex to produce a percentile: the 50th percentile means exactly average for that age, while the 15th to 85th percentile range covers the middle, typical majority of babies.
For example, a 4-month-old girl weighing 14 lbs works out to about the 46th percentile for weight, just under average and well within the typical range, with a Size 1 as her primary diaper recommendation. Percentiles are calculated for ages 0 to 24 months; for older toddlers, diaper sizing still works but the percentile figure won't be shown. These percentiles are for general reference only. Always defer to your pediatrician's official growth chart at checkups, which tracks your baby's specific history over time rather than a single point-in-time snapshot.
Frequently asked questions
When should I size up diapers?
Size up once diapers start leaving red marks at the waist or legs, gapping at the legs, or your baby's weight is at the very top of the current size's range, whichever comes first. Weight is the best guide, but fit matters more than the number if a diaper looks tight or is leaking regularly.
Is my baby's weight normal for their age?
This calculator uses the same WHO growth reference data pediatricians use for children under 2, and reports where your baby's weight falls compared to other babies the same age and sex. A percentile in the wide 15th-85th range is considered typical; percentiles below the 3rd or above the 97th are usually flagged for a closer look at a checkup, not because they're automatically a problem, but because they're less common.
How often do babies change diaper sizes?
On average, babies move through 4 to 5 diaper sizes in their first two years as they gain weight steadily, though the exact timing varies a lot by baby. Fast-growing babies might size up every month or two early on, while growth, and diaper size changes, naturally slows down after the first year.
Why does diaper sizing use weight instead of age?
Diapers need to fit snugly around the waist and legs to work, and weight predicts fit far better than age does, since two babies the same age can weigh very different amounts. That's also why the size ranges overlap: a baby's shape and the brand's specific cut both affect which size fits best at a given weight.
What if my baby is between two diaper sizes?
Try the smaller size first if your baby is between two ranges, since a snug fit that doesn't leak matters more than extra room. If you're seeing leaks, gapping, or red marks with the smaller size, size up even if your baby's weight is still technically within the smaller size's range.
Are these growth percentiles the same as what my pediatrician uses?
The underlying data source, the WHO Child Growth Standards, is the same reference most US and international pediatricians use for children under 2. The results here are the same style of percentile you'd see on an official growth chart, but treat them as a reference between checkups, not a replacement for your pediatrician's own measurements and chart, which account for your baby's full history.
What if I skip entering my baby's sex?
Selecting "Prefer not to say" averages the boy and girl growth reference data for that age, which gives a reasonable general estimate but is less precise than selecting your baby's sex, since boys and girls grow at slightly different rates. Use it if you'd rather not specify, but expect the percentile to shift a few points either way from what a sex-specific chart would show.
Tracking growth between checkups? The Greater Goods Digital Baby Scale is a well-reviewed way to weigh your baby at home without a trip to the pediatrician's office.
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