Baby growth percentile calculator

Pick US units (lb, oz, inches) or metric (kg, cm); the values you've typed convert when you switch. Choose your baby's sex and date of birth. If your baby was born before 37 weeks, enter the week of birth as well. Then enter the date of the measurement and any of the three measurements: weight, length (measured lying down, as at the pediatrician's office) and head circumference.

For each value you get:

• the percentile and which band it falls in
• the z-score (how many standard deviations from the median)
• the median for that age and sex, so you can see the middle of the range

The page also shows your baby's age at the measurement in weeks and months, plus the corrected age for babies born early. Tap Save this measurement to add it to the growth chart and to a table of saved visits. The chart shows the 3rd, 15th, 50th, 85th and 97th percentile lines for weight, length or head size.

The math runs in your browser, so nothing is uploaded. Saved measurements stay in this browser's local storage until you delete them or clear your browsing data.

A percentile compares your baby with 100 healthy babies of the same age and sex. The 50th percentile is the median: half are smaller, half are larger.

Example: a girl who is exactly 6 months old weighs 14 lb 8 oz (6.58 kg). On the WHO weight-for-age chart, that's about the 20th percentile: roughly 20 out of 100 girls her age weigh less. The median for her age is about 16 lb 1 oz. If she's 26 in long (66 cm), her length is around the 55th percentile. A long, lean baby like this is common and usually nothing to worry about.

Band	What the calculator says
Below 3rd	Below the 3rd percentile
3rd–15th	Lower range
15th–85th	Where most babies are
85th–97th	Upper range
Above 97th	Above the 97th percentile

A high or low percentile isn't good or bad on its own. What your pediatrician watches is whether your baby keeps following a similar curve over time.

The calculator uses the WHO Child Growth Standards for birth to 24 months. They're based on healthy, breastfed children growing in good conditions, so they show how babies grow when their needs are met. The CDC recommends the WHO charts for children under 2 in the United States, and its own CDC growth charts from age 2 onward. That's why this tool stops at 24 months.

Under the hood, each percentile comes from the WHO's LMS method: for the baby's age, the table gives three values (L, M and S), and the z-score is z = ((X/M)^L − 1) / (L × S), where X is the measurement. The z-score is then turned into a percentile. Values between monthly table rows are interpolated by age in days, and very high or very low weights use the adjusted calculation the WHO specifies.

If your baby was born before 37 weeks, growth is compared using corrected age until 24 months: the calculator subtracts the weeks your baby arrived before 40 weeks.

Example: a girl born at 32 weeks is now 6 months old (183 days) and weighs 13 lb (5.9 kg). She was born 8 weeks early, so her corrected age is about 4.2 months. At that age, 13 lb is around the 21st percentile. Using her calendar age instead would put her near the 4th percentile, which would look far more worrying than it is.

The same idea applies to developmental milestones: preemies are usually judged by corrected age in the first two years.

Show the results to your pediatrician if a value is below the 3rd or above the 97th percentile, or if your baby's curve starts crossing percentile lines from one visit to the next. Plenty of healthy babies sit near the edges, so a single number isn't a diagnosis. Your doctor looks at the trend, your baby's feeding and overall health, and the parents' size.

Before you worry, double-check the basics: the units (pounds vs. kilograms is the most common mix-up), the date of birth, and whether length was measured lying flat. Home scales and tape measures are less precise than clinic equipment.

Keeping a baby tracker of feeds and wet diapers gives your pediatrician useful context alongside the growth curve. Curious about later growth? Try the height predictor.

This calculator is informational and isn't medical advice.