About the Baby Growth Percentile Calculator
This baby percentile calculator shows where your baby’s measurements fall on the World Health Organization growth charts. Enter your baby’s sex, age in months and any of weight, length and head circumference — in metric or US units — and it returns the percentile and z-score for each, plus the WHO 3rd to 97th percentile values for that exact age so you can see the typical range.
A percentile compares your baby with a reference group of healthy, breastfed children of the same age and sex. The 60th percentile for weight means that 60% of babies in the WHO standard weigh less and 40% weigh more. The CDC and AAP recommend the WHO standards for all children under 2 in the United States, and they are used in most countries worldwide.
No single percentile is “good” or “bad”. What matters to paediatricians is steady growth along a similar curve over time and proportion between measurements. Use this tool to understand numbers from a checkup; sudden changes across two major percentile lines are worth discussing with your baby’s doctor. For premature babies, use their adjusted age.
How to use the baby growth percentile calculator
- 1Choose your baby’s sex and enter their age in months (adjusted age for preemies).
- 2Pick metric or US units.
- 3Enter weight, length and head circumference — set any you don’t have to 0.
- 4Read the percentile and z-score for each measurement.
- 5Compare with the WHO reference table for the typical range at that age.
Formula and method
The WHO growth standards describe each measurement at each age with three numbers: L (skewness, a Box-Cox power), M (the median) and S (the coefficient of variation). A measurement X is converted to a z-score with the LMS formula above (or ln(X/M)/S when L = 0), and the percentile is the standard normal cumulative probability of that z-score.
Values between whole months are linearly interpolated. For weight beyond ±3 SD, the WHO restricted-tail adjustment is applied so extreme values are not exaggerated. Length and head circumference have L = 1, so their distribution is normal with mean M and standard deviation M × S.
- X
- Your baby’s measurement
- L, M, S
- WHO skewness, median and coefficient of variation for age and sex
- Φ
- Standard normal cumulative distribution
Worked examples
6-month-old boy: 8.2 kg, 68 cm, 43.3 cm head
The WHO median weight for a 6-month-old boy is 7.93 kg, so 8.2 kg gives a z-score of about 0.30 — the 62nd percentile. A length of 68 cm is around the 57th percentile and a 43.3 cm head is just below the median (49th).
12-month-old girl in US units
At 12 months, 18 lb (8.16 kg) is around the 23rd percentile for girls and 28.5 in (72.4 cm) around the 26th. A 17.5 in (44.45 cm) head is about the 37th percentile — all proportional and within the typical range.
Newborn girl, weight only
A 2.5 kg newborn girl is at about the 4th percentile of the WHO birth-weight standard, whose median is 3.23 kg. Length and head were left at 0, so only weight is scored.
Frequently asked questions
What does it mean if my baby is in the 50th percentile?+
It means your baby’s measurement is exactly at the median: half of babies of the same age and sex in the WHO reference are smaller and half are larger. It is average, not a target.
What is a normal baby weight percentile?+
Anything between roughly the 3rd and 97th percentile (WHO charts) is generally considered within the normal range. Doctors care more that a baby follows a consistent curve over time than about any single number.
Should I use WHO or CDC growth charts?+
The CDC and AAP recommend WHO growth standards for children from birth to 2 years, and CDC growth charts from age 2 to 19. This calculator uses the WHO standards, so it covers 0–24 months.
Why does my baby’s percentile change between visits?+
Measurement error, especially for length, is common, and many babies shift percentiles in the first months before settling on their own curve. A drop or rise crossing two major percentile lines is worth discussing with your doctor.
Should I use corrected age for a premature baby?+
Yes. For babies born before 37 weeks, plot growth using adjusted age — age since the due date — until about 2 years, so they are compared with babies of the same developmental maturity.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.
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