Baby Percentile Calculator
Enter your baby's sex, birth date and measurements to get weight-for-age, length-for-age and head circumference percentiles and z-scores from the WHO Child Growth Standards, plotted on a growth chart. All calculations run in your browser.
Source: WHO Child Growth Standards (2006). For information only; your pediatrician or health visitor interprets growth over several measurements.
How to use the Baby Percentile Calculator
- Choose boy or girl and your units.
- Enter the date of birth and the date the measurements were taken. If your baby was born before 37 weeks, enter the gestational age at birth to use corrected age.
- Enter weight, length and/or head circumference. You can leave any of them empty.
- Press Calculate percentiles to see each percentile and z-score.
- Switch the chart between weight, length and head to see where your baby sits among the WHO percentile curves.
How the percentile is calculated (WHO LMS method)
The WHO Child Growth Standards summarise each measurement at each age with three numbers: L (skewness, a Box-Cox power), M (the median) and S (the coefficient of variation). For a measurement X the z-score is:
z = ((X ÷ M)L − 1) ÷ (L × S)
and the percentile is the standard normal cumulative probability of z (z = 0 is the 50th percentile, −1.88 the 3rd and +1.88 the 97th). For length and head circumference L = 1, so the formula reduces to z = (X − M) ÷ (M × S). For weight beyond ±3 SD the calculator uses the WHO "restricted" adjustment, as in the WHO Anthro software. The tables are embedded at monthly steps (1 month = 30.4375 days), and L, M and S are interpolated linearly for your baby's exact age in days.
WHO medians and 3rd–97th percentile ranges
| Age | Boys weight (kg) | Girls weight (kg) | Boys length (cm) | Girls length (cm) | Boys head (cm) | Girls head (cm) |
|---|---|---|---|---|---|---|
| Birth | 3.3 (2.5–4.3) | 3.2 (2.4–4.2) | 49.9 (46.3–53.4) | 49.1 (45.6–52.7) | 34.5 (32.1–36.9) | 33.9 (31.7–36.1) |
| 6 months | 7.9 (6.4–9.7) | 7.3 (5.8–9.2) | 67.6 (63.6–71.6) | 65.7 (61.5–70.0) | 43.3 (41.0–45.6) | 42.2 (39.7–44.6) |
| 12 months | 9.6 (7.8–11.8) | 8.9 (7.1–11.3) | 75.7 (71.3–80.2) | 74.0 (69.2–78.9) | 46.1 (43.6–48.5) | 44.9 (42.3–47.5) |
| 24 months | 12.2 (9.8–15.1) | 11.5 (9.2–14.6) | 87.8 (82.1–93.6) | 86.4 (80.3–92.5) | 48.3 (45.7–50.8) | 47.2 (44.6–49.8) |
Values: median (3rd–97th percentile), computed from the WHO LMS tables. Source: WHO Multicentre Growth Reference Study Group, WHO Child Growth Standards, World Health Organization, 2006 (length/height-for-age, weight-for-age) and 2007 (head circumference-for-age). Length at 24 months is recumbent length; standing height is about 0.7 cm less.
Reading the result sensibly
- The trend matters more than one number. Babies normally move a little between percentile lines, especially in the first weeks and after illness. Crossing two major lines (for example from above the 50th to below the 15th) is worth checking with a professional.
- Proportion matters. A long, light baby and a short, heavy baby can both be healthy; clinicians also look at weight-for-length.
- Parents' size matters. Small parents often have smaller children; see the child height predictor for mid-parental height.
- Measure carefully. Weigh without clothes or nappy and measure length lying flat; a 1 cm error in length can move the percentile by 10 points or more.
After age 2, growth is usually plotted on the CDC 2000 charts (United States) or the WHO 2007 growth reference for 5–19 years, which this tool does not cover. Note that adult BMI categories, as in the BMI calculator, do not apply to children.
Frequently asked questions
What does a baby's percentile mean?
A percentile compares your baby with the WHO reference population of healthy, breastfed children of the same age and sex. The 40th percentile for weight means 40% of babies in the reference weigh less and 60% weigh more. Any percentile can be normal; what matters most is steady growth along a similar curve over time.
Which growth chart does this use?
The WHO Child Growth Standards (2006), which describe how healthy children grow under optimal conditions. They are recommended for all children from birth to 2 years, including by the CDC in the United States and in the UK-WHO charts.
Is a low or high percentile a problem?
Not by itself. Values below the 3rd or above the 97th percentile, or a child who crosses two or more major percentile lines, are worth discussing with your pediatrician or health visitor, who will look at the whole picture, including parents' size, feeding and development.
Should I use corrected age for a premature baby?
Yes. For babies born before 37 weeks, growth is usually assessed using corrected age (actual age minus the weeks born early) until about 2 years old. Enter the gestational age at birth and the calculator subtracts the difference from 40 weeks.
How do I measure length and head circumference?
Length is measured lying down from the top of the head to the heels with the legs straightened, ideally on a measuring board. Head circumference is measured with a non-stretch tape just above the eyebrows and ears around the widest part of the back of the head. Home measurements are less precise than clinic ones.
Why is the percentile different from my baby's app or red book?
Different charts (WHO, CDC 2000, UK-WHO, Fenton for preterm), rounding of age to whole months and measurement error all cause small differences. This calculator uses exact age in days and the WHO LMS formula.