Baby growth percentile calculator

How is a growth percentile calculated?

The calculator uses the LMS method of the 2000 CDC growth charts. For each sex and age the CDC publishes three numbers: L (a power that makes the curve symmetric), M (the median) and S (the spread). The z-score is z = ((X ÷ M)^L − 1) ÷ (L × S), where X is the measurement (when L is 0 it is ln(X ÷ M) ÷ S), and the percentile is the share of the standard normal curve below z: z = 0 is the 50th percentile, +1 the 84th, −1 the 16th. The CDC tables have one row for each month of age, placed in the middle of the month, so for ages in between the calculator blends the two neighboring rows, as the CDC suggests.

Example: a 6-month-old boy

A boy exactly 6 months old weighs 17 lb 10 oz (7.995 kg). Halfway between the CDC rows for 5.5 and 6.5 months, L = 0.0278, M = 7.897 kg and S = 0.1172. Then z = ((7.995 ÷ 7.897)^0.0278 − 1) ÷ (0.0278 × 0.1172) = 0.105, which is the 54th percentile: heavier than about 54 of 100 boys that age in the reference, and a little above the median of 17 lb 6.5 oz. At 26.5 inches (67.3 cm) long he is at the 55th percentile for length, and a head of 17 inches (43.2 cm) puts him at the 34th.

What is a good or normal percentile for a baby?

There is no “good” percentile. By definition 9 of 10 children in the reference fall between the 5th and 95th percentiles, and plenty of healthy children are outside that band. One reading shows where a child sits among the reference children, not how healthy the child is. The CDC says growth monitoring is based on a series of accurate measurements over time, so doctors look at the pattern: does the child keep following a curve, and do weight and length fit together? A sudden change in your child’s own pattern is what to bring up with the pediatrician.

The CDC recommends WHO charts under age 2. Why does this page use CDC charts?

For children younger than 24 months the CDC recommends the WHO growth standards, and the CDC charts from 24 months on (MMWR 2010). The two differ in kind: the WHO charts are a standard that describes how healthy children grow in optimal conditions, and the CDC charts are a reference that describes how a group of U.S. children grew. The CDC notes that from about 3 months the WHO curves show slower weight gain than the CDC charts, so a baby’s CDC percentile can differ from the WHO percentile in the pediatrician’s chart, mostly for weight. This calculator uses the CDC 2000 charts for every age because that data is in the U.S. public domain; the WHO tables carry WHO’s copyright, which does not allow reuse on commercial websites without permission. For a WHO-based reading under age 2, ask your pediatrician.

Length lying down or height standing: which one do I enter?

Until age 2 children are measured lying down (length); from age 2, standing (height). The CDC says standing height is about 0.8 cm (¼ inch) less than lying length, and the reference tables show it: at 30 months the median for boys is 91.8 cm lying down and 91.0 cm standing. Between 24 and 36 months the CDC has both charts, so the calculator asks how your child was measured. Use the same method your pediatrician used.

What is weight-for-length?

Weight-for-length (weight-for-height from age 2) compares a child’s weight with that of children of the same sex and the same length, whatever their age, so a long, lean baby and a short, sturdy one are read fairly. The CDC tables cover lengths from 45 to 104 cm (17.7 to 41 inches) lying down and heights from 77 to 122 cm (30.3 to 48 inches) standing; this calculator uses the standing chart up to the fifth birthday. For older children use BMI-for-age: see the child BMI calculator.

What do the z-score and the “>99.9” or “<0.1” mean?

The z-score counts how many standard deviations a measurement is above (+) or below (−) the median: +2 is the 97.7th percentile and −2 the 2.3rd. Beyond about ±3, the 99.9th and 0.1st percentiles, the calculator shows only “>99.9” or “<0.1” and the z-score, because the reference has very few children out there. Such a value is worth measuring again and showing to a doctor, since it can be a measuring slip or something that needs a clinician’s attention.

My baby was born early. Which age do I enter?

For a baby born early the American Academy of Pediatrics advises using the corrected (adjusted) age during the first 2 years. Work it out with the adjusted age calculator and enter that age here. Your pediatrician may also use special charts for premature babies.

How accurate is the result?

The arithmetic follows the CDC’s published method; our check against the printed CDC tables agrees to within about half a percentile point. The larger uncertainty is the measuring: a quarter-inch (0.6 cm) slip in the length of that 6-month-old boy moves him from the 55th to the 64th percentile. The reference itself is a snapshot of U.S. children measured in national surveys from 1963 to 1994, so it is not the same as how children grow today.

More for parents: the child BMI calculator (ages 2 to 19), the adjusted age calculator for premature babies, the sleep calculator, the body surface area calculator and the child height predictor.

Sources: 2000 CDC Growth Charts for the United States, data files at cdc.gov/growthcharts (Source: CDC and the National Center for Health Statistics; the charts are available there free of charge); Kuczmarski RJ et al., “2000 CDC Growth Charts for the United States: Methods and Development,” Vital Health Stat 11(246), 2002; Flegal KM, Cole TJ, “Construction of LMS parameters for the CDC 2000 growth charts,” National Health Statistics Reports no. 63, 2013; CDC, “Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months in the United States,” MMWR Recomm Rep 2010;59(RR-9). This calculator is not endorsed by or affiliated with the CDC, NCHS, HHS or the U.S. government.

For general information only — not medical advice. Talk with your child’s pediatrician or another health professional about your child’s growth and health; a percentile on its own does not diagnose anything. How we check our calculators.