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BMI Percentile (Child) Calculator

Enter age, sex, height, and weight to estimate BMI-for-age percentile.

How it is calculated

First the tool computes BMI: BMI = weight (kg) ÷ height (m)², with height typed in centimetres. It then takes a built-in average BMI and spread (SD) for the child's age and sex, interpolated in a straight line between anchor ages 2, 5, 10, 15 and 20. The z-score is z = (BMI − mean) ÷ SD and the percentile comes from the normal distribution, percentile = 100 × Φ(z). Categories: below 5 underweight, 5 up to 85 healthy weight, 85 up to 95 overweight, 95 or above obese. Ages outside 2 to 20, heights outside 50 to 272 cm and weights outside 2 to 650 kg show no result. A blank sex uses the male values, and a sex other than male or female gives no result. The reference values are a rough fit, not the official clinical growth tables, so the result can differ from a paediatrician's chart; it is a screening aid only.

Worked example

A 10-year-old girl, 140 cm, 36 kg. Height in metres: 1.40, squared 1.96. BMI = 36 ÷ 1.96 = 18.37. For girls aged 10 the built-in mean is 17.8 and the SD is 2.8, so the z-score is (18.37 − 17.8) ÷ 2.8 ≈ 0.20. The normal curve turns that into a percentile of 58.03th, exactly as the tool prints it. That falls between 5 and 85, so the category is Healthy weight (5th-85th percentile).

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Common mistakes

  • Entering age in months, or a baby's or toddler's age; the tool only accepts ages from 2 to 20 years, and under 2 it shows no result.
  • Typing height in metres or inches instead of centimetres. A height under 50 is rejected, and an inch value that slips through gives an absurd BMI.
  • Applying adult BMI cut-offs (18.5, 25, 30) to the child's BMI value; for children only the percentile category is meaningful.
  • Leaving sex blank. A blank sex is calculated with the male averages, and the same BMI lands on a different percentile (63.07 instead of 58.03 for this girl).
  • Reading the ordinal suffix in English as a rank among real children; the percentile is a model estimate from rough reference averages, not a measured chart position.

Sources

About this calculator

A child's BMI-for-age percentile shows where their body mass index falls relative to other children of the same age and sex, as the official growth-chart reference curves do, such as those the CDC publishes for ages 2-20, the age range this calculator covers. It does not use the CDC tables themselves but an approximate normal-curve model fitted between five age points, and the separate WHO curves for younger children are not included. Unlike adult BMI, which uses fixed cutoffs (18.5, 25, 30), children's BMI must be read against a growth curve, because what counts as ‘normal’ body composition changes dramatically as children grow. A child at the 85th percentile has a BMI higher than 85% of same-age, same-sex peers in the reference population.

Pediatricians and school health screenings use BMI-for-age percentile as one screening tool for weight-related risk, with published clinical cutoffs typically defining underweight as below the 5th percentile, healthy weight as the 5th up to (but not including) the 85th percentile, overweight as the 85th up to the 95th, and obesity as at or above the 95th percentile — though pediatric guidelines stress that percentile alone shouldn't drive a diagnosis without a full clinical evaluation, since growth patterns, timing of puberty, and body composition vary widely among healthy children.

From a child's age, sex, height and weight it gives a rough estimate of where their BMI ranks among children of the same age and sex, a general reference only, not a replacement for an official pediatric growth chart or a clinician's assessment.

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