How is BMI calculated for a child?

The BMI number is the same formula, weight divided by height squared, but it is then compared with the CDC growth charts for the child's age and sex to give a percentile. Healthy weight is the 5th to 85th percentile.

Can I use adult BMI categories for a teenager?

No. Under 20, adult cut-offs misclassify children, often labelling healthy children as underweight. The app switches to percentiles automatically for ages 2 to 19.

Is BMI accurate for athletes?

Not very. Muscle is dense, so muscular people can read as overweight. Waist-to-height ratio and body fat estimates, both in the app, give a better picture.

BMI Calculator for Adults, Kids & Teens — CDC Percentiles, Body Fat

Children and teens, done properly

Body composition changes month by month during growth, so the same BMI means something different at 6 and at 16. Enter a date of birth for a percentile accurate to the month, with the healthy BMI and weight range for that age shown alongside.

Adult BMI

The full WHO table, from severe thinness to the three obesity classes
Your healthy weight range for your height and how far you are from it
BMI prime, ponderal index and body surface area
Metric and imperial: cm, feet and inches, kg, pounds, stones

A full set of body calculators

Body fat: US Navy tape-measure method or a BMI-based estimate
Calories: BMR and TDEE by Mifflin-St Jeor, Harris-Benedict or Katch-McArdle
Macros: balanced, low carb, high protein or ketogenic
Waist-to-height and waist-to-hip ratios, ideal weight by four formulas, daily water intake
A goal planner whose curve flattens as you get lighter, rather than a straight line that promises too much

Track it over time

Save measurements and see the trend against the healthy band, filter by month, quarter or year, and export everything as CSV. Your measurements stay on your phone; there is no account.

Honest about BMI's limits

BMI cannot tell muscle from fat, says nothing about where fat sits, and its thresholds were drawn mostly from European populations. The app explains this rather than pretending one number settles the question. It provides general information and is not medical advice.

Questions

How is BMI calculated for a child?: The BMI number is the same formula, weight divided by height squared, but it is then compared with the CDC growth charts for the child's age and sex to give a percentile. Healthy weight is the 5th to 85th percentile.
Can I use adult BMI categories for a teenager?: No. Under 20, adult cut-offs misclassify children, often labelling healthy children as underweight. The app switches to percentiles automatically for ages 2 to 19.
Is BMI accurate for athletes?: Not very. Muscle is dense, so muscular people can read as overweight. Waist-to-height ratio and body fat estimates, both in the app, give a better picture.

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BMI calculator for adults, children and teens