Child BMI Calculator
Quick BMI check for children ages 2–20 using official CDC BMI-for-age percentiles. Enter age, height and weight to get BMI, percentile and weight category — plus what parents can do next.
Educational tool only — not medical advice. A pediatrician interprets the result in context.
For parents: this calculator is educational. BMI is a screening tool, not a diagnosis. Always discuss the result with your child's pediatrician before taking any action.
Calculate your child's BMI percentile
Result
–
BMI (kg/m²)
–
–
0255075100th percentile
- What it means
- –
- BMI-for-age z-score
- –
What parents can do next
Screening result only — your pediatrician interprets it together with growth history, diet, activity and health.
Scope boundary — which BMI tool do you need?
- This tool (Child BMI Calculator): quick BMI + percentile check for children ages 2–20, using CDC BMI-for-age percentiles (NOT adult categories). Built for parents who want a fast answer plus next steps.
- Adult BMI? Use our BMI Calculator — WHO adult categories (18.5 / 25 / 30), healthy weight range, BMI Prime. Adult cutoffs must never be applied to children.
- Want the deep analysis? Try Child BMI Percentile — full growth-curve chart, monthly interpolation detail and the complete CDC reference table.
Why children use percentiles, not adult cutoffs
Children's body composition changes as they grow, so a fixed number like "25" is meaningless for a child. Instead, a child's BMI is ranked against other children of the same age and sex:
- Below 5th percentile
- Underweight range
- 5th – below 85th
- Healthy weight range
- 85th – below 95th
- Overweight range
- 95th and above
- Obesity range
These are CDC screening categories. Crossing into a higher range is a prompt to talk with a pediatrician — not a diagnosis.
Parent guide: by result category
Below 5th — underweight range
- Offer regular meals and nutrient-dense snacks; never force-feed.
- One low reading can be normal for a naturally slim child — watch the trend.
- See a pediatrician if there is weight loss, poor appetite or fatigue.
5th–85th — healthy range
- Keep up about 60 minutes of active play daily and regular sleep.
- Keep water the default drink and meals balanced across food groups.
- Recheck at routine checkups — growth patterns matter more than one reading.
85th–95th — overweight range
- Change the whole family's habits; avoid diets and weight-focused talk.
- Swap sugary drinks for water, add active play, keep consistent bedtimes.
- Book a pediatrician visit to review growth history and rule out underlying issues.
95th+ — obesity range
- This is a screening flag — only a clinician can evaluate your child's health.
- Focus on sustainable family habits, not rapid weight change.
- Ask your pediatrician about family-based lifestyle programs if recommended.
Talk to your pediatrician if…
- The result is at/above the 95th or below the 5th percentile.
- Your child's percentile line is crossed quickly between checkups.
- You are considering any diet or structured weight plan.
- You have concerns about eating habits, growth or puberty.
Frequently asked questions
What is a healthy BMI for a child?
There is no single healthy BMI number for children. Kids are assessed with BMI-for-age percentiles from the CDC growth charts: 5th to below 85th percentile is the typical healthy-weight range, below 5th is underweight, 85th to below 95th is overweight, and 95th and above is in the obesity range. The percentile compares your child with children of the same age and sex.
How is my child's BMI percentile calculated?
First BMI is computed as weight in kilograms divided by height in meters squared. Then the CDC LMS method converts that BMI into a z-score using age- and sex-specific L, M and S parameters from the CDC 2000 BMI-for-age tables: z = ((BMI/M)^L - 1) / (L*S). The z-score is mapped to a percentile with the standard normal distribution. This calculator uses the full monthly CDC table for ages 2 to 20.
Why can't I use an adult BMI calculator for my child?
Adult cutoffs (18.5 / 25 / 30) do not apply to children because body composition changes as kids grow. A BMI of 17 can be perfectly typical for a 6-year-old but low for a 16-year-old. Children must be evaluated with age- and sex-specific percentiles, which is what this tool does. For adults aged 20 and over, use an adult BMI calculator instead.
My child's result is in the overweight or obesity range. What should I do?
Treat the result as a screening flag, not a diagnosis. Avoid restrictive dieting or commenting on your child's weight. Focus the whole family on habits instead: active play every day, water instead of sugary drinks, regular sleep, and balanced meals. Then talk with your child's pediatrician, who can review growth history, diet, activity and health before recommending anything.
Why did my child's percentile change when their BMI barely moved?
Percentiles are relative to other children of the same age and sex. During growth spurts the reference median shifts, so the same BMI can map to a different percentile a few months later. Small shifts are normal. What matters more is the trend across checkups, not a single reading.
Is BMI accurate for athletic or muscular children?
Not always. BMI cannot tell muscle from fat, so a very muscular child can land in a high percentile without excess body fat. Pediatricians may use additional measures such as waist circumference, skinfold measurements or growth history. Use this result as a starting point for a conversation with your pediatrician, not a final verdict.
What ages does this calculator cover, and what about babies under 2?
This tool covers ages 2 to 20 years, the range of the CDC BMI-for-age charts. For children under 2, pediatricians use WHO weight-for-length charts instead of BMI, so this calculator does not give a result below age 2.
Free tool by NRTechStudio