The Child & Adolescent BMI Calculator is specially designed to measure body mass index for children aged 2-18.
Unlike adult BMI, child BMI takes age and sex into account because body fat composition changes as children grow. Results are adjusted according to WHO growth standards.
Calculator information
📋 How to use this calculator
- Select the child's sex (boy or girl) - CDC and WHO growth standards differ by sex.
- Enter the child's age in years and months (e.g., 7 years 4 months). The calculator is valid for ages 2-18.
- Enter weight in pounds or kilograms to one decimal place (e.g., 49.6 lb / 22.5 kg), ideally measured in the morning after using the bathroom.
- Enter height in inches or centimeters. For children under 2, use recumbent length; for ages 2 and up, use standing height.
- Click Calculate to see BMI and its percentile on the CDC/WHO growth curve.
- Read the category: underweight (<5th percentile), normal (5-85), overweight (85-95), obese (>95). Consult your pediatrician if results fall outside the normal range.
🧮 Child Body Mass Index (BMI) with growth percentile
BMI = weight (kg) / height^2 (m^2), then plot BMI on the percentile curve for the child's age and sex
- Weight = body mass in kilograms
- Height = body height in meters (cm divided by 100)
- Percentile = the child's position relative to 100 children of the same age and sex
For children, absolute BMI is not enough. The CDC Growth Charts are the standard reference for ages 2-19 in the US; the WHO Child Growth Standards are used for ages 0-2.
💡 Worked example: Boy, age 8 years
Given:- Age = 8 years 0 months
- Sex = boy
- Weight = 28 kg (~61.7 lb)
- Height = 128 cm (~50.4 in) = 1.28 m
Steps:- Square the height: 1.28 x 1.28 = 1.6384 m^2
- Compute BMI: 28 / 1.6384 = 17.09 kg/m^2
- Plot on the CDC BMI-for-age boys chart at age 8: 17.09 lands around the 75th percentile
- The 75th percentile falls within the 5-85 range (normal weight)
Result: BMI 17.09 (75th percentile) - normal weight status, appropriate for age and height.
❓ Frequently asked questions
Why don't we use the adult BMI cutoffs of 18.5-24.9 for kids?
Body composition changes substantially as children grow, and healthy body fat differs greatly between a 5-year-old and a 15-year-old. Pediatric BMI is interpreted using percentiles compared to peers of the same age and sex. Adult cutoffs would miscategorize nearly every child.
What is the difference between WHO and CDC growth curves?
WHO Growth Standards (2006) were built from a multinational study and are the global reference for ages 0-5. WHO also publishes growth references for ages 5-19. The CDC charts use US population data collected in the 1960s-1990s and are the default for ages 2-19 in US pediatric practice. The American Academy of Pediatrics recommends CDC charts after age 2 and WHO standards before.
My child is heavy but very active. Does a high BMI mean obesity?
BMI does not distinguish muscle from fat. An active child (swimming, soccer) may have a high BMI from muscle mass rather than fat. If BMI is in the overweight range but the child is active and healthy with normal waist circumference, it is usually not a concern. Ask the pediatrician about more accurate body-fat measurement (skinfold or bioimpedance).
When should I worry about my child's BMI?
If BMI sits below the 5th percentile (underweight) across two consecutive measurements, or above the 95th (obese), schedule a pediatric visit. Other red flags: sudden weight loss, rapid waist expansion, or extreme changes in eating patterns. Stunting (short stature for age) should also be evaluated, especially in toddlers.
How often should I recalculate my child's BMI?
Infants and toddlers are typically measured at every well-child visit. School-aged children only need a BMI check every 3-6 months unless there is a medical reason for more. Measuring more frequently doesn't add useful information because growth is gradual. Record results in your pediatrician's portal so trends can be reviewed at each visit.
📚 Sources & references
Last updated: May 11, 2026