Why pediatric BMI is reported as a percentile
Adult BMI categories (under-, normal-, over-, obese) use fixed cutoffs because adult height and body composition are relatively stable. Children grow rapidly, and what counts as a healthy BMI for a 5-year-old (around 15) is severely underweight for a 15-year-old. The CDC 2000 growth charts solve this by ranking each child against a reference cohort of same-sex / same-age peers from the NHANES surveys, then reporting a percentile.
What the bands mean (and don’t)
The CDC classifies under the 5th percentile as underweight, 5th–84th as healthy weight, 85th–94th as overweight, and 95th+ as obese. These are screening categories that prompt a clinical conversation, not standalone diagnoses. A pediatrician will look at the growth-curve trajectory over time (sudden drops or spikes matter more than a single point), family history, puberty stage, diet, activity and mental-health context before making any clinical call.
Limits — especially for athletes and growing teens
BMI cannot tell muscle from fat. A 14-year-old hockey player carrying significant lean mass can read in the 95th+ percentile without being clinically obese; a sedentary teen with low muscle mass and high body fat can read in the healthy band. If you have any doubt, ask the pediatrician for a skinfold assessment or a referral for body-composition measurement. Eating-disorder history in the family is another reason to interpret BMI percentile cautiously.
For infants and toddlers under 2
Use the WHO Growth Standards (weight-for-age, weight-for-length, length-for-age) for children under 24 months. The Canadian Paediatric Society and Dietitians of Canada recommend WHO charts for under-2s and CDC charts for ages 2 and older. Don’t use this calculator on infants.
Sources
- CDC Growth Charts (United States): 2 to 20 years BMI-for-age percentiles. Centers for Disease Control and Prevention.
- Dietitians of Canada — Promoting healthy growth in children: Joint statement with CPS, Canadian Paediatric Society, College of Family Physicians of Canada and Community Health Nurses of Canada.
- Canadian Paediatric Society. The use of growth charts for assessing and monitoring growth in Canadian infants and children. Position statement, last reviewed 2022.