BMI for Children Calculator (Ages 2-20)
Body mass index for children and teenagers aged 2 to 20 is assessed differently from adult BMI. Rather than comparing a child's BMI to a fixed scale, the CDC plots it on sex-specific growth charts to produce a percentile rank: a number showing how that child compares to a reference population of children the same age and sex. This is called BMI-for-age. The four CDC weight status categories for children are: underweight (below the 5th percentile), healthy weight (5th to below the 85th percentile), overweight (85th to below the 95th percentile), and obese (95th percentile and above). To use this calculator, enter the child's age in whole years (2 to 20), sex, weight in kilograms, and height in centimetres. The tool calculates BMI using the standard formula (weight in kg divided by height in metres squared), then estimates an approximate CDC percentile range by linearly interpolating between key age-based BMI cutoff values derived from the CDC growth charts. This estimate is suitable for general screening only. A pediatrician using the full CDC growth chart data will produce a more precise result. This calculator is not a substitute for professional medical assessment.
BMI: -- — Category: --
How BMI-for-age is calculated
The BMI value itself uses the same formula as adult BMI. The difference lies in interpretation: rather than fixed cut points, the BMI is compared to the distribution of BMI values from a reference population of same-age, same-sex children to produce a percentile.
BMI = weight (kg) / (height (m))^2
height (m) = height (cm) / 100
Percentile estimated by linear interpolation of CDC growth chart cutoffs
Worked example
10-year-old boy, 35 kg, 140 cm:
- Height in metres = 140 / 100 = 1.40 m
- BMI = 35 / (1.40 × 1.40) = 35 / 1.96 = 17.86
- For boys age 10: p5 = 14.2, p85 = 19.0. BMI of 17.86 is between p5 and p85.
- Category: Healthy weight (approximately 5th to 85th percentile)
CDC weight status categories for children
| CDC Category | Percentile range | Description |
|---|---|---|
| Underweight | Below 5th percentile | May indicate nutritional deficiency or underlying health issue; consult a pediatrician |
| Healthy weight | 5th to below 85th percentile | BMI within the expected range for age and sex |
| Overweight | 85th to below 95th percentile | BMI higher than most peers; lifestyle and diet review may be beneficial |
| Obese | 95th percentile and above | BMI among the highest for age and sex; medical review recommended |
Approximate CDC BMI cutoffs used in this calculator
| Age | Sex | 5th percentile BMI | 85th percentile BMI | 95th percentile BMI |
|---|---|---|---|---|
| 2 | Boys | 14.7 | 17.0 | 18.0 |
| 5 | Boys | 13.8 | 16.5 | 17.9 |
| 10 | Boys | 14.2 | 19.0 | 22.0 |
| 15 | Boys | 16.0 | 22.9 | 26.2 |
| 20 | Boys | 18.5 | 25.0 | 30.0 |
| 2 | Girls | 14.2 | 17.0 | 18.0 |
| 5 | Girls | 13.6 | 16.8 | 18.4 |
| 10 | Girls | 14.0 | 19.4 | 22.8 |
| 15 | Girls | 16.0 | 23.5 | 27.5 |
| 20 | Girls | 18.5 | 25.0 | 30.0 |
Values between age points are linearly interpolated. Source: CDC BMI-for-age growth charts (2000).
BMI for children: frequently asked questions
Why is BMI interpreted differently for children than adults?
For adults, a single BMI scale applies to all ages and sexes. For children and teens aged 2 to 20, BMI must be interpreted relative to other children of the same age and sex, because body fatness changes substantially as children grow and differs between boys and girls. The CDC uses sex-specific BMI-for-age growth charts to express a child's BMI as a percentile rank among peers.
What does a BMI percentile mean for a child?
A BMI percentile indicates how a child's BMI compares to children of the same age and sex in the CDC reference population. For example, a child at the 75th percentile has a BMI equal to or greater than 75 percent of children in that reference group. Percentile categories: below the 5th is underweight, 5th to below 85th is healthy weight, 85th to below 95th is overweight, and 95th and above is obese.
How accurate is this calculator's percentile estimate?
This calculator uses linear interpolation between approximate BMI cutoffs at ages 2, 5, 10, 15, and 20 derived from CDC growth chart data. It gives a reasonable approximation of the CDC percentile category for screening purposes. For a precise percentile, use the CDC's own BMI percentile calculator or consult a healthcare provider who has access to the full growth chart tables.
What should I do if my child's BMI is outside the healthy range?
A BMI percentile outside the healthy range (5th to 85th) is a screening signal, not a diagnosis. Many factors affect a child's BMI including growth spurts, puberty timing, muscle mass, and measurement accuracy. If you are concerned, consult a pediatrician who can assess growth trends over time, evaluate diet and activity levels, and determine whether any follow-up is needed.
Does this calculator apply to children under 2 years old?
No. For children under 2 years old, the CDC and WHO use weight-for-length charts, not BMI-for-age charts. BMI-for-age is only used from age 2 onwards. For infants and toddlers under 2, consult a healthcare provider and refer to the WHO child growth standards.
Official sources
- CDC BMI-for-age growth charts: CDC Growth Charts.
- CDC child BMI guidance: CDC BMI for Children and Teens.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology. General information only, not medical advice.