BMI Calculator for Kids

For adults, BMI categories (underweight, normal, overweight, obese) are fixed numerical thresholds that apply to everyone over 18. For children ages 2 to 19, the same BMI value is interpreted differently depending on age and sex because body composition changes substantially during growth and development. A BMI of 18 is healthy for a 10-year-old boy but would be underweight for a 19-year-old. The CDC uses sex-specific BMI-for-age percentile charts to account for these changes.

According to CDC growth chart reference data, a BMI between the 5th and 85th percentile is considered healthy weight for children. Below the 5th percentile is classified as underweight. Between the 85th and 95th percentile is overweight. At or above the 95th percentile is obese. These categories apply to children ages 2 to 19 and are sex-specific because boys and girls have different growth trajectories.

The CDC BMI-for-age percentile categories are: Underweight (below the 5th percentile), Healthy Weight (5th to below 85th percentile), Overweight (85th to below 95th percentile), and Obese (at or above the 95th percentile). These categories were established from NHANES reference data collected on US children across multiple survey periods and published in the CDC 2000 Growth Charts.

BMI for children uses the same formula as adults: weight in kilograms divided by height in metres squared. For a child weighing 35 kg and 140 cm tall: height in metres = 1.40; BMI = 35 / (1.40 x 1.40) = 35 / 1.96 = 17.9. The difference from adult BMI is in interpretation - 17.9 needs to be compared to the CDC percentile chart for the child's age and sex to determine if it is a healthy range.

A BMI at or above the 85th percentile but below the 95th percentile is classified as overweight per CDC guidelines. This means your child's BMI is higher than 85 percent of children of the same age and sex in the CDC reference population. It is a flag for closer monitoring and discussion with a pediatrician, but not necessarily an indication that a child needs to lose weight. A full assessment of growth trajectory, diet, activity level, and any health markers provides a more complete picture.

Not always. A child who is very muscular or tall for their age may have a high BMI percentile without excess body fat. BMI does not directly measure body fat. A pediatrician will use BMI percentile alongside growth trajectory charts, waist circumference, blood pressure, and lab values to assess health risk. A child who is consistently at the 90th percentile across multiple measurements is generally less concerning than one who has moved rapidly from the 50th to the 90th percentile over a year.

Consult your child's pediatrician before making any significant changes to their diet or activity level. The focus for children is almost always on healthy habit development rather than weight loss, especially for younger children who are still growing. Recommendations typically include reducing screen time, increasing daily physical activity (60 minutes of moderate to vigorous activity is recommended), improving diet quality (more vegetables, whole grains, and lean protein, fewer ultra-processed foods and sugary drinks), and ensuring adequate sleep.

Yes. A child with a healthy BMI percentile can still have poor fitness, a nutrient-deficient diet, inadequate sleep, or other health risks not captured by BMI. BMI is a population-level screening tool. Conversely, a child with a slightly elevated percentile who is physically active, eats a varied diet, has good cardiovascular fitness, and is growing along their growth curve may be perfectly healthy. Health is not defined by a single number.

For a 10-year-old boy at average height (138 cm), the 5th percentile BMI cutoff is approximately 14.1 and the 85th percentile cutoff is approximately 19.4. This translates to a healthy weight range of roughly 26.9 to 37.0 kg. These values differ for girls of the same age (female percentile cutoffs are slightly different) and vary with actual height. Use the Healthy Weight Range mode in this calculator for precise values based on your child's actual height.

Boys and girls have different growth and body composition trajectories during childhood and adolescence. Girls typically accumulate more body fat at earlier ages, particularly during puberty, while boys tend to have a longer period of lean mass growth. The CDC growth charts are sex-specific to account for these biological differences. Using male charts for a girl or vice versa would produce inaccurate percentile classifications.

The BMI formula itself (weight / height squared) is exact. The percentile classification uses approximate CDC BMI-for-age cutoff values at the 5th, 85th, and 95th percentiles from published CDC growth chart reference data. These are representative values for the ages 2 to 19 range. For precise percentile scores (e.g., 'exactly the 78th percentile') the official CDC BMI Percentile Calculator or a healthcare provider using the full LMS statistical method would be required. This tool provides accurate category classification for the four standard CDC weight-status groups.