BMI Calculator for Kids

Understanding BMI for Children and Teens

BMI for children and teenagers works differently than it does for adults. While the mathematical formula is the same — weight in kilograms divided by height in meters squared — the interpretation relies on age- and sex-specific percentile charts rather than fixed categories. This is because body fatness changes naturally as children grow, and it differs between boys and girls at each stage of development. A BMI number that is normal for a 12-year-old might indicate underweight in a 16-year-old. The CDC growth charts, based on data from large population studies, allow you to compare your child's BMI to that of other children of the same age and sex.

This calculator provides a raw BMI number based on the height and weight you enter. To determine your child's percentile, you will need to plot that BMI value on a CDC BMI-for-age growth chart appropriate for your child's sex. Your pediatrician does this at every well-child visit, and the CDC provides free printable charts and an online percentile calculator. A single BMI reading is a useful snapshot, but tracking the trend over multiple measurements is far more informative for assessing your child's growth trajectory.

BMI Percentiles and CDC Growth Charts

The CDC recommends using BMI-for-age percentiles to screen children and teens aged 2 through 19 for weight status. The percentile indicates where your child's BMI falls relative to other children of the same age and sex. A child at the 50th percentile has a BMI higher than half and lower than half of their peers — it does not mean they are at 50% of a "target" BMI. The weight status categories for children are: underweight (below the 5th percentile), healthy weight (5th to 84th percentile), overweight (85th to 94th percentile), and obese (95th percentile and above).

Age-Specific Interpretation

Body composition changes substantially as children mature. Infants and toddlers naturally have higher body fat percentages, which decrease during the preschool years. Around age 5-6, body fat begins to increase again in a normal process called the "adiposity rebound." Children who experience this rebound earlier than age 5 may have a higher risk of obesity later in life. During puberty, girls typically gain more body fat while boys gain more lean muscle mass. These normal developmental changes mean that BMI percentiles — not raw BMI numbers — are the appropriate tool for assessing children's weight. A jump in raw BMI during a growth spurt is often completely normal.

When to Consult a Pediatrician

Schedule a conversation with your child's pediatrician if their BMI falls below the 5th percentile or at or above the 85th percentile. You should also raise concerns if you notice a significant shift in percentile rank between visits — for example, moving from the 50th to the 80th percentile within a year. Other reasons to consult include signs of disordered eating, excessive concern about weight or body image, or a family history of obesity-related conditions. A pediatrician can evaluate the full clinical picture, including growth velocity, pubertal development, dietary habits, physical activity levels, and family medical history, before recommending any intervention.

CDC BMI-for-Age Reference — 5th, 50th, and 95th Percentiles

The table below lists approximate BMI cut-points from the CDC 2000 BMI-for-age growth charts — the same charts your pediatrician plots on. The 5th percentile marks the boundary with underweight, the 85th the start of overweight, and the 95th the start of obesity. Between the 5th and 84th percentiles is considered a healthy weight. Values are in kg/m² and rounded to one decimal; for a clinical percentile calculation, use the CDC's online tool or consult your pediatrician.

Notice the "adiposity rebound" in the data: BMI at the 50th percentile decreases from age 2 to age 6, then rises through adolescence. A rising BMI in a 7-year-old is normal; a rising BMI in a 4-year-old (early rebound) is a known risk factor for later obesity and worth discussing with a pediatrician.

Healthy Weight Range by Height for Common Ages

For a quick sanity check without percentile charts, here is the healthy weight range (5th-85th percentile) for common height/age combinations. These are reference points only — individual variation is significant, and growth should be evaluated on trend rather than a single measurement.

Frequently Asked Questions

How is BMI calculated for children?

BMI for children is calculated using the same formula as adults — weight in kilograms divided by height in meters squared. However, the result is interpreted differently. Instead of fixed categories, children's BMI is plotted on age- and sex-specific growth charts to determine a percentile ranking. A child at the 60th percentile, for example, has a BMI higher than 60% of children of the same age and sex.

What BMI percentile is considered healthy for kids?

For children and teens aged 2-19, a BMI between the 5th and 84th percentile is considered a healthy weight. Below the 5th percentile is classified as underweight, the 85th to 94th percentile is overweight, and the 95th percentile or above is classified as obese. These categories account for the natural changes in body fat that occur as children grow.

Why is BMI measured differently for children than adults?

Children's body fat levels change naturally as they grow, and they differ between boys and girls at various ages. A BMI of 18 might be perfectly healthy for a 10-year-old but underweight for an adult. Age- and sex-specific percentiles account for these normal developmental variations, making BMI a more meaningful screening tool for young people.

At what point should I consult a pediatrician about my child's BMI?

Consult a pediatrician if your child's BMI falls below the 5th percentile (underweight) or at or above the 85th percentile (overweight). Also seek guidance if there has been a significant jump in BMI percentile between checkups, if your child shows signs of disordered eating, or if you have concerns about their growth pattern. A pediatrician can assess the full picture including growth velocity, family history, and pubertal stage.

Does puberty affect BMI in children and teens?

Yes, puberty significantly affects body composition and BMI. Girls typically gain more body fat during puberty, while boys gain more muscle mass. Growth spurts can temporarily change BMI as height and weight increase at different rates. A teen's BMI percentile may shift during puberty without indicating a health problem. Tracking BMI over time on a growth chart reveals trends more meaningful than any single measurement.