Calculating BMI for kids uses the same basic formula as adults, but interpreting the result is completely different. Instead of fixed categories, a child’s BMI is plotted on age- and sex-specific growth charts to produce a percentile ranking. This percentile compares your child to other kids of the same age and sex, which accounts for the natural changes in body fat that happen as children grow.
The BMI Formula
The math itself is straightforward. If you’re using pounds and inches, the formula is:
BMI = (weight in pounds ÷ height in inches ÷ height in inches) × 703
If you’re using kilograms and meters:
BMI = weight in kilograms ÷ (height in meters × height in meters)
For example, a child who weighs 60 pounds and stands 42 inches tall would have a BMI of: (60 ÷ 42 ÷ 42) × 703 = 23.9. That number alone doesn’t tell you much, though. A BMI of 23.9 means something very different for a 6-year-old girl than for a 14-year-old boy. That’s where percentiles come in.
Why Percentiles Matter More Than the Number
Adults fall into fixed BMI brackets (under 18.5 is underweight, 25 and above is overweight, and so on). Children don’t. Because kids are constantly growing, and because boys and girls develop at different rates, a raw BMI number can’t be compared to a static cutoff. Instead, you need to know where that number falls relative to other children of the same sex and age. A BMI at the 70th percentile means your child’s BMI is higher than 70% of kids in that reference group.
The CDC growth charts used for this comparison cover children ages 2 through 19. Here are the standard categories:
- Underweight: below the 5th percentile
- Healthy weight: 5th to below the 85th percentile
- Overweight: 85th to below the 95th percentile
- Obesity: 95th percentile or above
Severe obesity is defined as a BMI at or above 120% of the 95th percentile value, or an absolute BMI of 35 or higher, whichever threshold is lower for a given age and sex. This definition, recommended by the American Heart Association, aligns pediatric severe obesity with the adult category most strongly linked to long-term health risks.
How to Get Accurate Measurements at Home
The formula is only as good as the numbers you put into it. Small errors in height or weight can shift a child’s percentile significantly, especially for younger kids. The CDC offers specific guidance for getting reliable measurements.
Measuring Height
Have your child remove shoes, hats, and any hair accessories or hairstyles that add height. They should stand on a hard floor (not carpet) against a flat wall with no baseboard molding. Feet should be flat, together, and against the wall, with legs straight, arms relaxed at their sides, and shoulders level. Ask them to look straight ahead.
Ideally, the child’s head, shoulders, buttocks, and heels should all touch the wall, though body shape sometimes prevents all four contact points. Place a flat, rigid object like a hardcover book against the wall and lower it until it rests firmly on the top of the head. Mark that spot lightly on the wall, then measure from the floor to the mark with a metal tape measure. Record to the nearest 1/8 inch or 0.1 centimeter.
Measuring Weight
Use a digital scale placed on a hard, flat surface. Spring-loaded bathroom scales are less reliable. Have your child remove shoes and heavy clothing like sweaters, then stand with both feet centered on the scale. Record the weight to the nearest decimal, such as 55.5 pounds or 25.1 kilograms.
Plotting the Percentile
Once you have a BMI number, the easiest next step is the CDC’s online BMI calculator for children and teens. You enter your child’s date of birth, sex, height, and weight, and it returns both the BMI and the exact percentile. This tool uses the CDC’s growth charts, including the extended BMI-for-age charts released in 2022, which provide more precise tracking for children at very high BMI levels.
If you prefer to do it manually, the CDC publishes downloadable growth charts. You find your child’s age along the bottom axis, their BMI on the side axis, and see where the point falls relative to the curved percentile lines. There are separate charts for boys and girls.
What BMI Can and Can’t Tell You
BMI measures excess weight relative to height. It does not measure body fat directly. This distinction matters for kids especially. A child with a stocky, muscular build may have an elevated BMI without carrying excess fat, because BMI can’t distinguish between fat mass and lean mass like muscle and bone. Ethnicity, stage of puberty, and individual growth patterns all influence how well BMI reflects actual body composition.
Research consistently shows that BMI works better as a screening tool than a diagnostic one. It’s good at flagging kids who may benefit from further evaluation, but a high BMI alone doesn’t confirm a health problem. Similarly, a normal BMI doesn’t guarantee a child has a healthy level of body fat. It’s one piece of information, not the full picture.
What Happens if the Percentile Is High
The American Academy of Pediatrics recommends that all children ages 2 through 18 be screened for overweight and obesity at least once a year using BMI percentiles from the CDC growth charts. If your child’s BMI falls at or above the 85th percentile, a pediatrician will typically look at the broader context: family medical history, activity levels, eating patterns, access to healthy food, mental health, and social factors that influence weight.
Rather than focusing on a single measurement, doctors track BMI percentile over time. A child who has consistently been at the 88th percentile since age 4 tells a different story than one who jumped from the 50th to the 88th in a year. Trends matter more than any single data point. For children with elevated BMI, follow-up visits every 3 to 6 months help track whether lifestyle changes are having an effect.
For children 6 and older with a BMI in the overweight or obese range, treatment generally centers on health behavior and lifestyle changes set collaboratively with the family. This often uses the “5-2-1-0” framework: five or more servings of fruits and vegetables daily, two hours or less of recreational screen time, one hour or more of physical activity, and zero sugar-sweetened beverages. The goal for most kids isn’t weight loss but rather growing into their weight as they get taller.
BMI for Children Under 2
Standard BMI-for-age charts begin at age 2. For infants and toddlers, pediatricians use weight-for-length charts from the World Health Organization instead. If your child is younger than 2, BMI isn’t the right tool. Your pediatrician tracks growth using different measurements at well-child visits during those early years.