BMI calculator for kids and teens

The BMI formula is the same for children: weight divided by height squared. What it means is very different. Body composition keeps changing as kids grow: BMI falls in early childhood, reaches its lowest point around age 5 or 6 (called adiposity rebound) and then rises through the teen years. Boys and girls also follow different curves.

So a BMI of 18 means one thing for an 8-year-old and something else for an adult. For kids and teens, BMI is expressed as a percentile: the share of children of the same sex and age with a lower BMI. The 70th percentile means your child's BMI is higher than 70% of the reference group and lower than 30%.

The calculator uses the CDC 2000 growth reference, which gives three parameters (L, M and S) of the BMI distribution for every month of age from 2 to 20. We use the LMS method to turn your child's BMI into a z-score and then a percentile, interpolating linearly between months.

In 2022 the CDC added a method for very high values. Above the 95th percentile the original LMS curves compress the differences, so there we use the extended BMI-for-age percentile. We also show BMI as a percentage of the 95th percentile for your child's age, which is easier to track than percentiles like 99.9 when obesity is severe.

We checked the math against the CDC's published 5th, 50th, 85th and 95th percentile values. The results match at all ten age test points.

The severe obesity classes come from the American Academy of Pediatrics (AAP) 2023 clinical practice guideline. A category is a screening result, not a diagnosis, and it doesn't measure how much of your child's weight is fat or muscle.

A 10-year-old boy is 4 ft 7 in (140 cm) tall and weighs 79 lb (36 kg).

• BMI = 36 ÷ (1.40 × 1.40) = 18.4
• On the CDC chart for 10-year-old boys, that's about the 76th percentile, in the healthy range.
• The 95th percentile at this age is about 22.2, so his BMI is about 83% of the 95th.

The same BMI would be about the 37th percentile for a 14-year-old boy, but about the 95th percentile, the top of the overweight range, for a 6-year-old. That's why adult cut-offs (18.5, 25, 30) don't work for kids.

• One reading isn't enough. Pediatricians look at how your child tracks along the chart over the years. A sudden jump across percentile lines says more than a single high value.
• Measure carefully: height without shoes against a wall and weight in light clothing. Half an inch or a pound can shift a young child's percentile noticeably.
• Muscular, athletic teens may get a BMI that overstates body fat.
• Don't put your child on a diet on your own. For a growing child, the goal is often not weight loss but letting height catch up while weight rises more slowly. Decide that with your pediatrician or a registered dietitian. That's why this calculator doesn't give a weight-loss target or a calorie budget.
• Avoid weight talk in front of your child. To help prevent eating disorders, focus on family habits like shared meals, activity and sleep rather than the number on the scale.

The CDC recommends its 2000 growth charts for children 2 and older in the United States, and the WHO growth standards for children under 2. Other countries use their own references, such as the WHO 5–19 reference or the UK90 charts in the UK, and their percentiles can differ by a few points. If your pediatrician's chart shows a slightly different number, that's not a mistake. We chose the CDC reference because its LMS tables are public and precise, and it comes with the extended method for severe obesity.

For children under 2, doctors assess weight-for-length instead of BMI; you can log measurements in our baby growth tracker. From age 20, use the adult BMI calculator.