BMI Calculator for Kids and Teens

Why a child's BMI comes with a percentile

An adult BMI of 25 means the same thing at 25 and at 65. A child's BMI does not work that way, because the child is still growing and because boys and girls add height and weight on different schedules. A BMI of 19 is ordinary for a 6-year-old and low for a 15-year-old. That is why every pediatric office converts the raw number into a percentile before saying anything about it.

The percentile answers a narrow question: compared with 100 children of the same sex and the same age, how many have a lower BMI? A BMI at the 40th percentile means about 40 of those 100 children have a lower BMI and about 60 have the same or a higher one. The number is a ranking, not a measurement of body fat, and it says nothing about how a single child is doing physically.

The CDC publishes the reference curves that most US clinics use, and it defines the categories from those curves rather than from fixed BMI cutoffs. Its child and teen BMI categories page was last reviewed in June 2024 and lists five bands for ages 2 through 19. The same page notes that race is not a factor in the percentile calculation, so the tool here does not ask for it.

The formula, step by step

The arithmetic is the same one adults use. Weight in kilograms divided by height in meters squared gives BMI. In pounds and inches the working form is 703 times the weight in pounds, divided by the height in inches squared, and the 703 is just the unit conversion folded into the formula.

For a 10-year-old girl who is 4 ft 10 in and 78 lb, the steps run like this:

• 78 lb divided by 2.20462 is 35.38 kg
• 58 in times 0.0254 is 1.4732 m
• 1.4732 squared is 2.1703 square meters
• 35.38 divided by 2.1703 is a BMI of 16.30

Then the percentile step begins, and it is the part people cannot do in their head. The CDC stores three numbers for every age in months and every sex: a power, a median and a spread. Those three numbers turn a BMI into a z score, and the z score turns into a percentile. The median BMI for a 10-year-old girl is 16.838, so her BMI of 16.30 sits below the middle of the curve, which is where the 40th percentile comes from.

The calculator on this page does that lookup for every half year of age and interpolates between the stored points, so a child of 10 years 3 months gets a value between the 10-year and 10.5-year rows rather than a rounded one.

The four CDC categories and where the lines fall

Once the percentile is known, the label comes from fixed percentile lines, not from fixed BMI numbers. The table below is the CDC definition for children and teens aged 2 through 19.

The 120% rule is the one that catches people out. For a 10-year-old girl the 95th percentile is a BMI of 22.9, so severe obesity begins at 27.5, which is 22.9 times 1.2. A BMI of 26 in that same girl is obesity but not severe obesity, even though 26 would be well into the adult obesity range.

The American Academy of Pediatrics uses an expanded split inside that top band, with class 2 obesity running from 120% to below 140% of the 95th percentile and class 3 at 140% or above. Those classes come from its 2023 clinical practice guideline on childhood obesity, which is also the source the CDC cites for the expanded definition.

Worked example: three children, same height

Height held constant makes the percentile easier to see. All three children below are 4 ft 10 in tall. The first is a 10-year-old girl at 78 lb, the second is the same girl at 105 lb, and the third is a 10-year-old boy at 100 lb.

The girl at 105 lb carries 27 lb more than the girl at 78 lb, and her BMI rises from 16.3 to 21.9. That is a move of 5.6 BMI points, which sounds small in absolute terms and is a jump of 53 percentile points on her curve. The boy at 100 lb has almost the same BMI as the girl at 105 lb, 20.9 against 21.9, yet he sits one percentile lower, because the reference curve for boys runs slightly higher at that age.

This is the whole argument for using percentiles. A parent comparing two children on BMI alone would call the boy and the girl identical. The chart says they are both above the 85th percentile for their own group, and it also says the girl crossed further into the band.

The 85th percentile for a 10-year-old girl is a BMI of 19.9 and the 95th is 22.9. For a 10-year-old boy the same lines are 19.4 and 22.1. The gap between the sexes is about half a BMI point at this age, and it widens through the teen years.

The 85th and 95th percentile lines by age

Because the lines move with age, a BMI that is fine at 6 can be high at 12. The table gives the BMI that sits on the 85th and 95th percentile for each age, so you can see how far the goalposts travel.

Read the 95th percentile column from top to bottom. For boys it climbs from 19.3 at age 2 to 28.9 at age 18, a rise of 9.6 BMI points. For girls it climbs from 19.1 to 30.3, a rise of 11.2 points. A 15-year-old with a BMI of 26 is below the 95th percentile for his age, while a 6-year-old with the same BMI is far above it. Same number, different verdict.

The lines dip slightly around age 5 and 6, which is the normal childhood minimum, then rise through puberty. That dip is one reason a single reading at age 5 can look alarming and settle down two years later.

One reading is a snapshot, the curve is the story

A percentile taken once describes one day. Pediatric growth charts are built to be read as a series, because the direction of travel carries more information than the position of a single point. A child who has tracked along the 60th percentile for four years is doing something different from a child who sat at the 60th and jumped to the 90th in twelve months, even though both have the same number today.

Two patterns are worth writing down and taking to an appointment. The first is a line that crosses two percentile bands upward or downward within a year, which the CDC charts are designed to make visible. The second is a BMI percentile that moves while height stays flat, because that points at weight rather than at a growth spurt.

Puberty timing scrambles the chart on purpose. A boy who matures early can post a high BMI at 13 and settle by 16, and a girl who matures late can look thin at 13 and average at 16. The chart already accounts for age and sex, but it cannot know how far along a child is in puberty, and that is the single largest source of misreading in the early teen years.

Measurements drift too. A height taken with shoes on adds about half an inch, and half an inch at 4 ft 10 in moves BMI by roughly 0.3 points. A weight taken after lunch instead of before breakfast can add a pound or two. Neither error matters much on its own, and both matter when you are comparing two readings taken months apart.

What the number cannot tell you

BMI is a ratio of weight to height. It cannot separate muscle from fat, and it cannot see where fat sits. A 12-year-old gymnast and a 12-year-old who plays video games can share a BMI and have different bodies. A child in a growth spurt can gain weight ahead of height for a few months and post a percentile that looks worse than the underlying trend.

The CDC says plainly that its calculator is not a substitute for professional medical advice, and the same applies here. The percentile is a screening flag that tells a clinician where to look. It is not a diagnosis, and it is not a target to hit. A child at the 20th percentile is not failing, and a child at the 60th is not passing.

Two groups need a different chart entirely. Children under 2 are measured on the World Health Organization growth standards rather than the CDC BMI curves, because BMI is not the right index for infants. Athletes and children with conditions that affect growth, such as thyroid disease or a genetic syndrome, need a clinician to read the chart with the rest of the record.

For the 2 to 19 range, the CDC reference data behind this page comes from its BMI-for-age training material for health care providers, which also documents the 2022 extended charts used above the 97th percentile.

What to do with a result you did not expect

If the percentile lands outside the 5th to 85th band, the useful next step is not a diet. It is a repeat measurement and a conversation. Bring the last two or three heights and weights from checkups, note the dates, and let a pediatrician read the trend against the chart. A single reading above the 85th percentile in a child who has been there for years is a different conversation from a jump of two bands in six months.

For a child in the overweight or obesity band, the AAP guideline points at family level changes rather than child level restriction: fewer sugar sweetened drinks, more sleep, more movement, and a shift in what the whole household eats. The guideline is explicit that restrictive dieting is not the first move for most children, and that weight talk in front of a child tends to backfire.

Numbers that should send you to a clinician sooner rather than later include a BMI percentile below the 5th, a drop of two bands in a year, a BMI above 120% of the 95th percentile, or any sudden change in weight alongside fatigue, thirst or appetite. Those are flags for a professional, not for a calculator.

This page is an educational estimate, not medical advice. Percentiles come from the CDC 2000 growth charts, with the 2022 extended curves for high BMIs, and a clinician should interpret any result alongside the full growth record.