This BMI calculator for kids works out a child's BMI from height and weight, then explains how to read it on the growth charts that paediatricians use. A child's BMI cannot be judged by adult bands. It is plotted on a chart for that exact age, with separate charts for girls and boys, which gives a percentile.
For Indian children aged 5 to 18, doctors use the IAP charts for BMI. For children under 5, the WHO standards apply. The calculator needs age in years and months, sex, height in cm and weight in kg.
Quick numbers
Ages
2 to 18 yearsUnderweight
below the 5th percentileOverweight (IAP)
above the adult 23 equivalent lineObese (IAP)
above the adult 27 equivalent lineUnder 2 years
weight for length is used instead
How the BMI calculator for kids works
- BMI is worked out with the adult formula: kg divided by height in metres squared.
- The value is then placed on a chart for the child's exact age and sex.
- The chart line it falls near gives the percentile.
- The calculator above shows the BMI and tells you which chart and line to look at for that age.
A 9 year old girl, 132 cm and 30 kg: BMI = 30 ÷ (1.32 × 1.32) = 17.2. On a girls' chart at 9 years, that sits above the middle but well inside the healthy band.
Reading the result
| Where BMI falls | What it suggests |
|---|---|
| Below 5th percentile | Underweight; check diet and health |
| 5th to adult 23 equivalent line | Healthy |
| Between adult 23 and 27 equivalent lines | Overweight |
| Above adult 27 equivalent line | Obese |
The IAP charts draw lines that end at BMI 23 and 27 at age 18, matching Indian adult cut-offs. WHO charts use percentiles or standard deviations with similar meaning.
Why kids need age and sex charts
BMI naturally falls from about age 1 to 6, as toddlers lose baby fat and grow taller, then rises again through childhood and puberty. Girls and boys follow different curves because they mature at different times. So a BMI of 18 is high for a 6 year old but normal for a 14 year old.
BMI calculator for teens and girls
During puberty, weight often goes up before height, and girls add some healthy body fat. A temporary jump in BMI is common and usually settles. Teens should never diet on adult calorie numbers. If you are worried, ask a paediatrician to plot the BMI for age and look at the trend.
Baby BMI and infants
For babies and toddlers under 2, doctors use weight for length rather than BMI. A baby BMI calculator in kg and cm can give a number, but it has little meaning on its own. Use the baby weight chart or the newborn weight page instead.
Healthy habits for any number
- Three meals and one or two healthy snacks, with dal, curd, eggs, fruit and vegetables.
- Water and milk instead of sweet drinks.
- At least one hour of active play every day.
- Screens limited to one to two hours of leisure time.
- Enough sleep: 10 to 13 hours for preschoolers, 9 to 12 for school children, 8 to 10 for teens.
The kids weight by age chart gives typical weights for ages 1 to 10.
When to see the doctor
See a paediatrician if your child's BMI is under the 5th percentile or above the overweight line, if weight changes quickly, if they complain about their body a lot or avoid food, or if they snore loudly or tire easily. Never start a child on calorie cutting unless the doctor advises it.
Frequently asked questions
How do I calculate BMI for a child?
Divide weight in kg by height in metres squared, then read the result on a BMI for age chart for your child's sex.
Which BMI is healthy at age 10?
It depends on sex and exact age. On most charts, a BMI of about 14 to 20 is within the healthy band at 10.
Do adult BMI bands work for children?
No. Children's BMI changes with age, so adult bands would mislabel many healthy children.
Is BMI accurate for children?
It is a good screening tool. A paediatrician will also look at growth over time, family size and puberty.
What is a BMI percentile?
It places a child among 100 others of equal age, boys with boys and girls with girls. The 50th percentile is the middle.
Can a baby have a BMI?
The number can be worked out, but doctors use weight for length charts for babies under 2.
Sources
- Indian Academy of Pediatrics growth charts
- WHO Child Growth Standards
- WHO growth reference 5 to 19 years