How to use the child BMI calculator
- Choose units and the child's sex.
- Enter their age in years and months – children's BMI changes quickly with age, so the months matter.
- Enter height and weight, measured without shoes and in light clothes.
- Press Calculate for the BMI percentile, the weight category, the healthy range for their age and height, and a growth chart.
Why children use BMI percentiles
Body fat changes a lot as children grow, and differs between boys and girls, so the adult cut-offs (25 for overweight, 30 for obesity) don't apply. Instead, a child's BMI is compared with children of the same age and sex on the CDC growth charts. The percentile says what share of those children have a lower BMI: at the 70th percentile, the BMI is higher than 70 out of 100.
Weight categories
| BMI-for-age percentile | Category |
|---|---|
| Below the 5th | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th or above | Obesity |
| 120% of the 95th percentile or more | Severe obesity (class 2) |
| 140% of the 95th percentile or more | Severe obesity (class 3) |
Categories follow the US Centers for Disease Control and Prevention (CDC). For BMIs above the 95th percentile, the CDC now grades obesity by how far the BMI is above the 95th percentile, because very high percentiles on the 2000 charts are not precise.
How the percentile is calculated
The CDC publishes three values – L, M and S – for every half-month of age from 2 to 20 years, separately for boys and girls. They turn a BMI into a z-score, and the z-score into a percentile:
z = \dfrac{(\text{BMI}/M)^{L} - 1}{L \times S}The percentile is the share of a normal distribution below z: z = 0 is the 50th percentile, 1.04 the 85th and 1.645 the 95th. The calculator uses the CDC's own data table and interpolates between half-months.
At 120.5 months the CDC values are L = -2.766, M = 16.65, S = 0.1201. So z = ((19.0 / 16.65)^{-2.766} - 1) / (-2.766 \times 0.1201) = 0.92, about the 82th percentile – a healthy weight.
BMI-for-age chart
BMI at key percentiles from the CDC growth charts:
| Age | Boys 5th | Boys 85th | Boys 95th | Girls 5th | Girls 85th | Girls 95th |
|---|---|---|---|---|---|---|
| 2 years | 14.7 | 18.1 | 19.3 | 14.4 | 18.0 | 19.1 |
| 4 years | 14.0 | 16.9 | 17.8 | 13.7 | 16.8 | 18.0 |
| 6 years | 13.7 | 17.0 | 18.4 | 13.4 | 17.1 | 18.8 |
| 8 years | 13.8 | 18.0 | 20.1 | 13.5 | 18.3 | 20.7 |
| 10 years | 14.2 | 19.4 | 22.2 | 14.0 | 20.0 | 23.0 |
| 12 years | 15.0 | 21.0 | 24.2 | 14.8 | 21.7 | 25.3 |
| 14 years | 16.0 | 22.7 | 26.0 | 15.8 | 23.3 | 27.3 |
| 16 years | 17.1 | 24.2 | 27.6 | 16.8 | 24.7 | 28.9 |
| 18 years | 18.2 | 25.7 | 29.0 | 17.6 | 25.7 | 30.3 |
Limits
- BMI doesn't separate muscle from fat, and children mature at different ages – a single result is less useful than the trend over several years.
- The CDC charts are the US standard. Many other countries, including India and the UK, use WHO or national charts, which can give slightly different percentiles.
- Under 2 years, weight-for-length charts are used instead, and from age 20 the adult BMI calculator applies.
Never put a child on a weight-loss diet without medical advice. Healthy habits for the whole family – regular meals, plenty of fruit and vegetables, fewer sugary drinks, an hour of activity a day and enough sleep – are the usual first step.
Frequently asked questions
What is a healthy BMI percentile for a child?
Between the 5th and the 85th percentile for their age and sex. Below the 5th is underweight, the 85th to 94th overweight, and the 95th or above obesity.
Why is BMI different for children?
Children's body fat changes as they grow and differs between boys and girls, so their BMI is compared with other children of the same age and sex rather than with fixed adult cut-offs.
What does the 90th percentile mean?
The child's BMI is higher than that of 90 out of 100 children of the same age and sex. For BMI, that falls in the overweight range.
From what age can I use this calculator?
From 2 to 19 years. Under 2, doctors use weight-for-length charts; from 20, use the adult BMI calculator.
Should I put my child on a diet if their BMI is high?
Not without talking to their doctor. Growing children usually need healthier habits rather than calorie cutting, and the doctor will look at growth over time.