Child BMI Calculator
Work out a child's BMI from their height and weight. For children aged 2–18, BMI must be read against age and sex centiles — this shows the BMI figure and explains what it means.
Child BMI figure
Read against age & sex centiles.
A guide only. For the exact centile and category, use the NHS child BMI tool and speak to a GP or health visitor.
How child BMI is different
A child's BMI is calculated the same way as an adult's — weight in kilograms divided by height in metres squared — but it is read very differently. Because children's body fat changes naturally as they grow, the fixed adult cut-offs (18.5, 25, 30) do not apply. Instead, a child's BMI is compared against age- and sex-specific centiles. This tool gives you the BMI figure; the exact centile comes from the NHS chart.
Calculate the BMI here, then read it against centiles: roughly the 2nd–91st centile is a healthy weight for the child's age and sex, 91st–98th is overweight, and above the 98th is obese. The NHS child BMI tool plots the precise centile for you.
How the centiles work
| BMI centile | Category |
|---|---|
| Below 2nd | Underweight |
| 2nd–91st | Healthy weight |
| 91st–98th | Overweight |
| Above 98th | Obese |
Centiles compare a child to others of the same age and sex. Source: NHS — children's weight.
Looking after your child's health
If you are worried about your child's weight, a GP or health visitor can plot the exact centile and offer support. For adults in the household, the BMI calculator gives the standard categories and the ideal weight calculator shows a healthy range. The calorie calculator can help plan balanced family meals.
Frequently asked questions
How is a child's BMI calculated?
A child's BMI is worked out the same way as an adult's — weight in kilograms divided by height in metres squared. The difference is how it is interpreted: for children it must be compared against age and sex centile charts, not the fixed adult categories.
Why is child BMI read against centiles?
Children's body fat changes a lot as they grow, so a healthy BMI differs by age and sex. The NHS compares a child's BMI to others of the same age and sex using centiles — for example the 91st centile is the overweight threshold and the 98th is obese.
What is a healthy BMI centile for a child?
A healthy weight is generally between the 2nd and 91st BMI centile for the child's age and sex. Below the 2nd is underweight, the 91st–98th is overweight, and above the 98th is obese. A clinician can plot the exact centile.
Can I use the adult BMI categories for my child?
No. The adult cut-offs of 18.5, 25 and 30 do not apply to children aged 2 to 18. You need the age- and sex-specific centiles, which is why this calculator gives the BMI figure and directs you to the NHS tool for the centile.
From what age can I check a child's BMI?
BMI centiles are used for children from age 2 up to 18. For babies under 2, growth is monitored differently using weight-for-length and head circumference charts in the personal child health record.