Why include age in a kg BMI calculator?
Adding age does two useful things. First, it lets the tool route correctly:
if the age falls between 2 and 19, the result switches to child/teen
guidance, because young people need BMI-for-age percentiles rather than the
adult bands. Second, for adults it provides context for a conversation with
a clinician - even though the adult categories themselves do not change with
age.
The metric formula
BMI = weight (kg) ÷ [height (m) × height (m)]. The calculator turns your
centimetres into metres automatically, so you only enter kg and cm. Your
result is rounded to one decimal place.
Adult BMI categories (ages 20+)
Adult BMI categories for ages 20 and older | Category | BMI range |
| Underweight | Below 18.5 |
| Healthy Weight | 18.5 - 24.9 |
| Overweight | 25.0 - 29.9 |
| Obesity | 30.0 and above |
| Class 1 Obesity | 30.0 - 34.9 |
| Class 2 Obesity | 35.0 - 39.9 |
| Class 3 Obesity | 40.0 and above |
Children and teens (ages 2-19)
Different rules for under-20s
If you enter an age between 2 and 19, this calculator shows the BMI number
but not an adult category. Official interpretation needs age- and
sex-specific percentiles. Use our
child BMI calculator and the CDC tool
for the percentile category.
Reading your result
For everyone, BMI is a screening measure rather than a diagnosis. It is a
quick, private way to check a number in kilograms - pair it with
professional advice for decisions about your health.
Frequently asked questions
Can I calculate BMI in kg and cm?
Yes. Switch the toggle to metric units and enter your height in centimetres and your weight in kilograms. The calculator converts your height to metres internally and returns your BMI rounded to one decimal place, with the matching adult category.
Does BMI change by age?
For adults 20 and older, the BMI categories stay the same regardless of age - a BMI of 24 means the same thing at 25 as it does at 65. Age still matters for interpretation, because muscle and fat distribution change over time. For children and teens 2-19, BMI must be read against age- and sex-specific growth charts instead of adult categories.
Why is a child's BMI handled differently?
Children and teens are still growing, so a healthy BMI range changes with age and differs between boys and girls. Instead of fixed adult categories, child BMI is interpreted using BMI-for-age percentiles from growth charts. This page can calculate the BMI number, but the official percentile category should come from the CDC Child and Teen BMI Calculator.
How do you calculate BMI?
In metric units, BMI = weight in kilograms ÷ (height in metres × height in metres). In US units, BMI = (weight in pounds ÷ (height in inches × height in inches)) × 703. The calculator above does this for you and rounds the result to one decimal place.
Is my height and weight kept private?
Yes. The calculator runs entirely in your browser using JavaScript. Your height, weight, age, and any other input are never sent to a server or stored by us, so your measurements stay on your device.