What is a BMI calculator?
A BMI calculator estimates body mass index, a value that relates an adult's weight to their height. It gives a quick screening measure that can help place a weight measurement into a standard adult category. Enter your height and weight, choose metric or imperial units, and the calculator shows your BMI and its corresponding category.
This page calculates BMI locally in your browser. The values you enter are not uploaded, stored or shared. It supports kilograms and centimetres for metric measurements, or pounds with feet and inches for imperial measurements.
How to use this BMI calculator
- Select Metric or Imperial units.
- In metric mode, enter your weight in kilograms and height in centimetres.
- In imperial mode, enter your weight in pounds, plus your height in feet and inches.
- Read your calculated BMI and the adult screening category.
- Use the result as a starting point for a conversation with a qualified healthcare professional if you have concerns about your weight or health.
For example, a person weighing 65 kg at 170 cm has a BMI of about 22.5. The calculator works immediately as you edit an input, so you can compare values without submitting a form.
How BMI is calculated
In metric units, BMI equals weight in kilograms divided by height in metres squared. In imperial units, the calculation is weight in pounds divided by height in inches squared, multiplied by 703. Both formulas describe the same relationship and give equivalent results when the measurements are converted accurately.
For adults aged 20 and older, standard BMI categories are: underweight below 18.5, healthy weight from 18.5 to below 25, overweight from 25 to below 30, and obesity at 30 or above. Obesity can be further grouped into classes at 30, 35 and 40. This calculator displays the more specific class when it applies.
Important limits of BMI
BMI is a screening tool, not a diagnosis of health or body fat. It does not directly measure body composition and may not reflect individual circumstances such as substantial muscle mass, pregnancy, age, ethnicity, or certain medical conditions. A BMI category should be considered alongside other health information and professional advice.
These adult thresholds are not intended for children or teenagers. Young people use age- and sex-specific BMI percentiles rather than the adult categories shown here. If you are concerned about a BMI result, or about unintended weight change, speak with a healthcare professional who can consider your full health picture.