BMI Calculator

What is BMI?

BMI — Body Mass Index — is a number that compares your weight to your height. Doctors and health workers use it as a quick first check for whether a person is underweight, at a healthy weight, overweight or obese. It is a common screening tool, not a final diagnosis — but it instantly tells you which range you are in.

How BMI is calculated

The formula is simple: BMI = weight in kilograms ÷ (height in metres)². For example, a person who weighs 70 kg and is 170 cm (1.70 m) tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2, which is in the healthy range.

The tool also shows your healthy weight range. For a height of 170 cm that range is 53.5 kg to 72.0 kg — any weight inside it gives a BMI between 18.5 and 24.9.

Tips and common mistakes

• Measure correctly. Weigh yourself in the morning, without shoes, and measure height standing straight against a wall. Even 2 cm of error changes the BMI.
• BMI is not the whole story. It cannot tell muscle from fat, so athletes can score as overweight while being very fit. Waist size matters too — for men, a waist above 102 cm and for women above 88 cm adds risk even at a normal BMI.
• Do not use it for children. Children's BMI is judged on age-and-gender growth charts, not these adult ranges.
• Use it as a starting point. If your BMI is outside the healthy range, a small change — a daily 30-minute walk and less sugary drinks — is the safest first step. For medical advice, see a doctor.

When to use a BMI calculator

Use it as a regular health check — once a month is enough. It is most useful at the start of a fitness journey, before starting a diet or a gym routine, and at an annual medical check-up, so you can see whether your weight is moving in the right direction. Parents sometimes check a teenager's BMI, but remember that children's BMI uses different growth charts, so this adult calculator is not suitable under age 18. It is also useful before a medical test or an insurance application, where weight and height are routinely recorded. Combine the result with two other simple checks — your waist measurement and how you feel day to day — and you have a much better picture of your health than any single number can give.