BMI Calculator

What this calculator does & how it works

Body Mass Index (BMI) is a simple ratio of your weight to your height. The formula was popularised as a population screening tool and is defined by the World Health Organization as your weight in kilograms divided by the square of your height in metres: BMI = kg / m². If you enter imperial units, this tool converts pounds and feet/inches to metric first, so the result is identical either way.

Once it has your BMI, the calculator places it into the WHO adult categories and works backwards to show the weight range that would put you in the "normal" band (a BMI of 18.5 to 24.9) for your specific height. That range is often more useful than the single number, because it gives you a concrete target rather than an abstract score.

How to interpret your results

The WHO adult categories are: under 18.5 — underweight; 18.5 to 24.9 — normal weight; 25 to 29.9 — overweight; 30 and above — obese (often split into class I, II and III). These cut-offs describe statistical risk across large populations; they are not a verdict on any individual. A BMI in the "overweight" band, for instance, does not by itself mean you are unhealthy.

BMI is best read alongside other simple measures. Your waist-to-height ratio captures where you carry weight, which matters for metabolic risk, and your daily calorie needs help you turn a target weight into a daily plan.

Limitations and when to consult a professional

BMI cannot tell muscle from fat. A muscular athlete and someone carrying excess fat can share an identical BMI. It also does not account for sex, age, ethnicity, bone density, or fat distribution, and the standard adult cut-offs are not appropriate for children, pregnant people, or frail older adults. Some populations carry higher metabolic risk at lower BMI values. For these reasons, clinicians use BMI as one input among many — never on its own.

If you are trying to lose or gain weight, are concerned about your health, or have a condition that affects body composition, talk to a doctor or a registered dietitian. They can interpret your numbers in the context of your full health picture, which a calculator cannot do. Nothing here is medical advice.

Frequently asked questions