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Calculate your Body Mass Index (BMI) instantly. Find out if you are underweight, normal, overweight, or obese based on your height and weight.
The Body Mass Index (BMI) is one of the most widely used screening tools in modern preventive medicine. By dividing your weight in kilograms by the square of your height in meters, it produces a single number that broadly categorises your weight status — whether underweight, healthy, overweight or obese. While BMI is not a perfect diagnostic tool and does not account for muscle mass or body fat distribution, it remains a valuable first checkpoint endorsed by the World Health Organization and the NHS alike.
Understanding your BMI is most meaningful when combined with other health metrics. For example, after you calculate your BMI here, visit our Calorie Calculator to discover how many calories your body genuinely needs each day based on your weight, age and activity level. The two tools together give you a far more actionable picture of your health than either can alone.
It is also worth noting that BMI should always be interpreted alongside lifestyle factors. Our Daily Water Intake Calculator can help you determine the optimal hydration level for your body weight — a factor that directly influences both metabolism and weight management. Small, consistent changes informed by data are the cornerstone of lasting well-being.
What is BMI and how is it calculated?
Body Mass Index is calculated by dividing your weight in kilograms by the square of your height in metres: BMI = kg ÷ m². The WHO defines four categories: Underweight (below 18.5), Normal (18.5–24.9), Overweight (25–29.9) and Obese (30 and above). The formula was developed by Belgian mathematician Adolphe Quetelet in 1832 and remains the most widely used population-level screening tool endorsed by the World Health Organization, the NHS and the CDC. For children and teenagers, age- and sex-specific percentile charts are used instead of adult thresholds.
Why BMI alone is not enough
BMI does not distinguish between fat mass and lean muscle mass, which means a muscular athlete may register as "overweight" while a sedentary person with excess visceral fat may score in the "normal" range. For a more complete picture, doctors combine BMI with waist circumference (risk increases above 88 cm for women and 102 cm for men), body fat percentage, and blood markers such as fasting glucose and cholesterol. Use our Calorie Calculator alongside your BMI result to understand your true energy needs, and our Water Intake Calculator to support your metabolism effectively.