BMI Calculator (Body Mass Index)

Body Mass Index (BMI) relates a person's weight to their height. In metric units it's calculated as BMI = weight (kg) / height (m)². For example, someone weighing 70 kg at 1.70 m has a BMI of 70 / (1.70 × 1.70) = 24.2. In imperial units the formula is BMI = 703 × weight (lb) / height (in)². It was developed in the 19th century by Belgian statistician Adolphe Quetelet and is used today by the World Health Organization (WHO) as a population-level reference.

For adults, the WHO defines: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9) and obesity (30 or above, split into class I from 30 to 34.9, class II from 35 to 39.9, and class III or severe obesity from 40 up). These cutoffs are the same for adult men and women, though some Asian populations use somewhat lower thresholds due to higher metabolic risk at the same BMI.

No. BMI only relates weight to height, without distinguishing muscle mass from fat mass. An athlete or bodybuilder with high muscle mass may score "overweight" or "obese" without excess body fat. It's also unreliable during pregnancy, for children and teenagers (who need age- and sex-specific percentile charts), and for older adults, who often lose muscle and gain fat in ways BMI doesn't capture well. It's a population screening tool, not an individual diagnosis.

Weight is solved from the BMI formula using the 18.5 and 24.9 cutoffs: minimum weight = 18.5 × height (m)² and maximum weight = 24.9 × height (m)². For example, at 1.70 m the healthy range is roughly 53.5 kg to 72.0 kg. This calculator does that automatically and also shows how much weight you'd need to lose or gain to fall within range.

BMI is an indirect calculation based only on weight and height. Body fat percentage directly measures how much of the body is fat tissue, using bioelectrical impedance, skinfold calipers, DEXA scans or other methods. Two people with the same BMI can have very different body compositions: BMI works as a first population-level filter, but body fat percentage (together with waist circumference) gives a more precise picture of individual health risk.

The BMI formula doesn't distinguish by sex because it was designed as a simple population measure. In practice, though, women tend to carry a higher body fat percentage than men at the same BMI, due to typical differences in body composition. That's why health professionals pair BMI with other measures (waist circumference, skinfold thickness) when assessing an individual patient rather than relying on this number alone.

An underweight BMI is associated with higher risk of malnutrition, osteoporosis and anemia. An overweight or obese BMI is statistically associated with higher risk of type 2 diabetes, hypertension, cardiovascular disease and certain cancers. These are population-level associations from epidemiological studies, not an individual prognosis: lifestyle, genetics and other health factors matter as much as, or more than, the BMI number itself.

Doctors and nurses use it as a quick first filter during general health checkups. Nutritionists and personal trainers reference it when designing diet or training plans, always alongside other measures. Health insurers use it in some countries as an actuarial reference. Public health researchers track it to monitor population weight trends over time. And anyone can use it as a rough first reference about their weight, without it replacing a full medical assessment.

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