Calculate your Body Mass Index (BMI) in metric or imperial units and see your healthy weight range.
BMI (Body Mass Index) is a simple ratio of weight to height squared. It's a screening tool, not a diagnostic, used to flag potential weight-related health risks. A single BMI reading doesn't account for muscle mass, bone density, age, or where fat is stored.
The standard adult ranges are: Under 18.5 = Underweight, 18.5–24.9 = Healthy weight, 25–29.9 = Overweight, 30 and above = Obese. These thresholds were set by the WHO and are used globally, though some guidelines use slightly different cut-offs for certain ethnic groups.
Athletes with high muscle mass often register as 'overweight' or even 'obese' on BMI scales. Conversely, someone with low muscle and high body fat ('skinny fat') may read as healthy. For a fuller picture, waist circumference and body fat percentage are also useful.
The metric formula is: BMI = weight (kg) ÷ height (m)². The imperial formula is: BMI = (weight (lbs) × 703) ÷ height (inches)². Both produce the same result. They are just two ways of expressing the same calculation.
Rather than targeting a specific BMI number, aim for the healthy weight range for your height. For example, someone 175 cm (5'9") tall has a healthy weight range of roughly 57–79 kg (126–174 lbs). Small changes in weight can shift your category.
BMI for under-18s is interpreted differently; it is plotted on age- and sex-specific growth charts as a percentile rather than a fixed range. The tool below is designed for adults (18+). For children, consult a GP or use a paediatric BMI tool.
Calculate your Body Mass Index in metric or imperial units. Enter your height and weight to instantly see your BMI, category, and the healthy weight range for your height.
70 kg at 175 cm (metric)
BMI: 22.9 — Healthy weight
90 kg at 170 cm (metric)
BMI: 31.1 — Obese
150 lbs at 5 ft 8 in (imperial)
BMI: 22.8 — Healthy weight
55 kg at 162 cm (metric)
BMI: 20.9 — Healthy weight
Metric BMI: 80 kg at 178 cm
BMI: 25.25 — Overweight
Imperial BMI: 160 lbs at 5 ft 9 in
BMI: 23.63 — Healthy weight
BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and was originally called the Quetelet Index. It was adopted by the WHO as a population health screening measure in the 1980s. Quetelet himself never intended it to diagnose individual health — it was designed purely for statistical studies of populations.
A BMI of 18.5 to 24.9 is considered healthy for most adults. Under 18.5 is underweight, 25–29.9 is overweight, and 30 or above is classified as obese. BMI is a useful screening tool, not a medical diagnosis.
BMI equals weight in kilograms divided by height in metres squared (kg/m²). For example, someone weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9, which falls in the healthy range.
BMI has limitations — it doesn't distinguish muscle from fat, so athletes may appear overweight. It also doesn't account for age, sex, or fat distribution. Use it as a general guide alongside other health indicators.