Our free BMI Calculator computes your Body Mass Index instantly from height and weight, in either metric (kg/cm) or imperial (lbs/ft/in) units. Beyond the raw number, you get a colour-coded visual gauge showing exactly where you fall on the scale, the healthy weight range for your specific height, a plain-English explanation of what the result means, and two secondary metrics — BMI Prime and the Ponderal Index — for a more complete picture.
Body Mass Index is a simple screening measurement calculated from height and weight. The metric formula is: BMI = weight (kg) ÷ height (m)². For imperial units, the equivalent is: BMI = 703 × weight (lbs) ÷ height (in)². For example, a person who is 70 kg and 175 cm has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
The World Health Organisation classifies adult BMI into four main bands: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), and Obese (30.0 and above). The Obese band is further subdivided into Class I (30–34.9), Class II (35–39.9), and Class III or morbid obesity (40 and above), though this calculator shows the four main categories for clarity.
The calculator works backwards from the Normal BMI range (18.5–24.9) to give you the actual weight range in your chosen unit. For example, a person who is 175 cm tall has a healthy weight range of roughly 56.7–76.2 kg. This is shown directly in the result panel after you enter your measurements.
BMI Prime is the ratio of your BMI to the upper limit of the Normal range (25). It is calculated as BMI Prime = BMI ÷ 25. A value of 1.00 means you are exactly at the top of the Normal range. Values below 1.00 are in the Normal or Underweight range; values above 1.00 indicate Overweight or Obese. BMI Prime makes it easy to see by how much a BMI deviates from the boundary: a BMI Prime of 1.20 means 20% above the Normal upper limit.
The Ponderal Index (PI) is an alternative leanness metric calculated as PI = weight (kg) ÷ height (m)³. Unlike BMI, which uses height squared, the Ponderal Index uses height cubed, making it somewhat more accurate for very tall or very short individuals. The typical normal range for adults is 11–14 kg/m³. Values below 11 suggest a lean or thin build; values above 14 suggest higher weight relative to height.
BMI is a population-level screening tool, not a direct measure of body fatness or health. It does not distinguish between muscle and fat, so a muscular athlete can have an "Overweight" BMI while being very lean. It also does not account for fat distribution (visceral vs. subcutaneous), age-related changes in muscle mass, or ethnic differences in body composition. Use BMI as a starting point, not a definitive health verdict.