BMI Calculator

How to use this tool

1. Choose a unit system. Select metric (kg/cm) or imperial (lb/ft+in). The calculator defaults to the system used in your location.
2. Enter your weight and height. Type your current weight and height. For imperial, enter feet in the first box and remaining inches in the second.
3. Read your result. Your BMI and BMI Prime appear with the WHO classification, healthy weight range, and your distance from that range. The bar shows where your BMI falls on the scale. BMI is a screening measure, not a diagnosis.

BMI categories

The World Health Organization defines five adult BMI classifications:

• Below 18.5: Underweight. Associated with nutritional deficiency and increased risk of certain conditions including osteoporosis and anemia. WHO tables further describe mild thinness (17 to below 18.5), moderate thinness (16 to below 17), and severe thinness (below 16); these remain screening categories, not diagnoses.
• 18.5–24.9: Normal weight. The range associated with the lowest all-cause mortality risk in most large epidemiological studies.
• 25–29.9: Overweight. Modestly elevated risk of cardiovascular disease and type 2 diabetes in population-level data.
• 30–34.9: Obesity class I. Substantially elevated metabolic risk.
• 35–39.9: Obesity class II. High risk; often qualifies for weight-loss intervention.
• 40 and above: Obesity class III (severe obesity). Very high risk of metabolic, cardiovascular, and musculoskeletal complications.

How BMI is calculated

BMI is your weight divided by the square of your height. In metric units:

BMI = weight (kg) ÷ height (m)²

In imperial units the formula adjusts for unit conversion:

BMI = weight (lb) × 703 ÷ height (in)²

A 70 kg person who is 1.75 m tall: 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9. That falls in the normal weight range. The healthy weight range shown is the reverse: the weight that would give a BMI of exactly 18.5 and exactly 24.9 at your height.

BMI Prime is BMI divided by 25, a dimensionless comparison to the upper boundary of the standard healthy range. A Prime of 1.00 corresponds to BMI 25; for example, a BMI of 22.5 has a Prime of 0.90. It is another way to describe the same height-and-weight estimate, not a separate diagnosis or personal target. The result also shows how far the entered weight is from the healthy range calculated for that height.

Limitations of BMI

BMI was developed in the 1830s by mathematician Adolphe Quetelet as a statistical tool for describing populations, not for diagnosing individuals. Several limitations are well documented:

• Muscle vs. fat: BMI measures weight, not composition. A trained athlete with low body fat and high muscle mass can register as overweight or obese. Conversely, a sedentary person at a "normal" BMI may carry high visceral fat.
• Age: Older adults tend to have more body fat at the same BMI than younger adults. The thresholds were derived from studies skewed toward working-age populations.
• Ethnicity: The WHO cutoffs were calibrated on studies dominated by non-Hispanic white populations. Studies of South Asian, East Asian, and some other populations suggest metabolic risk rises at lower BMIs. Some guidelines use adjusted cutoffs for these groups.
• Sex: Women typically carry more body fat than men at the same BMI; standard cutoffs don't account for this.

Despite its limitations, BMI remains in wide clinical use because it is quick, free, and correlates reasonably well with metabolic risk at the population level. For individual assessment, clinicians pair it with waist circumference, lipid panels, blood pressure, and blood glucose.

BMI vs. other body composition measures

BMI is a starting point, not a complete picture. Clinicians often pair it with: