BMI Calculator

Body Mass Index (BMI) is a numerical value derived from a person's weight and height that serves as a simple screening tool for categorizing body weight. The formula, first proposed by Belgian mathematician Adolphe Quetelet in the 1830s, is defined as: BMI = weight (kg) / height (m)^2. For example, a person weighing 70 kg with a height of 1.75 m has a BMI of 70 / (1.75 x 1.75) = 22.9. In imperial units, the formula becomes: BMI = (weight in pounds x 703) / (height in inches)^2.

The World Health Organization (WHO) classifies BMI into four primary categories: Underweight (below 18.5), Normal weight (18.5 to 24.9), Overweight (25.0 to 29.9), and Obese (30.0 and above). These thresholds were established through large-scale epidemiological studies correlating BMI ranges with mortality and morbidity risks. The normal range of 18.5-24.9 is associated with the lowest statistical risk of weight-related health conditions including type 2 diabetes, cardiovascular disease, and certain cancers.

While BMI is widely used due to its simplicity and low cost, it has well-known limitations. It does not distinguish between muscle mass and fat mass, meaning highly muscular athletes may be classified as overweight despite having low body fat. It also does not account for fat distribution -- visceral fat around internal organs carries higher health risks than subcutaneous fat. Age, sex, and ethnicity also influence the relationship between BMI and actual body fat percentage. For these reasons, BMI is best used as an initial screening metric rather than a definitive diagnosis.

Step-by-step instructions:

1. Select your preferred unit system: Metric (kg/cm) or Imperial (lb/ft) using the tabs at the top.
2. For Metric mode, enter your height in centimeters and weight in kilograms.
3. For Imperial mode, enter your height in feet and inches, and your weight in pounds.
4. Your BMI is calculated instantly and displayed with a color-coded category (Underweight, Normal, Overweight, or Obese).
5. The healthy weight range for your height is shown below the BMI value, giving you a target range to aim for.
6. A visual BMI scale bar shows where your value falls relative to all categories, with a marker indicating your exact position.

• Health screening: Doctors use BMI as a first-pass screening tool during routine checkups to identify patients who may be at risk for weight-related conditions.
• Weight management goals: Knowing your BMI and the healthy range for your height provides a concrete numerical target for weight loss or gain programs.
• Life insurance underwriting: Insurance companies factor BMI into premium calculations, with higher BMIs often corresponding to higher premiums due to increased health risk.
• Public health surveillance: Government health agencies track population-level BMI distributions to monitor obesity trends and allocate public health resources.
• Fitness baseline tracking: While imperfect, tracking BMI over time alongside other metrics (body fat percentage, waist circumference) provides a useful longitudinal picture of body composition changes.
• Surgical risk assessment: Anesthesiologists and surgeons evaluate BMI as part of pre-operative risk assessment, since extreme BMI values (very low or very high) are associated with increased surgical complications.
• Pediatric growth monitoring: For children and teens, BMI-for-age percentiles (adjusted for sex and age) help pediatricians track healthy growth trajectories.