BMI Calculator

Body Mass Index is a numerical value derived from your weight and height. It provides a quick screening tool to categorize weight status as underweight, normal, overweight, or obese, though it does not measure body fat directly.

BMI can overestimate body fat in athletes and people with high muscle mass, since muscle weighs more than fat. For these individuals, body fat percentage or waist-to-hip ratio may be more meaningful measurements.

The standard WHO categories are: Underweight (below 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obese (30 and above). These thresholds apply to adults and may differ for children and teens.

BMI is a screening tool, not a diagnosis. If your BMI is outside the normal range, consult a healthcare provider who can consider additional factors like body composition, diet, exercise, and overall health before making recommendations.

BMI equals weight in kilograms divided by height in meters squared (kg/m²). For imperial units, the formula is (weight in pounds × 703) / (height in inches)². Both formulas produce the same result.

BMI often classifies athletes and bodybuilders as overweight or obese because it cannot distinguish between muscle mass and fat mass. A person with significant muscle mass may have a high BMI but very low body fat. In these cases, body fat percentage, DEXA scans, or skinfold measurements provide more accurate assessments.

For children and teens aged 2–19, BMI is interpreted using age- and sex-specific percentile charts rather than fixed thresholds. A child at the 85th to 94th percentile is considered overweight, and at or above the 95th percentile is considered obese. This accounts for the natural variation in body composition as children grow.

BMI Prime is your BMI divided by 25 (the upper limit of the normal range). A BMI Prime of 1.0 means you are exactly at the upper boundary of normal weight. Values below 1.0 indicate normal or underweight, and values above 1.0 indicate overweight or obese. It provides a quick way to see how far you are from the normal range.

Several measures can complement or improve upon BMI: waist circumference measures abdominal fat specifically, waist-to-hip ratio assesses fat distribution, body fat percentage (via DEXA, calipers, or bioimpedance) directly measures fat mass, and the Body Roundness Index accounts for body shape. No single measure is perfect — healthcare providers often use multiple indicators together.

Research shows that health risks associated with BMI vary by ethnicity. People of South Asian and East Asian descent tend to have higher body fat percentages at lower BMI values, leading some guidelines to use lower thresholds (e.g., overweight at 23 instead of 25). Conversely, people of Polynesian descent may carry more muscle mass at the same BMI.

The Ponderal Index (PI) is calculated as weight (kg) divided by height (m) cubed, rather than squared as in BMI. This makes it less sensitive to height — very tall and very short people get more accurate results with PI than with BMI. A normal Ponderal Index typically falls between 11 and 14 kg/m³.

Large-scale studies show a U-shaped relationship between BMI and mortality: both very low and very high BMI values are associated with increased mortality risk. The lowest risk generally falls in the BMI range of 20–25, though some studies suggest slightly higher BMI may be protective in older adults. BMI alone is a weak predictor — overall fitness, diet quality, and other health markers matter more.