BMI Calculator

How Does the Formula Work?

Body Mass Index (BMI) is a simple screening metric that uses your height and weight to estimate body fat. It was developed by Belgian mathematician Adolphe Quetelet in 1832 and is used worldwide by the World Health Organization (WHO) as a population-level indicator of weight status.

The WHO classifies BMI into categories: underweight (below 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30 and above). These thresholds were established from large epidemiological studies linking BMI ranges to health outcomes like cardiovascular disease, type 2 diabetes, and mortality risk.

Important Limitations

BMI is a screening tool, not a diagnostic one. It does not distinguish between muscle and fat — a muscular athlete may have a "overweight" BMI despite low body fat. It also does not account for age (older adults naturally have more body fat), sex (women typically have more body fat than men at the same BMI), or ethnicity (Asian populations may have higher health risks at lower BMI thresholds, which is why some countries use adjusted cutoffs).

For a more complete picture of health, healthcare professionals use BMI alongside waist circumference, body fat percentage, blood pressure, blood sugar levels, and lipid profiles. The American Heart Association and the National Institutes of Health both recommend using BMI as one of several tools — never as the sole indicator.

BMI for Children and Teens

This calculator is designed for adults aged 20 and older. For children and adolescents (ages 2–19), BMI is interpreted using age- and sex-specific percentile charts from the CDC or WHO growth standards. A pediatrician should evaluate a child's BMI in the context of their growth pattern.

What Is BMI and How Is It Calculated

Body Mass Index (BMI) is a simple screening tool that estimates body fat based on height and weight. The formula divides weight in kilograms by height in meters squared: BMI = kg / m². For imperial units, the formula is BMI = (lbs × 703) / (inches²). A BMI of 18.5 to 24.9 is considered normal weight. Below 18.5 is underweight. 25.0 to 29.9 is overweight. 30.0 and above is classified as obese. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) use these thresholds for adult screening. BMI does not directly measure body fat — it is a proxy based on population statistics. Athletes with high muscle mass may have a high BMI despite low body fat, while elderly individuals may have a normal BMI with high body fat due to muscle loss.

BMI Categories and Health Risk

Research consistently shows that BMI outside the normal range correlates with increased health risks. Underweight (below 18.5) is associated with malnutrition, osteoporosis, immune deficiency, and fertility issues. Overweight (25 to 29.9) increases risk of Type 2 diabetes, hypertension, and cardiovascular disease. Obesity (30 plus) significantly elevates risk of heart disease, stroke, Type 2 diabetes, certain cancers, sleep apnea, and osteoarthritis. However, BMI is a screening tool, not a diagnostic tool — a healthcare provider should evaluate individual risk factors including waist circumference, blood pressure, cholesterol levels, blood sugar, family history, and physical activity level. The American Heart Association recommends regular BMI screening as part of annual check-ups.

BMI for Different Populations

Standard BMI thresholds were developed primarily from European populations and may not apply equally to all ethnic groups. The WHO recommends lower cutoffs for Asian populations: overweight starts at 23 (not 25) and obesity at 27.5 (not 30). This is because studies show that Asian individuals tend to carry more visceral fat at lower BMI values, increasing metabolic risk. For children and teens aged 2 to 19, BMI is calculated the same way but interpreted using age and sex-specific percentile charts from the CDC. A child at or above the 85th percentile is considered overweight, and at or above the 95th percentile is considered obese. This calculator is designed for adults aged 20 and older.

Limitations of BMI

BMI has significant limitations as a health metric. It does not distinguish between muscle mass and fat mass — a bodybuilder and a sedentary person of the same height and weight have identical BMIs despite vastly different body compositions. It does not account for fat distribution — visceral fat around organs is more dangerous than subcutaneous fat under the skin. It does not consider age, sex, or ethnicity in its basic formula. Alternative metrics include waist-to-hip ratio (WHR), waist-to-height ratio (WHtR), body fat percentage (measured by DEXA scan, bioelectric impedance, or skinfold calipers), and the Body Roundness Index (BRI). Despite these limitations, BMI remains the most widely used population screening tool due to its simplicity and strong statistical correlation with health outcomes at the population level.

Using BMI as a Starting Point

Think of BMI as a conversation starter with your healthcare provider, not a definitive health verdict. If your BMI is in the overweight or obese range, discuss it with your doctor along with other risk factors. The National Institutes of Health (NIH) recommends weight loss for individuals with a BMI of 30 or higher, or 25 or higher with at least one obesity-related risk factor. A modest weight loss of 5 to 10 percent of body weight can significantly reduce health risks. Your doctor can help develop a plan combining dietary changes, physical activity, behavioral therapy, and in some cases medication or surgical options.