BMI Calculator

BMI (Body Mass Index) is a quick screening number that compares your weight to your height. The formula is weight in kilograms divided by height in meters squared (or in US units, pounds divided by inches squared, multiplied by 703). It was developed in the 1830s by Belgian mathematician Adolphe Quetelet and adopted by the World Health Organization and US CDC as a population-level health indicator.

The CDC uses this scale for adults: underweight under 18.5, normal weight 18.5 to 24.9, overweight 25.0 to 29.9, obese class 1 from 30.0 to 34.9, obese class 2 from 35.0 to 39.9, and obese class 3 (severe obesity) 40 or higher. These thresholds were standardized in 1998 and apply to most adults aged 20 and older.

BMI is a useful population-level screening tool but a blunt instrument for individuals. It can't distinguish fat mass from muscle mass, bone density, or fluid. A muscular athlete and a sedentary person of the same height and weight will have identical BMI but very different body composition. For most sedentary US adults, BMI tracks health risk reasonably well. For athletes, very muscular individuals, the elderly, and certain ethnic groups, additional measures like waist circumference or body fat percentage give a better picture.

Often poorly. NFL linemen, Olympic sprinters, and serious recreational lifters frequently register as overweight or obese by BMI despite having very low body fat. Muscle is denser than fat, so a lean, muscular body weighs more for its height. If you train seriously with weights, treat BMI as one data point and pair it with body fat percentage or waist-to-height ratio.

Less well. After about age 65, muscle mass declines (sarcopenia) and fat distribution changes. The same BMI can mean different things at 25 vs 75. Some research suggests slightly higher BMI (25 to 27) is actually associated with better outcomes in older adults — the so-called 'obesity paradox' in geriatric populations. Always discuss with your physician rather than treating BMI as definitive.

Not with the adult scale. For people under 20, doctors use BMI percentile based on age and sex, plotted on CDC growth charts. The same BMI of 22 means very different things for a 10-year-old vs an 18-year-old. This calculator is built for adults aged 20 and older.

BMI is a height-weight ratio — it doesn't measure fat directly. Body fat percentage is the actual proportion of your weight that's fat tissue. Body fat is more informative for individuals but harder to measure accurately (DEXA scans, hydrostatic weighing, or skinfold calipers are most accurate; bathroom scales and tape methods are estimates). Try our Body Fat Calculator for the US Navy tape method.

Mathematically, only two ways: reduce weight or grow taller. Since the second isn't an option past adolescence, sustainable weight loss is the path. A combined approach of moderate caloric deficit (250 to 750 calories per day below maintenance), resistance training to preserve muscle, and consistent sleep produces the most lasting reduction. Crash diets often lower BMI temporarily by stripping muscle and water, then rebound. Talk to a physician or registered dietitian before significant changes, especially if you have underlying conditions.

At a population level, yes — BMI correlates with risk of type 2 diabetes, cardiovascular disease, certain cancers, sleep apnea, and joint problems. The strongest associations are at the extremes (very low or very high BMI). For individuals in the normal-to-overweight range, BMI is a weak predictor; lifestyle factors like activity, diet quality, sleep, smoking, and stress matter more. Waist circumference is often a better predictor of cardiometabolic risk than BMI alone.

Both, if possible. Waist circumference (measured at the navel) directly reflects abdominal fat, which is more metabolically active and risk-relevant than fat stored elsewhere. US guidelines flag risk at waist over 40 inches for men or 35 inches for women, regardless of BMI. Waist-to-height ratio (waist divided by height, both in same units) is another strong predictor — keep it below 0.5 for good cardiometabolic health.