BMI Calculator

BMI measures weight relative to height using a simple ratio — your weight in kilograms divided by your height in meters squared. A person who is 5'9" and weighs 150 pounds has the same BMI as someone 6'2" weighing 180 pounds, because the weight increases proportionally with height. The formula assumes that healthy weight scales with the square of height, not linearly — doubling your height means your healthy weight range roughly quadruples.

The tool converts all measurements to metric units internally for consistency, then applies the standard WHO classifications: underweight (under 18.5), normal (18.5-24.9), overweight (25-29.9), and obese (30+). These categories were established by studying health outcomes across large populations and identifying BMI ranges associated with lowest mortality and disease rates.

BMI assumes that weight consists primarily of fat and lean tissue in typical proportions. It doesn't measure body fat percentage directly — someone with high muscle mass and low body fat can have the same BMI as someone with high body fat and low muscle mass. This is why bodybuilders and serious athletes often score as overweight despite having excellent metabolic health.

Use BMI for initial health screening when you want a quick assessment of whether your weight falls within typical ranges for your height. It's most accurate for sedentary to moderately active adults between ages 20-65 with average muscle mass. BMI works well for tracking weight changes over time and setting realistic weight loss or gain targets.

Don't rely on BMI alone if you're very muscular, have an athletic build, are under 18, over 65, pregnant, or have a medical condition affecting body composition. Bodybuilders, powerlifters, and some endurance athletes will almost always score as overweight despite excellent health. Similarly, people who have lost significant muscle mass due to aging or illness may score as normal weight while actually having unhealthy body composition.

BMI also doesn't account for fat distribution — someone with excess abdominal fat faces higher health risks than someone carrying weight in hips and thighs, even at identical BMI levels. Waist circumference measurements provide better cardiovascular risk assessment for people in the overweight BMI range.

Users often enter their height in feet and inches incorrectly, forgetting that 5'10" means 5 feet and 10 inches, not 5.10 feet. This decimal confusion can shift BMI results by several points. Someone who is 5'10" (70 inches) but enters 5.10 feet (61.2 inches) would show an artificially high BMI of 28.5 instead of the correct 24.7.

Another common error is using BMI as a definitive health assessment rather than a screening tool. People with high muscle mass — weightlifters, rugby players, some manual laborers — may score as overweight while having low body fat and excellent cardiovascular health. Conversely, someone with normal BMI but high abdominal fat and low muscle mass may face higher health risks despite falling in the normal category.

Parents sometimes apply adult BMI standards to teenagers, which gives misleading results. Children and adolescents use age and gender-specific BMI percentiles rather than fixed categories because body composition changes dramatically during growth. A BMI that indicates obesity in an adult might be perfectly normal for a growing 15-year-old.

The BMI formula is BMI = weight(kg) ÷ height(m)². For someone weighing 70 kg and standing 1.75 meters tall: BMI = 70 ÷ (1.75)² = 70 ÷ 3.06 = 22.9. This calculation works because it adjusts for the fact that volume (and hence healthy weight) scales with the cube of linear dimensions, while BMI uses the square — a reasonable approximation for human body proportions.

The normal weight range for any height spans from BMI 18.5 to 24.9. For a 175 cm person, this means 18.5 × (1.75)² = 56.7 kg minimum and 24.9 × (1.75)² = 76.2 kg maximum. The 20-kilogram range allows for significant variation in bone density, muscle mass, and body frame while maintaining healthy proportions.

BMI categories create discrete boundaries, but health risks increase gradually. Someone with BMI 24.9 isn't fundamentally healthier than someone with BMI 25.1. The cutoff points represent statistical averages where population health outcomes begin to diverge significantly, not individual diagnostic thresholds.

The standard BMI thresholds misclassify Asian populations, who show increased diabetes and cardiovascular risk at lower BMI values. WHO published separate Asian cut-offs in 2004: normal weight 18.5-23, overweight 23-27.5, with obesity beginning at 27.5 rather than 30. This reflects genetic differences in body fat distribution and insulin sensitivity.