BMI Calculator

BMI (Body Mass Index) is a simple screening measure that estimates whether your weight is healthy for your height, using nothing more than two numbers. It's widely used by doctors, health organizations, and insurers as a quick, population-level indicator of weight-related health risk, though it doesn't directly measure body fat and can't account for muscle mass, bone density, or where fat is distributed on the body.

The formula itself dates back to the 1830s, when Belgian mathematician Adolphe Quetelet developed it while studying "the average man" for population statistics, not individual health assessment. It was originally known as the Quetelet Index and only became widely adopted in clinical and public health settings in the 1970s and 80s, after researchers found it correlated reasonably well with body fat across large populations, even though it was never designed as a diagnostic tool for any one person.

Because BMI is a screening number rather than a full picture of body composition, it's often paired with other measures — see the Body Fat Calculator for a circumference-based estimate of actual body fat percentage, or the Ideal Body Weight Calculator for a different, height-based approach to a healthy weight target.

The formula divides your weight in kilograms by the square of your height in meters. Squaring the height adjusts for the fact that taller people naturally weigh more even at a proportionally healthy size, making BMI a reasonable way to compare body weight across people of different heights without needing separate reference tables for every possible height.

Squaring, rather than simply dividing by height, was Quetelet's empirical choice — he tested several exponents against population data and found that height squared produced a number that stayed relatively stable across most adult heights for a given build. That said, the approximation still breaks down at the extremes: very tall or very short individuals, and people with unusually high muscle mass, can end up with BMI values that don't reflect their actual health risk as accurately as they would for someone closer to average height and build.

Your BMI places you into one of four categories — Underweight, Normal, Overweight, or Obese. This is a screening indicator, not a diagnosis: it flags whether your weight-to-height ratio is in a range associated with higher health risk, but factors like muscle mass, age, and body composition also matter.

Roughly: Underweight (under 18.5) can signal inadequate nutrition or an underlying health issue; Normal (18.5-24.9) is associated with the lowest average health risk in most population studies; Overweight (25-29.9) is linked with a moderately increased risk of conditions like high blood pressure and type 2 diabetes; and Obese (30 and above) carries a further increased risk of chronic disease. These are population-level associations rather than individual predictions — someone can sit outside the "Normal" band and still be healthy, or fall inside it and still carry meaningful health risk.