Free BMI Calculator
Enter height and weight to calculate BMI and view the standard adult category. BMI is a general screening estimate, not a diagnosis.
BMI Calculator
BMI Calculator gives a quick screening estimate from height and weight, using the same standard adult categories doctors reference — a two-century-old formula that's simple enough to calculate in seconds, with real limitations worth understanding before reading too much into the number.
What is BMI, and how is it calculated?
Body Mass Index is a screening measure that estimates weight relative to height. In metric units, the formula is weight in kilograms divided by height in meters squared; in imperial units, it's weight in pounds divided by height in inches squared, multiplied by 703 — both give the same result, just scaled for the units used.
What do the BMI categories mean?
For adults, the World Health Organization's standard thresholds are: under 18.5 is underweight, 18.5-24.9 is the normal or healthy-weight range, 25-29.9 is overweight, and 30 or above is classified as obese. These categories are identical for men and women — the formula itself doesn't account for sex, and that's part of why it's a general screening tool rather than a precise individual measurement.
Is BMI actually accurate, and does it account for muscle, age or sex?
It's useful as a quick population-level screening estimate, but it has real limits: BMI can't distinguish muscle mass from fat, so it can be misleading for athletes and very muscular people, and it doesn't show where body fat is actually distributed on the body, which body fat percentage measures more directly. The formula and categories are identical regardless of sex or age for adults — interpretation stays the same across an adult's lifespan, though children and teens are assessed differently.
What is a good BMI for a child or teenager?
Adult BMI categories don't apply directly to children and teens — instead, they're assessed by percentile compared to others of the same age and sex, not against a single fixed number, since growth rates and healthy body composition change substantially through childhood and adolescence.
Where did BMI actually come from?
The formula is nearly 200 years old: Belgian mathematician and astronomer Adolphe Quetelet developed it in the 1830s — originally called the Quetelet Index — not for individual health screening but for his broader study of "social physics," an attempt to statistically describe the characteristics of an "average man" across a population. It stayed a niche statistical tool for over a century until 1972, when American physiologist Ancel Keys and his team studied it against other weight-screening formulas, found it worked reasonably well at a population level, renamed it the "Body Mass Index," and championed it for large-scale public health studies — which is how a 19th-century statistical curiosity became the number doctors' offices still use today.
Is BMI Calculator free to use?
Yes. BMI Calculator is completely free, with no sign-up and no usage limits.
Do I need to install any software?
No. BMI Calculator runs directly in your web browser — nothing to download or install.
Is my data kept private?
Everything happens on your device. We never receive or store the file or text you enter into BMI Calculator.