BMI Calculator

About this calculator

A free Body Mass Index calculator that turns your weight and height into a screening number using the standard BMI formula. Choose metric (kg, cm) or imperial (lb, in); the tool computes BMI, places it in the World Health Organization adult categories — Underweight, Normal weight, Overweight, and Obesity — and shows the weight band that would land you in the healthy 18.5–24.9 range for your height. Everything runs in your browser and nothing you type is uploaded or stored. BMI is a screening estimate, not a diagnosis. It does not measure body fat directly and ignores muscle mass, frame size, age, and sex, so very muscular or older people can screen misleadingly. Treat the result as a starting point and consult a healthcare professional before making any health, diet, or fitness decision.

How the BMI formula works

Body Mass Index is a single ratio of weight to height. In metric units it is defined as BMI = kg / m² — your weight in kilograms divided by your height in meters squared — so the tool divides a centimeter height by 100 to get meters before squaring it. The result is the index most clinicians and health bodies use as a quick screening number.

Imperial units reach the same index without converting to kilograms or meters. The tool uses BMI = 703 × lb / in², where the 703 factor rescales the pounds-over-inches-squared ratio into the metric value. Whichever unit you enter, the underlying number and its category are identical.

What the WHO categories mean

The calculator places your index in the World Health Organization adult categories using fixed cut points: under 18.5 is Underweight, 18.5 to 24.9 is Normal weight, 25 to 29.9 is Overweight, and 30 or above is Obesity. It also shows the weight band that would keep BMI in the healthy 18.5–24.9 range for your height.

These ranges describe populations of adults and are not designed for children, pregnancy, or athletes. A category is a screening signal that may warrant a closer look, not a verdict on an individual's health.

The key limitation

BMI does not measure body fat. Because it only compares weight to height, it cannot tell muscle from fat, and it ignores frame size, age, and sex. A very muscular person can screen as Overweight while carrying little fat, and an older person who has lost muscle can screen as Normal while carrying more.

For these reasons BMI is a starting point, not a diagnosis. Treat the result as one screening estimate among several and consult a healthcare professional before making any health, diet, or fitness decision based on it.

By variant

• Metric (kg, cm)
• Imperial (lb, in)

Questions

Is the BMI calculator free?

Yes. It is free, needs no account, and calculates entirely in your browser — your weight and height are never uploaded or stored.

What BMI formula does this use?

Metric inputs use BMI = kg / m², where height in centimeters is divided by 100 to get meters. Imperial inputs use BMI = 703 × lb / in². Both produce the same index; the 703 factor converts pounds and inches to the metric ratio.

What do the BMI categories mean?

The tool uses the WHO adult cut points: under 18.5 is Underweight, 18.5 to 24.9 is Normal weight, 25 to 29.9 is Overweight, and 30 or above is Obesity. These ranges apply to adults, not children or pregnancy.

Is BMI a reliable measure of health?

BMI is a quick screening estimate, not a medical diagnosis. It does not measure body fat and ignores muscle, frame size, age, and sex, so athletes and some body types screen misleadingly. Use it as a starting point and consult a healthcare professional for advice.