How the BMI Calculator Works
Body Mass Index (BMI) is a simple screening measure that relates your weight to your height. It's calculated as your weight in kilograms divided by the square of your height in meters:
BMI = weight (kg) / height (m)²
The result is compared against four widely used World Health Organization categories: under 18.5 is Underweight, 18.5 to 24.9 is Normal weight, 25 to 29.9 is Overweight, and 30 or above is Obese. This calculator also works out the healthy weight range for your exact height — the weight span that would put you in the "Normal weight" category — so you can see how far you are from either edge of that range.
BMI is popular because it's quick to calculate and correlates reasonably well with body fat at a population level, which is why doctors use it as a first-pass screening tool. But it has real limitations: it doesn't measure body fat directly, doesn't account for muscle mass, bone density, or where fat is distributed on the body, and the standard categories were developed mainly from adult population data. Use it as a starting point for a conversation about health, not a standalone diagnosis.
Frequently Asked Questions
Is BMI an accurate measure of health?
BMI is a useful screening tool, but it doesn't distinguish between muscle and fat. Athletes and very muscular people can show a high BMI while having low body fat, and it may under-read body fat in older adults with reduced muscle mass. It's a starting point, not a diagnosis.
Does BMI apply the same way to everyone?
The standard BMI categories were developed primarily from adult population studies and don't fully account for age, sex, ethnicity, or body composition differences. They also don't apply to children or teenagers, who need age-specific growth charts instead.
What should I do if my BMI is outside the healthy range?
Talk to a doctor or registered dietitian before making major changes. BMI alone doesn't capture your full health picture — factors like waist circumference, diet, activity level, and existing conditions all matter too.