Free BMI Calculator
Calculate your Body Mass Index and see which weight category it falls into.
| Underweight | Below 18.5 |
| Normal weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obese | 30.0 and above |
Where the BMI formula comes from
Body Mass Index was never designed as a personal health verdict. It was developed nearly two centuries ago by the Belgian statistician Adolphe Quetelet, who was studying the average build of a population — not diagnosing individuals — and needed a simple number he could compute from just two measurements. That population-level origin is exactly why BMI is a blunt instrument for any one person: it can't tell whether your weight is muscle, fat, or bone density.
Why the imperial formula multiplies by 703
Both formulas measure the same thing — weight divided by height squared — but pounds and inches aren't kilograms and meters, so a conversion constant is needed to make the imperial version return the identical number the metric version would. One pound is about 0.453592 kg and one inch is 0.0254 m, so converting kg ÷ m² into lb ÷ in² means multiplying by 0.453592 ÷ 0.0254² ≈ 703.07. That's not an arbitrary rounding — it's the exact unit-conversion factor, which is why the 703 constant works for every input, not just the example below.
Worked example
Take the calculator's own default values: 70 kg at 175 cm. Convert height to meters (1.75 m), square it (3.0625), then divide weight by that: 70 ÷ 3.0625 = 22.9 — squarely inside the 18.5–24.9 "normal weight" band.
Frequently Asked Questions
How is BMI calculated?
Metric: weight in kilograms divided by height in meters squared (kg/m²). Imperial: 703 × weight in pounds ÷ height in inches squared. Both formulas give the same result for the same body.
Is BMI an accurate measure of health?
BMI is a quick screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so athletes and very muscular people are often misclassified as "overweight." Use it as one data point, not a verdict — talk to a doctor for a full picture.
Are the categories the same for everyone?
The standard WHO categories shown here are designed for adults. They don't apply the same way to children, teenagers, pregnant women, or very elderly or very muscular adults.
Does the healthy BMI range differ between men and women?
No — the standard 18.5–24.9 "normal weight" band is the same scale for adult men and women. Where the sexes typically differ is body fat percentage at a given BMI, not the BMI thresholds themselves.
Why do two people with the same BMI sometimes look completely different?
Because BMI only uses weight and height — it has no way to see body composition. A bodybuilder and someone carrying excess fat can land on the identical BMI number despite very different health profiles, which is the central limitation to keep in mind when reading the result.
Is my data sent anywhere?
No. The calculation happens entirely in your browser. Nothing is transmitted or stored.