BMI Calculator
A Ratio, Not a Diagnosis
Body Mass Index has been the default screening number in doctor's offices since the 1970s, not because it measures body fat directly, but because it's cheap to calculate from two numbers everyone already has: height and weight. It was never designed to judge an individual body — it was built by the statistician Adolphe Quetelet in the 1830s to describe population averages. Used correctly, as a quick screening flag rather than a verdict, it's still genuinely useful.
The Formula
For imperial inputs, weight in pounds is converted to kilograms (× 0.45359237) and height in inches is converted to meters (× 0.0254) before the calculation runs, so the result matches the metric formula exactly.
Standard BMI Categories
| BMI range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
These are the general reference ranges used worldwide for adults; they do not distinguish muscle mass from fat mass and are not diagnostic on their own.
Where BMI Is Actually Useful
- Population health tracking — public health agencies use BMI distributions to monitor obesity trends across large groups, where individual measurement error washes out.
- Initial clinical screening — a starting point that prompts further evaluation (waist circumference, body fat testing) rather than a standalone conclusion.
- Insurance and eligibility criteria — some programs and clinical trials use BMI thresholds as an entry filter.
- Personal tracking over time — watching your own BMI trend over months is more informative than any single reading, since it controls for your own body type.
How to Use This Calculator
- Choose Metric (kg/cm) or Imperial (lb/in) units.
- Enter your weight.
- Enter your height.
- Select Calculate to see your BMI value and category.
Related Calculations
BMI works best alongside the Body Fat Calculator for a composition-based view, or the Ideal Weight Calculator for a height-based target range.