BMI Calculator Guide: How Body Mass Index Works
BMI (Body Mass Index) is the most widely used screening tool for weight categories. It takes just two measurements — your height and weight — and produces a single number that estimates body fatness for most adults. This guide explains the formula, how to interpret your result, and where BMI can mislead you.
The BMI Formula
- BMI — body mass index (kg/m²)
- weight — body weight in kilograms
- height — height in meters
For imperial units: BMI = 703 × weight (lb) ÷ height (in)²
For example, a person who is 70 kg and 1.75 m tall has a BMI of 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 ≈ 22.9, which falls in the healthy range.
What Your BMI Number Means
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
When BMI Gets It Wrong
- Athletes & bodybuilders — muscle is denser than fat, so muscular people often score "overweight" despite low body fat.
- Older adults — BMI doesn't account for muscle loss or bone density changes with age.
- Pregnant or breastfeeding women — BMI is not meaningful during these stages.
- Different ethnicities — some populations face health risks at lower BMI thresholds; the WHO suggests a 23 cutoff for Asians.
BMI is a screening tool, not a diagnosis. Pair it with waist circumference and body-fat percentage for a fuller picture.
Put It Into Practice
Enter your height and weight in either metric or imperial units to see your BMI instantly, along with your category and the healthy weight range for your height.
Try the BMI Calculator
Supports both metric (kg/cm) and imperial (lb/in) units.
⚖ Open BMI Calculator →