BMI Calculator
Body Mass Index (BMI) is a quick screening measure that relates your weight to your height. Enter your height and weight in either metric or imperial units to get your BMI and category instantly.
How the BMI formula works
BMI is calculated differently depending on the unit system:
Metric: BMI = weight(kg) / height(m)² Imperial: BMI = 703 × weight(lb) / height(in)²
The result is a single number, categorized against standard World Health Organization thresholds.
Step-by-step calculation
- Convert height to meters (metric) or keep it in inches (imperial).
- Square the height value.
- Divide weight by the squared height (metric), or apply the 703 conversion factor (imperial).
- Round to one decimal place and compare against the BMI category table.
Worked example
A person who is 1.75 m tall and weighs 70 kg: BMI = 70 / (1.75 × 1.75) = 70 / 3.0625 ≈ 22.9, which falls in the 'Normal weight' category (18.5–24.9).
Frequently asked questions
Is BMI an accurate measure of health?
BMI is a useful population-level screening tool, but it doesn't distinguish muscle from fat and can be less accurate for athletes, older adults, or during pregnancy. Use it alongside other measures, not as a sole diagnosis.
What are the standard BMI categories?
Under 18.5 is classified as underweight, 18.5–24.9 as normal weight, 25–29.9 as overweight, and 30 or above as obese, per WHO adult thresholds.
Does BMI apply to children the same way?
No. Children and teens are assessed using age- and sex-specific BMI percentiles rather than the fixed adult category thresholds used here.
Which unit system should I use?
Either — the calculator produces the same BMI value regardless of whether you enter metric or imperial measurements, since it converts internally.