ToolNestFree Online BMI Calculator
Check your Body Mass Index in seconds. Our free BMI calculator works with metric or imperial units and tells you instantly whether your result falls in the underweight, healthy, overweight, or obesity range — with the exact formula shown so you can verify it.
Enter your weight and height, then calculate.
✓ Free forever · ✓ No signup · ✓ Runs in your browser — your data never leaves your device.
How to Use the BMI Calculator
- Choose metric (kg/cm) or imperial (lb/inches).
- Enter your weight and height.
- Click Calculate BMI for your index and category.
- Compare against the standard ranges shown below.
Why Use Our BMI Calculator?
- Metric & imperial — switch units without converting yourself.
- Instant categories — underweight, healthy, overweight, obesity per WHO cutoffs.
- Transparent formula — BMI = weight(kg) ÷ height(m)², shown on the page.
- Private — your numbers never leave your browser.
Frequently Asked Questions
What is BMI?
Body Mass Index — weight in kilograms divided by height in meters squared. It's a screening measure, not a diagnosis.
What are the BMI categories?
Under 18.5 underweight; 18.5–24.9 healthy; 25–29.9 overweight; 30+ obesity (WHO cutoffs).
Is BMI accurate for athletes?
Not always — muscle weighs more than fat, so very muscular people can score 'overweight' while being lean. Treat BMI as one signal among many.
How often should I check?
Monthly is plenty for tracking trends; daily fluctuations are mostly water weight.
Is my data private?
Yes — calculation runs entirely in your browser.