BMI Calculator
Calculate your Body Mass Index (BMI) and see which category it falls into — plus when BMI is useful and when it misleads.
BMI is a general health screening tool — it does not measure body composition, muscle mass, bone density, or fitness level. Athletes, older adults, pregnant people, and certain ethnic groups may need different interpretations.
What Body Mass Index (BMI) is
Body Mass Index (BMI) is a simple ratio of weight to height: kilograms divided by height in metres squared. It is used worldwide to screen populations for under- or over-weight.
BMI is a screening tool, not a diagnosis. It says nothing about body composition, fat distribution, or fitness on its own.
How the calculation works
Formula: BMI = weight (kg) / height (m)². The result is matched to WHO categories for adults aged 20+.
- Under 18.5 — underweight
- 18.5–24.9 — normal weight
- 25.0–29.9 — overweight
- 30.0 or more — obesity
How to read your result
Use BMI as a starting point and pair it with waist circumference, training status, and body-fat percentage for the full picture.
Athletes with high muscle mass often classify as 'overweight' by BMI despite low body fat.
Why BMI is still useful
- Cheap, fast, and reproducible — no calipers or scans needed
- Strong correlation with health risk at the population level
- Helpful trend marker when tracked over months
- Universal vocabulary across clinics and research
Common mistakes
- Treating BMI as a body-fat measurement
- Applying adult BMI categories to children or pregnant women
- Ignoring it for muscular athletes — BMI still flags weight changes
- Reading a single value rather than the trend
- Mixing units (pounds with metres, etc.)
Practical recommendations
- Measure weight in the morning, after the bathroom, before eating
- Combine BMI with waist circumference (men <94 cm, women <80 cm)
- Use a body-fat tool when BMI seems off for your build
- Track BMI monthly, not daily
- Discuss persistent extremes with a clinician
Related calculators
Frequently asked questions
Bottom line
BMI is a fast, free first check. Use it as one signal among many — never as the final word on health or appearance.