What is BMI?
Body Mass Index (BMI) is a simple calculation from your height and weight: BMI = weight (kg) ÷ height (m)². It is the most widely used screening measure for underweight, overweight and obesity in adults. BMI does not measure body fat directly, but across large groups of people it tracks the health risks linked to excess weight.
Featured Snippet: How do I calculate BMI, and what is a healthy BMI?
Divide your weight in kilograms by your height in metres squared. Example: 70 kg ÷ (1.75 × 1.75) = 22.9. For adults aged 18 and over, a healthy BMI is 18.5–24.9, for men and women alike. Use our BMI Calculator for an instant result.
How is BMI Calculated?
- Metric: BMI = weight (kg) ÷ height (m)²
- Imperial: BMI = 703 × weight (lb) ÷ height (in)²
To look up any height and weight without calculating, see the BMI chart.
BMI Classification Table (WHO)
| Weight Category | BMI Range | Risk of co-morbidities (WHO) |
|---|---|---|
| Underweight | < 18.5 | Low (other clinical problems more likely) |
| Healthy Weight | 18.5 – 24.9 | Average |
| Overweight | 25.0 – 29.9 | Increased |
| Obesity Class I | 30.0 – 34.9 | Moderate |
| Obesity Class II | 35.0 – 39.9 | Severe |
| Obesity Class III | 40.0 or more | Very severe |
If your result is outside the healthy range, the Ideal Weight Calculator shows the healthy weight range for your height.
Does a Healthy BMI Change With Age?
For adults the categories are the same at every age. In older adults, results are best read with a doctor: muscle loss can hide excess fat, and some research links a slightly higher BMI with lower risk later in life. Children and teenagers are assessed with BMI-for-age percentiles, not the adult cut-offs above.
Limitations of BMI
- Muscle vs. fat: BMI does not separate muscle from fat. Athletes can have a high BMI with a low body fat percentage.
- Fat distribution: It does not show where fat is stored; fat around the waist carries more risk.
- Age and sex: At the same BMI, women and older adults usually carry more body fat.
- Ethnicity: Some populations, for example people of South Asian origin, face higher risk at lower BMI values.
- Pregnancy: The adult categories do not apply during pregnancy.
Why BMI Still Matters
BMI is free, quick and needs no equipment, and at population level it is associated with the risk of conditions such as type 2 diabetes and heart disease. It works best as a first check, combined with waist measurement and body composition.
Beyond BMI: Other Measures
- Ideal Weight Calculator: a weight range for your height.
- Body Fat Calculator: an estimate of your body composition.
- Calorie Calculator: your estimated daily energy needs.
Tips for Maintaining a Healthy BMI
- Balanced diet: see sample meal plans built on whole foods.
- Regular activity: estimate what you burn with the Exercise Calculator.
- Fluids: get an estimate from the Water Intake Calculator.
- Calories: estimate your daily needs with the Calorie Calculator.
Conclusion
BMI is a useful starting point, not a diagnosis. Track it over time in your history, and discuss results outside the healthy range with a doctor.
