What is BMI?
Body Mass Index (BMI) relates your weight to your height. It was developed in the 19th century and is now used by organizations such as the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) as a population-level screening tool for weight status in adults.
BMI alone does not diagnose illness. It helps flag when further assessment—such as waist measurement, blood work, or a clinical exam—may be useful.
How to Calculate BMI
The standard formulas are:
Metric
Imperial
Use our free BMI calculator to compute your result in metric or imperial units. Calculations run in your browser.
BMI Categories
For adults 20 and older, WHO and CDC use these broad categories (see CDC adult BMI):
| BMI Range | Category | Notes |
|---|---|---|
| Below 18.5 | Underweight | May warrant clinical review |
| 18.5 – 24.9 | Normal weight | Typical reference range |
| 25.0 – 29.9 | Overweight | Higher associated risk |
| 30.0 – 34.9 | Obesity Class I | CDC obesity class |
| 35.0 – 39.9 | Obesity Class II | CDC obesity class |
| 40.0 and above | Obesity Class III | Severe obesity |
Children and teens use BMI-for-age percentiles, not these adult cutoffs. See CDC BMI for children and teens.
Limitations of BMI
- Muscle vs fat: Muscular people may have a high BMI with low body fat.
- Body composition: Two people with the same BMI can differ in fat percentage.
- Age: Older adults may lose height or muscle, affecting interpretation.
- Ethnicity: Some groups face health risks at lower BMI values than standard cutoffs suggest.
- Fat location: Waist size and visceral fat matter; BMI does not measure where fat is stored.
What BMI Can and Cannot Tell You
Higher BMI is associated with increased risk of conditions such as type 2 diabetes and cardiovascular disease in population studies. Very low BMI may signal undernutrition or other issues. These are statistical associations—not diagnoses for any one person.
A healthcare provider may combine BMI with blood pressure, lipids, waist circumference, family history, and lifestyle when assessing your health.
Next Steps
If your BMI is outside the normal range, consider discussing it with a doctor or registered dietitian before making major diet or exercise changes. Sustainable habits—balanced nutrition, regular activity, adequate sleep—matter more than chasing a number alone.
If BMI is high
- Ask your provider about safe weight-management goals.
- Focus on whole foods and consistent movement you can maintain.
- Track waist circumference if your provider recommends it.
If BMI is low
- Rule out underlying conditions with a clinical evaluation.
- Work with a professional on nutrient-dense meals and strength training if appropriate.
- Avoid extreme restriction without medical supervision.