Understanding BMI: What Your Number Really Means
BMI is a common health metric, but what does it actually tell you? We break down the categories, limitations, and what to do with your results.
The BMI Formula
Body Mass Index is calculated by dividing your weight by the square of your height:
BMI = weight (kg) / height (m)2
In imperial units: BMI = [weight (lbs) / height (in)2] x 703
BMI Categories
- Underweight: BMI below 18.5 — may indicate nutritional deficiency or other health issues
- Normal weight: BMI 18.5 to 24.9 — generally associated with the lowest health risks
- Overweight: BMI 25.0 to 29.9 — may carry increased risk for certain conditions
- Obese: BMI 30.0 and above — associated with higher risk of heart disease, diabetes, and other conditions
Important Limitations
BMI is a useful screening tool, but it has significant limitations you should understand:
- Muscle mass: Athletes and muscular individuals often register as "overweight" or "obese" despite being very fit, because muscle weighs more than fat by volume
- Age: Older adults may have a normal BMI but carry excess body fat due to age-related muscle loss
- Gender differences: Women naturally carry more body fat than men, but BMI does not account for this
- Body composition: BMI cannot distinguish between fat, muscle, bone, and water weight
When to Consult a Doctor
If your BMI falls outside the normal range, consider speaking with a healthcare provider. They can assess additional factors like waist circumference, blood pressure, cholesterol levels, and family history to give you a much more complete picture of your health than BMI alone.
Ready to calculate? Try the BMI Calculator →