What your BMI number means
Body Mass Index (BMI) is a simple ratio of weight to height, used as a quick screening tool to flag whether someone's weight falls in a range associated with higher health risk. The standard categories are: under 18.5 (underweight), 18.5–24.9 (normal weight), 25–29.9 (overweight), and 30 and above (obese).
Limitations of BMI
- It doesn't distinguish between muscle and fat, so muscular or highly athletic people are often classified as "overweight" despite low body fat.
- It doesn't account for age, sex, or body frame, all of which affect what a healthy weight looks like for a given height.
- It isn't designed for children, pregnant women, or the elderly, who need different reference standards.
Frequently asked questions
Is BMI accurate for muscular or athletic people?
Not reliably — muscle is denser than fat, so athletes can have a "high" BMI while carrying very little body fat. Body fat percentage or waist-to-height ratio are often better indicators for this group.
What should I use instead of BMI?
Many clinicians pair BMI with other measures like waist circumference, body fat percentage, or blood markers for a fuller picture, rather than relying on BMI alone.
Does a healthy BMI differ by age or sex?
The standard BMI categories are the same formula for all adults, but health risk at a given BMI can vary by age, sex, and ethnicity — which is one reason it's treated as a screening tool rather than a diagnosis.
This tool is for general informational purposes only and is not a substitute for professional medical advice. Speak with a doctor or registered dietitian about what's healthy for your individual body.