Body mass index (BMI) is the most widely used screening number for weight-related health risk. It is quick to calculate, needs only your height and weight, and is used by the CDC, the NIH and the World Health Organization. It is also easy to misread. This guide explains what a healthy BMI is for adults, how to interpret your result, and where BMI stops being useful.
How BMI is calculated
BMI divides your weight by the square of your height:
- Metric: BMI = weight (kg) ÷ height (m)²
- US units: BMI = 703 × weight (lb) ÷ height (in)²
For example, a person who is 5 ft 9 in (69 in) and weighs 165 lb has a BMI of 703 × 165 ÷ 69² ≈ 24.4.
Adult BMI ranges
For adults aged 20 and over, the CDC and WHO use the same categories for men and women:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class 1 |
| 35.0 – 39.9 | Obesity class 2 |
| 40.0 and above | Obesity class 3 |
So a "healthy" adult BMI is 18.5 to 24.9. For a 5 ft 9 in adult that corresponds to roughly 125–168 lb.
Some health bodies use lower cut-offs for people of Asian descent, because type 2 diabetes and heart-disease risk rise at lower BMIs in those populations. WHO guidance suggests treating 23 and above as increased risk and 27.5 and above as high risk.
What BMI doesn't tell you
BMI measures size, not body composition. It cannot tell muscle from fat, and it says nothing about where fat is stored. That leads to predictable blind spots:
- Muscular people such as athletes and lifters can score as "overweight" while having low body fat.
- Older adults often lose muscle as they age, so a "healthy" BMI can hide a high body-fat percentage.
- Fat distribution matters. Abdominal (visceral) fat carries more cardiometabolic risk than fat on the hips and thighs, and BMI can't see it.
- Pregnancy makes BMI meaningless as a weight-status measure.
Children and teens use BMI-for-age percentiles instead of the adult table, because healthy body composition changes as they grow.
Better together: BMI plus waist measurements
Pairing BMI with a waist-based measure fixes many of its blind spots. A simple rule of thumb is to keep your waist circumference below half your height (a waist-to-height ratio under 0.5). Waist-to-hip ratio and body-fat percentage add more detail.
When to talk to a doctor
BMI is a screening tool, not a diagnosis. It's worth a conversation with a healthcare professional if your BMI is under 18.5 or 30 and over, if it has changed quickly without explanation, or if you have a borderline BMI along with high blood pressure, high blood sugar or a family history of heart disease. A clinician can combine BMI with blood tests, blood pressure and your history to judge your real risk.
Key takeaways
- A healthy adult BMI is 18.5–24.9.
- BMI is a fast screen. It doesn't measure body fat or show where fat sits.
- Combine it with waist-to-height ratio or body-fat percentage for a fuller picture.
This article is for general information and is not medical advice.