Body mass index, or BMI, is one of the most common tools doctors use to screen for weight-related health risks. It’s quick and free to calculate. But BMI has real limitations, and on its own it doesn’t tell the whole story about your health. Pairing it with your waist size gives a more complete picture.
What is BMI?
BMI is a calculation based on your height and weight. It estimates whether your weight is in a range linked to higher or lower risk of certain health problems. You can find free BMI calculators online, including from the CDC.
Standard BMI categories for adults
According to the CDC:
- Underweight: less than 18.5
- Healthy weight: 18.5 to less than 25
- Overweight: 25 to less than 30
- Obesity: 30 or greater, divided into class 1 (30 to less than 35), class 2 (35 to less than 40) and class 3 (40 or greater)
What BMI doesn’t tell you
BMI is a screening tool, not a diagnosis. It has several important limitations:
- It doesn’t measure body fat directly. Very muscular people can have a high BMI without excess fat.
- It doesn’t show where fat is stored. Fat around the abdomen carries more health risk than fat on the hips and thighs.
- It varies by age, sex and ethnicity. Older adults may have less muscle at the same BMI, and some groups, such as people of Asian descent, may face health risks at lower BMIs.
- It doesn’t reflect fitness or health habits.
Why waist size matters
Your waist measurement helps estimate abdominal (visceral) fat, which surrounds organs and is linked to type 2 diabetes, heart disease and fatty liver disease. Health risk increases with a waist size of more than 40 inches for men and more than 35 inches for women, even if BMI is in a healthy range. A large waist is also one of the criteria for metabolic syndrome.
How to measure: stand, place a tape measure around your bare abdomen just above your hip bones, keep it snug without squeezing, and measure after you breathe out.
Other measures of health
Doctors also look at blood pressure, blood sugar, cholesterol, physical activity, diet, sleep and family history. Some people with a higher BMI are metabolically healthy, while some people with a normal BMI have health risks.
What this means for you
Use BMI and waist size as starting points for a conversation with your doctor, not as a judgment. If your numbers are high, small changes can make a difference. Losing even 5% of your body weight can improve blood pressure, blood sugar and cholesterol. Regular activity, guided by how much exercise you need, benefits your health at any size.
The bottom line
BMI is a useful screening tool, but it doesn’t measure body fat or show where it’s stored. Adding a waist measurement and other health markers gives a clearer picture of your risk.
Common questions
Is BMI accurate for athletes?
Often not. Muscle weighs more than fat, so athletes and very muscular people can have a high BMI without excess body fat.
Is BMI used for children?
Yes, but children’s BMI is compared with others of the same age and sex using percentiles, rather than adult categories.
What’s a healthy waist-to-height ratio?
Some experts suggest keeping your waist measurement at less than half your height. It’s another simple way to check abdominal fat.
This article is for general information and isn’t medical advice. Talk with your healthcare provider about your individual needs.
