Your BMI estimates your body fat better than the number on a scale. But a tape measure is an equally important tool.

As your BMI rises above 25, so does your risk for developing high blood pressure, diabetes, and cardiovascular disease.
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The body mass index (BMI), a measurement derived from the relationship between your weight and your height (see box), is a common way to estimate body fat. It’s a simple, fast, and essentially free tool that enables doctors to help identify people who might be at risk for health problems—including cardiovascular disease—that are linked to excess body fat.

People with a healthy BMI (18.5 to 24.9) tend to have better blood sugar and blood lipid levels compared with people whose BMI values fall above that range. They’re also more likely to have normal blood pressure, which puts less strain on the heart and circulatory system. They’re less prone to joint and muscle aches and may even have more energy—factors that make heart-healthy exercise less of a chore.

For many people, BMI values tend to creep upward over the years, and that trend can be hard to stop. “Pay attention to your BMI, because if you progress from normal to overweight to obese, your risk of cardiovascular disease goes up,” says Dr. George Blackburn, professor of nutrition and associate director of the Division of Nutrition at Harvard Medical School. (See “Weight and BMI ranges for average-height men and women” for a sense of the span at two different heights.)

The limits of BMI

Still, the BMI has some limitations. It doesn’t reveal how much of your body weight consists of muscle and how much is fat. Muscle is much denser and therefore heavier than fat. As a result, well-muscled athletes with little body fat may have BMI values that put them in the overweight or even obese category. But these exceptionally fit (and rare) individuals are unlikely to have high blood pressure or other risks for heart disease.

The opposite scenario is more worrisome. People who aren’t very fit and have less muscle mass may have normal BMI values. But they may still have unhealthy levels of cholesterol and triglycerides in their blood—especially those who sport a “spare tire.” Excess fat is what contributes to heart disease risk, and visceral fat (the type found around the belly) is particularly dangerous, explains Dr. Blackburn.

Watch your waistline

In fact, if you compare people with the same BMI but different waist sizes, the risk of heart disease is much higher in people who have large waists. That’s why you should keep tabs on your waistline as well as your BMI. For an accurate measurement, exhale and wrap a measuring tape around your bare abdomen just above your belly button. For women, 35 inches or higher signals a high risk; for men, it’s 40 inches.

What about the notion that a little extra fat in old age might be a good thing? Earlier research hinted that slightly overweight or even mildly obese people had lower death rates than normal-weight people. But the overweight people had been compared against a range of people with BMIs in the “healthy” range. Some were lean and active, but others were thin because they were heavy smokers or had a chronic disease. As a result, the risk of death was underestimated in the heavier people, giving the false impression they live longer than normal-weight people.