What’s a Healthy BMI?

Q: What is a healthy BMI range? And what does that actually mean?

A: BMI stands for body mass index. We take a person’s weight and height and convert that into metric units, and then calculate weight per meter of body surface area – so it’s measured in kilograms per meters squared. This is used as an approximation of body composition, in particular the amount of total body fat.

It’s easy because we can measure your height and your weight and we can calculate this BMI, which standardizes weight across heights for adults. The BMI scale for adults is:

• Under 18: Underweight
• 18-25: Normal
• > 25-30: Have overweight
• > 30-40: Have obesity
• Above 40: Class III obesity (formerly known as morbidly obese)

However, BMI is only an estimation of body composition. So there are people who carry a lot of muscle mass whose BMI will be high because they weigh more, and it’s not really a good indication of what their body mass is. But for the vast majority of Americans, it’s a good rough estimate.

There’s an interesting paper that was published in The Lancet in August 2016 that combined the data from 239 clinical trials and found that, in people who never smoke cigarettes, increasing BMI was associated with all causes of mortality. So higher BMI is associated with increased risk for cancer, heart disease and stroke.

But there are other measurements that can help determine whether you’re at a healthy body weight. One that we probably don’t use enough is waist circumference. If you start an exercise program or a diet and you’re losing inches around your midsection, even if the scale doesn’t change a whole lot, that’s still a good sign.

— Family medicine physician Robert Bales, MD, MPH, FAAFP