Do I really need to know my body mass index? The short answer: yes, but it’s not the only medical number you should know. Here’s what it means to have a healthy BMI and why some people question it.
Body mass index (or BMI) is basically in the middle of a tug-of-war. On one side, medical experts yank you toward BMI being a top (even the top) marker of your health, signaling obesity if it’s high. On the other end, body-positive pros pull you toward the idea that well-being depends on way more than your weight, so you shouldn’t even pay attention to it.
The truth is, BMI holds steady somewhere in the middle. In isolation, BMI doesn’t mean all that much—it’s your weight in relation to your height. But it’s a helpful number. Finding out if you have a healthy BMI or a number in the overweight range can jumpstart a convo with your doctor about overall well-being. And that discussion could lead you toward more knowledge on how to live a longer, healthier life.
A high (or low) BMI doesn’t diagnose you with a condition, though—it’s just one corner of a bigger health picture. Above all, know that BMI most definitely doesn’t determine your value as a human being. It’s a health measure, not a human measure.
To understand the battle about BMI, you have to weigh both sides. Here’s what to know, including other tools that help paint a fuller picture of health.
What is BMI?
To determine your BMI, take your weight (in kilograms) and divide it by the square of your height (in meters). Or, for a faster calculation, check out our BMI calculator.
In the medical world, that number puts you in a certain category:
- BMI less than 18.5 = underweight
- BMI 18.5 to <25 = normal weight
- BMI 25.0 to <30 = overweight
- BMI 30.0+ = obese
What should I know about a healthy BMI as a marker of disease?
If you’re seeing for the first time that you don’t have a healthy BMI (a number in the normal range), keep calm. As mentioned, this is just one piece of the big health puzzle. You’re not doomed. However, research does reveal some links between high BMI (in that overweight and obese category) and certain health conditions.
Most notably, research shows those with normal range BMI (around 25) have the lowest risk of early death from all diseases1. When getting down to the specifics, science shows a link between obesity-level BMI and a two-year shorter life expectancy for non-smoking men in their 40s and a five-year reduced life expectancy for 40-year-old non-smoking women, compared to those in the healthy BMI category. Researchers also associate a BMI in the underweight category with a four-year lower life expectancy. In other words, BMI can swing both ways in terms of your longevity if you’re underweight or obese.
Obesity, in general, has also been linked to health conditions, which is why many medical pros tend to focus on BMI to assess your risk of disease. This includes heart conditions, type 2 diabetes, and high blood pressure2.
What are the drawbacks to only looking for a healthy BMI?
The main issue with BMI: It doesn’t consider your percent lean muscle mass, body fat, or where you carry that fat. A person with lots of lean muscle could throw the healthy BMI number off if the scale reveals a high number, even if they have no heightened risk for disease. On the other end, an older person who loses muscle mass, has more body fat, but still weighs less, might be at higher risk of disease than BMI reveals.
Science shows this downfall to BMI, saying muscle mass can weaken the relationship between BMI and mortality risk3. Also, different races and ethnicities may meet different health numbers. As the research notes, Black individuals tend to preserve more muscle and may have lower risk of poor health outcomes associated with a high BMI, compared to white people.
What else can we use to help measure health?
Of course, lab tests, like what you learn from the SoWell Weight Biology Kit, will tell you what’s going on under your skin. But something as simple as waist circumference—or the measurement taken around your belly—can also tell you more about your health than BMI alone.
Waist circumference provides some info on the fat on our bodies. Belly fat (or what the docs call “visceral fat”) can affect your metabolic health, upping disease risk like cardiovascular problems or type 2 diabetes. So taking a measurement of what you’re carrying around your middle is a helpful indicator of health and longevity. In fact, at least one large study found that women with a waist circumference of 35 inches or higher had double the risk of dying from heart disease, compared to those with a smaller waist size. They had higher risk of cancer and all-cause related death, too4. These higher risks remained, even when the women had a healthy BMI.
So what numbers should you know for waist circumference? Men with a waist circumference of more than 40 inches and women more than 35 inches have an increased risk of disease.
What does the science say about waist circumference v. healthy BMI?
Studies show that BMI and waist circumference measures vary for different races and ethnicities. For example, one American Heart Association study found that BMI might not work well to indicate obesity or heart disease for Asians, and may not be the best stand-alone health readers for Blacks, Hispanics, or Specific Islanders, either5. That’s why waist circumference thresholds are so important for these populations. Other science agrees, saying waist circumference may better predict diabetes risk, particularly for Asian women but also across all ethnic groups6.
Now that you’re more familiar with what it means to have a healthy BMI and how it can affect your well-being, it’s a good time to talk to your doctor about it. You also now have a full case for steering the convo beyond the scale and toward other health measures, too. Getting to the belly of the issue will help you live long and well.
1 The Lancet Diabetes & Endocrinology. Association of BMI with overall and cause-specific mortality: a population-based cohort study of 3·6 million adults in the UK. December 2018.
2 National Heart, Lunge and Blood Institute. “Classification of Overweight and Obesity by BMI, Waist Circumference, and Associated Disease Risks.”
3 PLOS ONE. Muscle mass, BMI, and mortality among adults in the United States: A population-based cohort study. April 2018.
5 Circulation. Identification of Obesity and Cardiovascular Risk in Ethnically and Racially Diverse Populations. July 2015.
6 Diabetes Care. Racial and Ethnic Differences in Anthropometric Measures as Risk Factors for Diabetes. January 2019.