Maintaining a normal body mass index (BMI) may help you protect your future health. A high BMI has been linked to an increased risk of diseases such as heart disease and type 2 diabetes, according to the Centers for Disease Control and Prevention (CDC). And having a low BMI and being underweight can lead to problems with your menstrual cycle, bone health, and mood.
But some experts consider BMI to be a flawed tool to measure your risk for disease, and the American Medical Association adopted a policy in 2023 that encourages doctors to not rely on BMI alone to diagnose obesity.
Body mass index, more commonly known as BMI, is a number that measures your weight relative to your height. Medical professionals have long used this calculation to categorize people as having underweight, normal weight, overweight, or obesity. Using this approach, more than 4 in 10 American adults are considered obese.
BMI can be tied to your health risk for certain diseases and health outcomes, including high blood pressure, heart disease, cholesterol problems, and certain cancers.
Some critics argue that the measurement is flawed and has limited use. Rather, if you have a high or low BMI, your physician may have you undergo additional health screenings, such as for waist circumference, high cholesterol, or high blood pressure.
If math isn’t your strong suit, you can opt to use the CDC’s BMI calculator to figure out your BMI.
To calculate your own number, take your weight in kilograms (kg) divided by the square of your height in meters (m). You can also use your height in inches squared, divide that by your weight in pounds, and multiply by 703.
For example, for a woman who is 140 pounds and 63 inches tall, we can calculate her BMI this way:
Height in inches squared: 63 x 63 = 3,969
Weight divided by height squared: 140/3,969 = .03527
.03527 x 703 = 24.79 BMI
Here’s what your number means, per the CDC:
Below 18.5: underweight
18.5 to 24.9: healthy weight
25 to 29.9: overweight
30 and over: obese
Although the BMI cutoffs are the same regardless of fitness and age, there are some differences in body composition that BMI doesn’t account for. The older we are, the more fat we carry on our bodies, which weighs less than an athletic body with denser muscle mass.
The chart below specifies healthy, overweight, and obese ranges based on your height in inches and body weight in pounds.
Measuring body fat is different in kids and teens, since they’re still growing. Your child’s pediatrician will usually calculate their BMI when they are around age 2, per the CDC.
Instead of following BMI ranges for adults, doctors screen children, adolescents, and teenagers for obesity using what’s called growth chart percentages, which use percentiles. Percentiles represent a child’s BMI relative to other U.S. children who are the same sex and age. This approach can help your doctor understand your child’s body weight and health risks as their bodies continue to grow and develop.
On an observational level, BMI is associated with obesity-related conditions, says Jessica Crandall, RDN, the owner and CEO of Vital RD in Centennial, Colorado. For example, a higher BMI is associated with an increased risk of:
But a higher BMI doesn’t always translate into poor health. One review of studies found that a higher BMI may have a protective effect on survival among older individuals and those with certain chronic conditions. The review suggests that a higher BMI may, in these populations, improve survival rates among dialysis patients, those suffering from heart attack or heart failure, and those with lung disease and type 2 diabetes, among others.
This phenomenon, as noted in some studies, is called the “obesity paradox” and may be partially explained by the fact that extra weight and muscle can minimize the metabolic consequences of some diseases. The researchers noted, however, the shortcomings of BMI and proposed that future research should consider body composition when defining obesity rather than simply BMI, to better understand this paradox.
A high BMI is related to a high body weight and excessive body fat. Fat accumulates when the energy (calories) taken in through food is more than the energy we expend. A low intake of fruits and vegetables is associated with a higher body weight because the calories contained in these foods are typically much lower than in other foods, particularly fatty ones.
How much your body moves on a regular basis plays into this risk equation, too. People who are physically unable to exercise or choose not to are more likely to have overweight or obesity.
Working a sedentary job can also reduce your body’s energy needs and lead to weight gain. Research has connected sitting for long periods of time with obesity and the cluster of health problems associated with that disease.
Losing weight is hard — and keeping it off can be even harder. Most people put on weight over a long period of time, so lowering your BMI may take a while as well. Gradual, practical changes to your lifestyle can ultimately lead to long-lasting results. If you need additional help, reach out to your primary care doctor or find help from a nutritionist skilled in weight loss support.
Make Smart Food Choices
Several healthy diet plans are at your disposal to help you take off extra pounds. But in general, eating more low-calorie, high-fiber foods such as nonstarchy vegetables, fruits, beans, and whole grains — and less fatty and processed food — is a tried-and-true way to lower your body weight.
Research shows that eating at home typically results in a reduction of sugar, fat, and overall calories. Not only is it healthier, but you’ll likely save money by eating in rather than ordering out.
Exercise Regularly
For most adults who are healthy, the guidelines are to get at least 150 minutes of moderate aerobic activity a week or at least 75 minutes of vigorous aerobic activity a week, plus strength training twice a week. Getting moderate levels of aerobic exercise for at least 300 minutes a week may help with weight loss.
Get Adequate Sleep
Research shows that not getting enough sleep is a risk factor for overweight and obesity. Most adults need seven to nine hours of shut-eye per night. If you’re falling shy of that mark, try these tips for getting a better night’s sleep.
Manage Stress Well
Stress is impossible to avoid completely, and not all stress is bad. But chronic stress causes strain on the body, which can increase your risk of obesity, high blood pressure, heart disease, and type 2 diabetes, as well as mental disorders like anxiety and depression. In some people, it can also lead to weight loss.
Stress management tips include regular exercise; yoga or mindfulness meditation; emotional support through friends and family; and having an open conversation with your healthcare team about your mental health.
BMI paints a picture of only a portion of a person’s health. Although BMI takes your weight and height into account, it doesn’t consider other factors that are known to affect disease risk, such as high blood pressure, blood sugar, and cholesterol.
“There is not just one data point” when it comes to assessing someone’s health risks, says Janice Baker, RDN, CDCES, a registered dietitian and certified diabetes educator in San Diego. She adds that it’s important for physicians to assess factors like muscle mass, activity level, and nutrition when determining how healthy or unhealthy a person is.
Research shows that a number of other approaches may be more appropriate for measuring body composition than BMI, including:
According to Harvard Health Publishing, the following characteristics may mean BMI isn’t an effective tool for measuring your health risks:
Being an athlete
Being pregnant
Being elderly
Being nonwhite
The Takeaway
Body mass index is a measure of your weight in relation to your height. Healthcare professionals have long used this number to determine if someone has underweight, healthy weight, overweight, or obesity.
Online calculators can help you determine your BMI. A number below 18.5 is considered underweight, 18.5 to 24.9 is a healthy weight, 25 to 29.9 is overweight, and 30 or over is obese.
BMI should be viewed as a simple measurement that can help doctors identify potential concerns, but it doesn’t definitively account for whether a person’s weight will cause health problems.
You can lower your BMI by making smarter food choices, exercising regularly, and managing stress, and getting enough sleep.
