All About BMI (Body Mass Index): Ranges, Calculations, Flaws

The field of medicine is filled with tons of complex and confusing medical terms. You’ve likely heard one of those terms — the abbreviation BMI — during a doctor’s appointment or even in the news.

You probably know BMI stands for body mass index and that it has to do with weight, but what exactly is BMI used for, and how seriously do you need to consider your BMI and the BMI of your family members?

Think of this as your comprehensive guide to all things BMI. Here, you’ll learn what BMI means; how to calculate your number; what a low, normal, or high BMI is; and why the measurement isn’t without its downsides.

What Is the Definition of BMI?

BMI is a common measure of body weight, and most medical professionals use BMI to categorize people as having a normal weight, being overweight, or being obese.

Using this approach, the Centers for Disease Control and Prevention (CDC) estimates that more than one-third of American adults are obese. The CDC also estimates that in 2019, obesity cost the United States $173 billion in healthcare that year. A person who is obese paid $1,861 per year more than someone with a normal weight, the CDC notes. (1)

BMI can be tied to your health risk for certain diseases and health outcomes, but the measurement is limited, some critics argue. (2)

The CDC warns that BMI shouldn’t be used to define body fat or diagnose health conditions. (3) Rather, if you have a high or low BMI, your physician may have you undergo additional health screenings, such as for high cholesterol or high blood pressure.

The CDC compared data gathered from 1999–2000 and 2017–2020 and found the obesity rate in the United States increased from 30.5 percent to 41.9 percent, and the rate of severe obesity increased from 4.7 percent to 9.2 percent during the same time span. (1)

Non-Hispanic Black people are the most likely to be obese, followed by Hispanics, white people, and non-Hispanic Asian people. (1)

Middle-aged adults are at the greatest risk for the disease, followed by older adults ages 60 and older, and young adults ages 20 to 39. (1)

Overall, the South has the highest prevalence of obesity, while the West and Northeast have the lowest rates of obesity. (5)

The Importance of a Healthy BMI

If you’re an adult, maintaining a normal BMI is crucial for your future health, because a high BMI has been linked to an increased risk for diseases such as heart disease and type 2 diabetes. (6)

In young adults, obesity may even be to blame for an increase in rates of colorectal, endometrial, gallbladder, kidney, multiple myeloma, and pancreatic cancer, according to a 2019 data analysis. 

RELATED: How Obesity and Heart Disease Are Connected

Calculating Your BMI

To calculate your BMI, take your weight in kilograms divided by the square of your height in meters.

If you live in the United States and don’t use the metric system, you can also take 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 (lb) and 63 inches (in) tall, we can calculate her BMI this way:

1. Height in inches squared: 63 x 63 = 3,969
2. Weight divided by height squared: 140/3,969 = .03527
3. .03527 x 703 = 24.79 BMI

If math isn’t your forte, you can opt to use the CDC’s BMI calculator to figure out your BMI.

What Is a Healthy BMI? A List of the Ranges

Here’s what your number means: (3)

Below 18.5: underweight

18.5 to 24.9: normal

25 to 29.9: overweight

30 and over: obese

Although the BMI cutoffs are the same regardless of gender and age, there are some differences. In general, women of all ages tend to have a higher body fat percentage compared with men the same age. (7) And the older we are, the weight we carry is typically higher in body fat and lower in muscle. (8)

Learn More About BMI in Adults

To determine whether your BMI is healthy, you can also review the below chart, which specifies healthy, overweight, and obese ranges based on your height and body weight in pounds.

Children aren’t immune to the United States obesity epidemic. In fact, according to 2017–2020 CDC data, the most recent data available, about 20 percent of children and adolescents ages 2 to 19 are considered obese or overweight. (9)

Your behavior as a parent, as well as your child’s community and geographical region, are closely connected to his or her obesity risk. The affordability of healthy food, the opportunity to be physically active in schools and daycare, and exposure to the marketing and promotion of healthy or unhealthy foods can also affect your child’s obesity risk. (6)

How to Calculate Your Child’s BMI

Your child’s pediatrician will usually calculate his or her BMI when he or she is around age 2 or 3. But 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.

A child between age 2 and 18 is considered overweight when the BMI is between the 85th and 94th percentile. A child in that same age range is considered obese if the BMI is at or greater than the 95th percentile.

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. (11)

Steps to Help Lower Your Child’s BMI

Weight loss is a sensitive subject regardless of age, but for kids, this topic requires some extra care to nurture your child’s mental health. Having obesity may increase your child’s risk of depression, and children and adolescents who are overweight or obese may have a negative self-image, especially if they are girls, research suggests. (12)

If your child needs to lower his or her BMI, consider approaching the topic by discussing lifestyle changes across the family unit rather than addressing your child’s weight loss directly. Doing so can help avoid damaging their self-esteem.

Healthy diet changes you can make as a family include an emphasis on lean protein, whole grains, fruits, vegetables, and low-fat dairy. Popular diets are not typically recommended, but changing what types of foods your child has access to is a good start. Do not control the amount your child eats — that’s his or her choice. Your role is to control the types of choices available.

Because weight loss requires taking in fewer calories than you burn, you can help your child lower his or her BMI by encouraging more physical activity. But instead of telling your child to get up and get some exercise, invite her to play a group sport like kickball or take a walk with you. Model the behaviors you want to see in your children by spending time being active as a family. (6,13)

Learn More About BMI in Children

When it comes to assessing the relationship between your BMI and your risk for disease or early death, study results are mixed and may vary by patient population, with some indicating that a higher BMI may at once increase your risk for negative health outcomes and have a protective effect on your chances for survival. (14)

What we know for sure, at least on an observational level, is that BMI correlates with obesity-related conditions, says Jessica Crandall, RDN, owner and CEO of Vital RD in Centennial, Colorado.

For example, a higher BMI is associated with an increased risk of: (15)

• Type 2 diabetes
• Heart disease
• Cancer
• Gallbladder disease
• Depression
• Nonalcoholic fatty liver disease
• Arthritis
• Sleep apnea
• COVID-19 related complications

A higher-than-normal body weight usually involves excess fat on the body, which is a basic cause of body systems malfunctioning. An overproduction of fat cells results in or is associated with an overproduction of blood glucose, hormones, plaque in the arteries, and stress on bones and organs. The body is overwhelmed and begins to do its best to balance itself out. Some people may see a delay in the onset of age-related disease because of their genetics. (16,17)

A higher BMI’s slightly protective effect on survival is interesting to note. Some research suggests that if you’re hospitalized for a chronic condition such as heart disease or cancer, having a higher BMI may offer a protective effect. This phenomenon is called the “obesity paradox” and may be partially explained by the fact that extra weight and muscle can compensate for lost weight and muscle mass during serious illness, ultimately helping speed recovery. (14,18,19,20)

Learn More About BMI and Your Health

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 burn through activity.

A low intake of fruits and vegetables is associated with a higher body weight because the energy density of these foods is lower than in other foods. On the other hand, high fat and sugar intake is associated with a higher BMI because the density of those foods is higher than that of fruits and veggies.

The amount your body moves plays into this energy balance equation. People who are physically unable to exercise because of illness or injury, or who can’t move around safely in their communities, may experience weight gain. Also, having a desk job can reduce your body’s energy needs, increasing your risk of being overweight or obese. (21,22,23,24) As mentioned, having a high BMI can take a toll on your health, so it’s no surprise that the more you sit and the less you move, the greater your risk for early death and chronic disease. (25)

Learn More About the Top Risk Factors for Having a High BMI

Remember that BMI is a calculation using your weight and your height. Adults who need to decrease their BMI must lose weight, because their height is fixed.

You can help lower your BMI by taking the following steps.

Make smart food choices. Several diets 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 is a tried-and-true way to help lower your body weight.

The U.S. Department of Agriculture’s (USDA) Dietary Guidelines suggest that variety, nutrient density, and portion sizes are important. For optimal health overall, limit added sugars, unhealthy fat, and sodium. Think 10-10-2,300 for these food groups: added sugars less than 10 percent of your total calorie intake, saturated fat less than 10 percent, and sodium less than 2,300 milligrams (mg) per day. (22)

Exercise regularly. The U.S. Department of Health and Human Services recommends 150 minutes of moderate-intensity aerobic physical activity (such as playing tennis or walking) per week or 75 minutes of vigorous-intensity aerobic physical activity (such as biking, swimming, running, or jogging). For even more health benefits, you can increase your moderate-intensity exercise to 300 minutes per week or your vigorous-intensity aerobic physical activity to 150 minutes. (23)

RELATED: Everything You Need to Know About Fitness

Get adequate sleep. Most adults need seven to nine hours of sleep. Chronic lack of sleep or poor sleep quality may increase your risk for poor health outcomes including high blood pressure, cardiovascular disease, diabetes, depression, and obesity. (26)

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 for obesity, high blood pressure, heart disease, and type 2 diabetes, as well as mental disorders like anxiety and depression. Stress management tips include getting regular exercise; trying yoga, meditation, or tai chi; finding emotional support through friends and family; and having an open conversation with your healthcare team about your mental health. (27,28)

Eat out less and cook more at home. Eating at home typically results in a reduction of sugar, fat, and unhealthy refined carbohydrates. Make a realistic goal of eating a meal at home just one to two days more than you do now. Keep the meals simple, and aim for at least three food groups per meal (such as whole grains, lean protein, and vegetables). A meal such as a chicken breast with brown rice and a side salad can be pulled together in less than 30 minutes. (29)

RELATED: 9 Hard Truths About Weight Loss That Can Help You Slim Down

Let’s face it: Losing weight is hard — and keeping it off is even harder. Many of us want that quick fix, which is why fad diets that promise quick weight loss are so tempting. But most people put on weight over a long period of time, so losing weight may take some time as well. Making gradual, practical changes to your diet and lifestyle can ultimately lead to the long-lasting results you’re seeking. (30,31,32,34) There can also be mental and emotional barriers to weight loss that can be explored if you’ve tried to lose weight several times and are still struggling.

Learn More About How to Lower Your BMI

The fact that BMI paints a picture of only a portion of a person’s health has drawn criticism from some health professionals. 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, RD, CDE, a certified nutrition support clinician based in Poway, California. 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.

To measure body composition, a number of other approaches may be more appropriate than BMI, notes past research: (35)

• Weight-to-height ratio
• Waist circumference
• Waist-to-hip ratio
• Body fat percentage

The following characteristics may mean BMI isn’t an effective tool for measuring your health risks: (36,37,38)

• Being an athlete
• Being pregnant or nursing
• Being over age 65
• Being Asian

Learn More About BMI Flaws