What Is BMI, and How Do You Calculate It?

The body mass index (BMI) has come under a lot of scrutiny in recent years — for good reason. It was originally developed by a Belgian man who wanted to see if he could use statistics to determine the size of a “normal” adult man. It was never intended to be used in a medical setting, let alone to assess individual health. 

But now, BMI has become a regular part of people’s medical records. And it's used for anything from routine health screenings to insurance approval. But on its own, BMI doesn’t say a lot about your overall health — or even your body fatness. So let’s take a closer look at what BMI means and what it can and can’t tell you about your health. 

What is BMI?

BMI is your weight (in kilograms) divided by your height (in meters squared). This number is then categorized into one of four different weight categories: 

• Underweight: below 18.5 

• Healthy weight: 18.5 to 24.9 

• Overweight: 25 to 29.9

• Obesity: 30 and above

Promotional Disclosureinfo_outlined

SPECIAL OFFER

Sponsored By

GLP-1 Nutrient Support

Explore dietitian-formulated, doctor-approved supplements from Whole Health Rx that work alongside GLP-1 medications for a balanced approach to weight loss.

Shop now

For a BMI above 30, there are three more categories:

• Mild obesity (class 1): 30 to 34.9

• Moderate obesity (class 2): 35 to 39.9

• Severe obesity (class 3): 40 or higher

As we’ll explore later in the article, someone’s BMI alone isn’t an accurate predictor of their body composition or health. Two people with the same BMI can have totally different bodies and health profiles. 

But for better or for worse, BMI is a simple and inexpensive screening tool. You don’t need any special technology or expensive equipment to calculate it. And so healthcare professionals use it as a quick way to assess if someone may be at risk for weight-related health conditions. 

How do you calculate your BMI?

You can calculate your BMI with this tool. To manually calculate BMI, divide your weight (in kilograms) by the square of your height (in meters):

weight (kg) / height (m)² = BMI

If you want to use pounds and inches, a conversion factor is necessary:

weight (lbs) / height (in)² x 703 = BMI

What does your BMI mean for your health?

The BMI doesn’t say much about your overall health. But it can be a quick way to assess if your weight might be putting you at risk for certain health conditions. This is because a lot of research over the years has shown that health complications increase as BMI increases. In fact, this is one of the reasons healthcare professionals still rely on the BMI — because it has become such a standard way to research weight. 

High BMI

Studies have found a link between the following health risks and a BMI in the overweight and obesity categories: 

• Type 2 diabetes

• High cholesterol

• High blood pressure

• Heart disease

• Stroke

• Blood clots 

• Sleep apnea

• Erectile dysfunction 

• Irregular periods, fertility problems, and complications in pregnancy 

• Osteoarthritis

• Body pain

• Mental health conditions like depression and anxiety

• Several types of cancer

EXPERT PICKS: WHAT TO READ NEXT

• Living well in a larger body: We want to have a different approach to weight at GoodRx — and to help you live a healthy life at any size.

• Have you experienced weight stigma? Learn how to advocate for yourself and your health at your next healthcare visit.

• On a mission: One woman opens up about her experience with weight bias — and how she now advocates for others in her position.

This list can feel daunting. But keep in mind that not every person with a bigger body is necessarily at increased risk for these conditions. Research looks at large populations of people, not individuals. And each person’s health is so much more complex. 

Can you have a high BMI and still be healthy?

Yes. Someone with a high BMI can still be healthy. Likewise, you can have a BMI in the normal range and have chronic medical conditions related to body weight.

Your BMI number is just one piece of a much larger puzzle. It needs to be interpreted with other information about you, like your: 

• Diet

• Fitness level

• Daily lifestyle

• Medical history

• Genetics

Read more like this

Explore these related articles, suggested for readers like you.

How Do You Measure Body Fat? There Are Better Ways Than BMI

Can You Be Overweight and Healthy? Why Waist Size May Matter More Than Body Fatness

The Fat Liberation Movement Has a New Advocate: Singer and Activist Mary Lambert

View more

To highlight this, a 2016 study questioned whether being labeled as overweight or obese means that someone is unhealthy. The study measured several different markers of health beyond weight and BMI. Researchers found that many people in the overweight and obesity categories were quite healthy. Meanwhile, over 30% of people in the “healthy” BMI category were at risk for many of the health conditions associated with a higher weight. 

Low BMI

It’s also important to note that not having enough fat on your body can also increase someone’s health risks. Low BMI has been linked to: 

• Certain heart diseases, like problems with the heart valves

• Lower immune function and respiratory infections

• Faster decline in lung function in people with chronic obstructive pulmonary disease (COPD)

• Decreased semen quality

• Irregular menses and infertility

• Osteoporosis and bone breaks

• Increased risk for certain types of cancers

• Surgical complications and poor wound healing

• Mental health conditions like depression and difficulty sleeping

And like a high number, low BMI has been associated with increased mortality risk (early death).

Problems with BMI

The BMI has evolved to be a tool to predict body fat and how this fat impacts someone’s health. But many times, it doesn’t predict either of these well. This is because BMI doesn’t account for:

• Fat versus muscle: Many of the health risks associated with a higher BMI are actually associated with a higher percentage of fat — not weight. But the BMI can’t measure this, and muscle weighs more than fat. So a muscular person in a larger body with very little body fat may have a BMI in the obesity range. 

• Fat distribution: It’s not just the percentage of body fat that makes a difference. The distribution of that fat on your body is also important. Someone who has more fat around the abdomen is at greater risk of health problems than someone with overall excess fat.

• Racial differences: The risk of health problems by BMI varies by race. Remember, it was first developed in white Belgian men. So, for example, Asian people have a higher risk of diabetes and heart disease at lower BMIs than white people. 

• Sex differences: Women generally have more body fat than men, even if their BMI is the same. This means that women may be at higher risk for conditions like heart disease at a lower or normal BMI.

Does BMI change with age?

The BMI is calculated the same way regardless of age. But it needs to be interpreted differently. The amount of fat on your body has different benefits and risks depending on if you're young or older. 

BMI in kids

BMI is interpreted differently in children and teens. It uses the same measurements, but the number is then reported as a percentage. This is done to take into account the natural differences in body fat between boys and girls at different ages. You can use this calculator to calculate BMI for anyone 2 to 19 years of age.

Even though the BMI is an imperfect tool, it's still important to track in children. This is because children and teens with excess weight are more likely to have similar health risks as adults, but at a younger age.

BMI in older adults

BMI in older adults — ages 65 and over — should be interpreted differently for a couple of reasons. 

First, BMI tends to underestimate fat in older people. As you age, you lose lean mass and gain more fat mass. So, for older adults, it’s more important to look at how the fat is distributed in the body — and whether weight changes rapidly.

Second, a higher BMI may be more beneficial in older adults. A large analysis of studies showed that people who qualified as overweight didn’t have an increased mortality risk. In fact, people with a lower BMI (less than 23) had a much higher mortality risk than people with an overweight BMI. The mortality risk didn’t increase for higher-weight people until their BMI was over 33, well into the obesity category. 

Studies have also found that older adults with a higher BMI are better able to perform their daily tasks. 

Alternatives to BMI

BMI is just one way to estimate body fatness. Other measurements can also help you understand if body fat is putting you at risk for health problems. These may do an even better job at estimating your body composition:

• Waist circumference: The fat that gathers around your middle is more dangerous to your health than fat in other areas. The risk of health problems may be higher for men with a waist size above 40 inches and for women with a waist size above 35 inches.

• Waist-to-hip ratio: This is another way to measure fat around the middle. It's calculated by dividing waist circumference by hip circumference. According to the WHO, this number should be less than 0.85 for women and less than 0.9 for men. 

• Body roundness index (BRI): This calculation attempts to estimate fat distribution — especially abdominal fat. The formula is complicated, but it only requires height and waist circumference. And early research suggests it may be a more accurate predictor of mortality than BMI. 

• Relative fat mass (RFM): This is a calculation that uses height, waist circumference, and gender. It may also be more accurate than BMI in determining body fat percentage. But studies are in progress to confirm its accuracy. 

• Body volume index (BVI): This is a new method that uses 3D body scanning. It measures the volume of the trunk and abdomen, limbs, and the entire body. And it may be a better way to measure abdominal fat.

There are other tools that attempt to calculate body fat percentage. But they aren't well established. And some may be less accurate in certain situations, like for people with excess fluid due to kidney or heart disease. So they’re not recommended as screening tools or for monitoring high BMI categories. They include:

• Body-fat scales 

• Underwater weighing

• Air displacement (BOD POD)

• Bioelectrical impedance analysis (BIA)

Why trust our experts?

Written by:

Katie E. Golden, MD

Katie E. Golden, MD, is a board-certified emergency medicine physician and a medical editor at GoodRx.

Reviewed by:

Sophie Vergnaud, MD

Sophie Vergnaud, MD, is the Senior Medical Director for GoodRx Health. An experienced and dedicated pulmonologist and hospitalist, she spent a decade practicing and teaching clinical medicine at academic hospitals throughout London before transitioning to a career in health education and health technology.

Our editorial standards

Meet our experts