Key takeaways:
Body mass index (BMI) is calculated by dividing your weight by your height. It’s often used as a quick way to assess someone’s body composition.
BMI provides a limited view of your body fat and your overall health. Researchers are uncovering better ways to tell if weight is a potential health risk.
Your BMI may help you understand your risk for certain conditions, like diabetes and heart disease. But it should be interpreted along with other, more meaningful details about your health.
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 determine the size of a “normal” adult man. It was never intended to be used in a medical setting, let alone to assess a person’s health.
But now, BMI has become a regular part of medical records. It’s used for anything from routine health screenings to insurance approval. But on its own, BMI doesn’t say a lot about a person’s overall health — or even their amount of body fat.
Let’s take a closer look at how you calculate BMI, what it means, and better ways to measure body composition.
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
For a BMI above 30, there are 3 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, BMI alone isn’t an accurate predictor of body composition or health. Two people can have the same BMI but 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. Because of this, healthcare professionals use BMI as a quick way to assess if someone may be at risk for weight-related health conditions.
How do you calculate your BMI?
This table provides an easy way to look up your BMI.
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 calculate your BMI using pounds and inches, you need a conversion factor:
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weight (lbs) / height (in)² x 703 = BMI
What does your BMI mean for your health?
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. This is one reason healthcare professionals still rely on the BMI — it has become such a standard way to research weight.
High BMI
Studies have found links between the following health risks and a BMI in the overweight and obesity categories:
Irregular periods, fertility problems, and complications in pregnancy
Body pain
Mental health conditions like depression and anxiety
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. 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 healthy range and have chronic medical conditions.
A 2016 study highlights this point well. It 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.
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
Daily lifestyle
Medical history
Genetics
Low BMI
It’s also important to know that too little body fat can also increase health risks. Low BMI has been linked to:
Certain heart diseases, like heart valve problems
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 BMI, a low BMI has also been linked to increased mortality risk (early death).
What are the limitations of BMI?
Over the years, healthcare experts have used BMI 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 take several important factors into account:
Fat versus muscle: Many health risks associated with a higher BMI are actually associated with a higher percentage of fat — not just higher weight. But BMI can’t measure the difference between fat and muscle.
Fat distribution: Experts now focus more on where the fat is stored, rather than how much there is. This is because research is revealing that fat around the abdomen is linked to a greater risk of health problems than excess fat in other areas.
Racial differences: The risk of health problems by BMI varies by race. For example, research suggests that Asian and Black people have a higher risk of diabetes at lower BMIs than white people.
Sex differences: Women generally have more body fat than men, even with the same BMI. 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. Body fat has different benefits and risks for young and older people.
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 isn’t a perfect 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 (age 65 and up) should be interpreted differently for a couple of reasons.
First, BMI tends to underestimate body fat in older people. As you get older, 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 older people in the overweight category didn’t have an increased mortality risk. In fact, older adults with a lower BMI (less than 23) had a much higher mortality risk than those with an overweight BMI. And the mortality risk didn’t increase for higher-weight people until 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 show 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 World Health Organization (WHO), this number should be less than 0.85 for women and less than 0.9 for men.
Waist-to-height ratio: This compares your waist size to your height. The measurement may be better than waist circumference since it’s less likely to be affected by differences in sex and race. It also changes with weight loss, unlike waist-to-hip ratio. Early studies suggest it’s better than BMI at predicting heart disease risk.
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 sex to estimate body fat. Initial studies suggest it may be more accurate than BMI in determining body fat percentage for women and men across multiple races.
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.
Other tools for calculating body fat percentage
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. Because of this, they aren’t recommended as screening tools or for monitoring high BMI categories. They include:
Underwater weighing
Air displacement (BOD POD)
Bioelectrical impedance analysis (BIA)
Frequently asked questions
The BMI categories are the same for men and women, so a normal BMI range for a woman is 18.5 to 24.9. For Asian women, a normal BMI is between 18.5 and 23.
But some experts recommend that BMI ranges should vary even more by race. For example, research looking specifically at the risk for diabetes across different race and ethnic groups found that risk increased at the following BMI levels (for both men and women):
White: BMI of 30
Black: BMI of 28
Chinese: BMI of 27
Arab: BMI of 26
South Asian: BMI over 24
It’s important to note that women tend to have a higher percentage of body fat than men with the same BMI. This means that women with a normal BMI may still be at higher risk for weight-related medical conditions, like heart disease.
A normal BMI for a man is considered to be 18.5 to 24.9 using the “healthy weight” category, and 18.5 to 23 for Asian men.
Right now, BMI categories don’t vary by age, so a BMI of 18.5 to 24.9 is considered “good” or “healthy” for most adults. But this may change soon. As people get older, they tend to lose muscle mass and gain body fat, especially around the belly where it may be more harmful. This means that older adults may be at increased risk of certain health conditions, even if they have a BMI in a healthy range. This is why healthcare experts are increasingly using other measures, like waist circumference and waist-to-height ratio.
The BMI categories are the same for men and women, so a normal BMI range for a woman is 18.5 to 24.9. For Asian women, a normal BMI is between 18.5 and 23.
But some experts recommend that BMI ranges should vary even more by race. For example, research looking specifically at the risk for diabetes across different race and ethnic groups found that risk increased at the following BMI levels (for both men and women):
White: BMI of 30
Black: BMI of 28
Chinese: BMI of 27
Arab: BMI of 26
South Asian: BMI over 24
It’s important to note that women tend to have a higher percentage of body fat than men with the same BMI. This means that women with a normal BMI may still be at higher risk for weight-related medical conditions, like heart disease.
A normal BMI for a man is considered to be 18.5 to 24.9 using the “healthy weight” category, and 18.5 to 23 for Asian men.
Right now, BMI categories don’t vary by age, so a BMI of 18.5 to 24.9 is considered “good” or “healthy” for most adults. But this may change soon. As people get older, they tend to lose muscle mass and gain body fat, especially around the belly where it may be more harmful. This means that older adults may be at increased risk of certain health conditions, even if they have a BMI in a healthy range. This is why healthcare experts are increasingly using other measures, like waist circumference and waist-to-height ratio.
The bottom line
The BMI has been the standard way to determine if someone’s weight puts them at risk for certain health risks. But it may soon be a thing of the past. Research is uncovering better ways to measure how weight affects health, like waist circumference and waist-to-height ratio. If your BMI number has been a source of stress or even weight bias, remind yourself that it’s just one piece of the puzzle. Healthcare professionals are also becoming more aware that BMI alone may not be a helpful way to assess health.
Why trust our experts?
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