Medically Reviewed by Zilpah Sheikh, MD on May 24, 2024
Written by WebMD Editorial ContributorsBody mass index (BMI) uses your height and weight measurements to estimate how much body fat you have. It’s a quick screening tool that can help find out if you are in a healthy weight range. It can also estimate your risk for weight-related illnesses such as heart disease, stroke, and type 2 diabetes.
As useful as BMI can be, it has limitations. For starters, it doesn’t consider physical characteristics beyond height and weight, like how muscular you are. What’s more, white European men were the original models for the BMI formula. Research shows that race, gender, age, and ethnicity can influence how much body fat you have and what amount is healthy for you.
For these reasons, the American Medical Association advises against using BMI alone to check for weight status and related health risks. They instead urge doctors to use BMI along with other health assessments — such as waist circumference, blood pressure, and blood glucose and cholesterol levels — to gain a fuller and more accurate picture of their patients’ health status.
If you use one of the readily available online BMI calculators, consider your results a basic snapshot of your weight and health. A less-than-ideal BMI may simply be a sign that you should check in with your doctor and take a closer look at what’s going on.
Who created BMI?
A Belgian statistician, mathematician, and astronomer named Adolphe Quetelet came up with the formula back in 1832. He believed that if he collected measurements from thousands of mostly white, European males, he could identify the characteristics of the “average man.” During his research, he found that weight generally increased with a person’s height. He used this equation to pinpoint a supposedly “normal” weight range. Quetelet called his formula the Quetelet Index. He used it for his social research but never intended it as a measure of obesity or health.
In 1972, scientist Ancel Keys used the formula in his Seven Countries Study, which explored the link between excess weight and cardiovascular disease.
Ultimately, Keys found that Quetelet’s Index — which he renamed Body Mass Index — was not a better predictor of heart disease than other measures of fatness. But he declared BMI to be the best available way to quickly screen for obesity.
Keys said in his findings that BMI was most valuable as a tool for population studies, not for assessing the weight of individuals. However, doctors embraced BMI as a screening tool, and they — along with fitness coaches, nutritionists, and the general population — continue to use it to this day as a quick, if imperfect, gauge of weight health and disease risk.
BMI uses the ratio of your height to your weight to estimate body fat. If it sounds complicated, don’t sweat it. A BMI calculator can do the math for you. The number you get will fall into one of four weight categories: underweight, healthy weight, overweight, or obesity.
The basic idea is that the higher your BMI number, the more body fat you have. Low body fat may be a red flag for malnutrition, which can decrease your immunity and raise your risk for osteoporosis -- a condition that causes weak bones -- among other issues. High body fat may raise your risk for a host of weight-related diseases.
The same formula is used to calculate BMI for adults as well as children and teens. However, we read the results differently. This is because body fat changes as kids develop and differs between sexes.
Interestingly, research suggests that body fat composition differs between adult sexes, too. You also tend to lose muscle mass and gain fat as you enter your later years. But at this point, BMI interpretations for adults over 19 years of age do not adjust for sex or age.
BMI formulas
These are the formulas for calculating BMI:
Metric
weight (kg)/[height (m)]2
This is your weight in kilograms divided by your height in meters squared. Since metric height is generally measured in centimeters, divide the height score in centimeters by 100 to get your height measurement in meters.
English
weight (lb)/[height (in)]2x703
Divide your weight in pounds by your height in inches squared and then multiply by 703.
Here’s the BMI breakdown for adults:
BMI for children and teens is expressed in terms of percentages, not raw BMI numbers. These percentages reflect a child’s BMI relative to other kids or teens of the same gender and age. For instance, a child assigned male at birth in the 85th percentile has a BMI that is higher than 85% of males his age.
Here's the BMI breakdown for young people aged 2-19:
As a screening tool, BMI does a decent job of estimating body fat, especially when BMI is 30 or above. Research also strongly suggests that low and high BMIs are linked to an increased risk of chronic diseases and premature death.
Still, much of this evidence comes from studying BMI patterns in the general population. When it comes to individuals, BMI on its own can be a far less accurate measure of how much fat you have in your body and what your related health risks are.
Here are some reasons:
BMI doesn’t differentiate fat from muscle mass. Since muscle is more dense than fat, an athlete, even if they have very little body fat, can have the same BMI as someone who is obese.
People assigned female at birth tend to have more total body fat and less muscle mass than people assigned male at birth. Even with the same BMI, the sexes can have very different body compositions.
BMI doesn’t account for where fat is. Excess fat in your midsection — giving you an “apple” shape — can significantly raise your risk for heart attack, type 2 diabetes, and stroke. This is true even if your BMI is normal.
BMI does not consider the physical characteristics of people with disabilities. For instance, if you have achondroplasia (a type of dwarfism), your BMI may underestimate your body fat because of your short stature and tendency to carry belly fat. It may also underestimate obesity in people with Down syndrome.
BMI is based on data from white populations. Studies show that it may underestimate body fat and disease risk for people who are of a different race or ethnicity. For instance, research suggests that a woman of Asian descent has more than double the risk of developing type 2 diabetes than a white non-Latina woman with the same BMI.
Older adults tend to lose muscle mass and gain fat. Your BMI may be healthy when your body composition isn’t. Interestingly, excess body fat is not linked to a higher risk of disease and death in adults over 65. But loss of lean body mass may pose a significant threat to your longevity.
Because of these and other limitations, BMI generally isn’t accurate if you’re an athlete or bodybuilder, are pregnant, over age 65, or have muscle loss due to an existing illness.
Many screening and diagnostic tools can estimate or directly measure body fat. Some are far more accurate than BMI. The downside is that they can be expensive and medical professionals generally have to perform them.
Some common and relatively simple measurements include:
Waist circumference. This is the measurement of your waist between your lowest rib and the top of your hip bone. It is an easy way to identify excess abdominal fat, which is closely linked to weight-related illnesses and mortality. Calculating your waist-to-hip ratio can also estimate abdominal fat.
Skinfold thickness. This test uses a special tool to pinch your skin and the fat beneath it at seven specific points on your body. By measuring the skinfolds, a trained professional can estimate your percentage of body fat.
More complex measurements include:
Bioelectric impedance. Technicians can differentiate fat from other tissue by passing a small electric current through your body.
Dual-energy X-ray absorptiometry (DEXA). This can show how X-ray beams pass through your body fat and other body tissue at different rates. This also helps you know how much fat vs. lean muscle you have.
Hydrostatic weighing. Also called underwater weighing, this test weighs your body in normal conditions and again underwater. Based on the difference between these two measurements, technicians can calculate your body density and body fat percentage.
Isotope hydrometry. This test requires you to drink isotopes diluted in water. You then provide body fluid samples that technicians use to calculate your body composition.
CT and MRI scans: Experts generally consider these imaging tools the gold standard for measuring organs, tissue, and total body fat. Using MRIs and CTs for this purpose is generally reserved for research purposes.
BMI can be a helpful way to quickly assess your weight status and risk for weight-related illnesses. The key is knowing what the limitations of BMI are and consulting your doctor if you have concerns. If needed, they can do further assessments and help you figure out what your next steps should be.
Does BMI go up with age?
BMI climbs as kids grow and their body composition changes. So, we talk about children’s and teens’ BMI in terms of percentages instead of raw BMI numbers. After age 19, BMI ranges and weight classifications stay the same regardless of age. Data shows that the average US adult gains about 30 pounds by age 50 and that people in midlife today are more likely to live with obesity than any other generation. So, as a trend, BMI increases for many people as they go through midlife. Beyond age 65, BMI isn’t generally accurate, as older adults tend to lose muscle and put on fat. As a result, an older adult can have a seemingly ideal BMI but may be carrying too much body fat. A high BMI also isn’t linked to increased risk in older adults, while loss of muscle mass is.
How can I lower my BMI?
First, don’t rely only on BMI as a measure of how healthy your weight is. If you’re concerned that your BMI is high, see your doctor. They can consider your BMI in context with other health assessments and tell you if you truly need to lose weight. If that turns out to be the case, follow the guidelines for healthy weight loss, such as exercising regularly, eating out less, cutting calories, and eating plenty of whole grains and fresh vegetables.
Does BMI work if you’re muscular?
BMI doesn’t differentiate fat from muscle mass. Since muscle is denser than fat, it isn’t uncommon for athletes to have a BMI in the obese range, even if they have very little body fat. For this reason, it’s particularly important to take body composition and other measurements into consideration if you are athletic or just very muscular.
