Science Center

Body Mass Index, explained

Body Mass Index (BMI) is a simple measure for weight-related health risk.  It is one of the measures that your doctor or nurse uses to determine obesity and your health risk in relation to your weight. It is a signal to your doctor to take a closer look and have a more detailed conversation with you about your health. The measure is used for adults aged 20 and older, and BMI-for-age, also known was BMI percentile, is used for children and teens to account for gender, age, and maturation level.

Specifically, BMI represents the relationship between weight and height to estimate body fatness.1 A higher percentage of body fat is proven to be associated with increased risk for developing certain diseases such as heart disease, high blood pressure, type 2 diabetes, breathing problems, certain cancers, and death.2, 3, 4


How is BMI calculated?

It is as simple as plugging your height and weight into an online calculator, such as the one hosted by the National Heart, Lung, and Blood Institute.

BMI formula: your weight (in kilograms) divided by your height squared (in meters), expressed as kg/m2

  • Formula: Weight (kg)/[height (m)]2
  • Example: weight =92 kg  and height= 1.78 m
    • Calculation: 92/ (1.78)2= 92/3.17=29.0

However, it can also be calculated using the English System:  your weight (in pounds) divided by your height squared (in inches) multiplied by 703.

  • Formula: [Weight (lb)/height (in)]2  x 703
  • Example: weight =202.8 lb and height= 70 in
    • Calculation: [202.8/ (70)2] x 703= [202.8/4900] x 703= 29.1

BMI is interpreted by using the following weight categories.

BMI (kg/m)2 Weight Range
< 18.5 Underweight
18.5-24.9 Healthy Weight
25.0-29.9 Overweight
30.0 or higher Obese


Overweight and obese are the ranges of weight that are greater than what is generally considered healthy for a given height, and are used as a signal for higher risk for disease.4


Why use BMI?

We know that BMI is not a perfect measure; some of the known limitations include:

  • It does not directly assess body fat and there are other measures such as waist circumference or DEXA scans that may be a better measure of body composition.
  • It is not as accurate for athletes as it may not accurately assess body composition (read: does not take into account the weight of bones, muscles, or organs5).
  • It is not as accurate for certain racial/ethnic groups such as those of Asian descent (the weight categories have different cut points for those of Asian descent; for example, overweight is defined as a BMI of 23.0-27.5 instead of 25.0-29.9 for those of non-Asian descent).6
  • People with a similar BMI can have very different health risks due to genetics, lifestyle, and their environment.7, 8, 9
  • Body fatness levels differ across demographic categories, and two people of the same BMI can have different levels of body fatness. For example, we know that women tend to have more body fat than men, older people tend to have more body fat than younger adults, and athletes tend to have less body fat than nonathletes.10

All of that being true, BMI continues to be the best estimation of weight-related health risk because of its scalability and accessibility—it is a simple calculation, it is based on easily measured values (height and weight), and it is relatively easy to understand.11, 15 Additionally, research has shown that BMI is strongly correlated with the gold-standard (read: best) methods for measuring body fatness.1, 12 Other measures of body composition, such as bioelectrical impedance, dual energy x-ray absorptiometry, skinfold thickness measurements, and underwater weighing, tend to be expensive, invasive, difficult to obtain, and require highly trained clinicians.15


Are there times when BMI is not the best measure?

For the people who work out vigorously, or athletes, other measures of body composition may be more useful than BMI to assess their health risk associated with their weight, such as body measurements (waist, hip, arm, thigh) or skinfold thickness with calipers. Body measurements can be tricky to measure the same way each time, especially if you are measuring yourself, so here are a few tips:1,15

  • Take body measurements at the same time of day (e.g., in the morning).
  • Measure once a month, not every day.
  • Use landmarks on your body that will not change as your body shape changes:
    • Upper arm circumference: halfway between the bend in your elbow and the top of your shoulder
    • Waist circumference: halfway between your hip bone and last rib
    • Thigh circumference: halfway between the kneecap and where your leg joins the rest of your body
  • When using a tape measure:
    • Don’t hold your breath.
    • Make sure the tape measure is straight across whatever body part you are measuring.
    • Pull the tape measure snug, but not too tight.
    • Make sure to use a flexible tape measure.


BMI and the weight-loss journey

BMI, like the scale or your energy level, is another tool that can be used to measure success. At the beginning of a weight-loss journey, people may find that BMI is useful to tell them where they are starting and provides an initial estimate of their weight-related health risk. The scientific literature suggests that even modest decreases in BMI can significantly improve health by lowering the risk for high blood pressure, type 2 diabetes, heart disease, obesity-related cancers, and early death.13,14,15 Additionally, a weight loss of 5-10% has been associated with improvements in sleep quality, mood, and mobility. 16,17

During the weight-loss journey, people tend to find that tracking the change in their weight (either in pounds or kilograms) is more meaningful, and easier to interpret, than changes in BMI. As with all other indicators of success, BMI is just one measure of success. The goal is not to get to a certain BMI, but to move along the continuum toward a happier, healthier you.

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