Since BMI was created centuries ago, many patients and practitioners wonder whether the measure is still an accurate picture of health. Even with its limitations, BMI provides an inexpensive and accessible way to estimate if changes in weight may be needed.
Learning how to calculate BMI can help you keep your health goals on track. Read on for answers to common questions like, “What is BMI?” and more!
What Is BMI?
Body mass index (BMI) is a measure used to estimate body fat. It’s a ratio of body weight to height, and the resulting number indicates whether a person falls within the underweight, normal weight, overweight, or obese range.
A Brief History of BMI
In the mid-1800s, a Belgian mathematician and statistician named Adolphe Quetelet created a formula (the Quetelet Index) to help measure and categorize the average man. The equation we know today as BMI was initially intended as a way to analyze society, not necessarily group people by weight.
In other words, it was likely that Quetelet intended for BMI to be used as a “big picture” for society instead of a method for proposing individual interventions. However, in the early 1970s, a physiologist named Ancel Keys renamed the Quetelet Index (calling it body mass index) and proposed that BMI was an accurate way to identify individuals who were overweight or obese.
Since then, BMI has been incorporated into government health policies and organizations (such as the National Institutes of Health) as a way to categorize individuals by weight. Despite its limitations as a health screening tool for individuals, it’s still widely used in medical settings because it’s an inexpensive and easily accessible way to estimate body fat.
How BMI Is Used Today
Certain BMI categories are linked with health risks, whereas a normal (healthy) weight is linked to better health outcomes.
In other words, BMI may be able to help predict health problems and provide you with empowering information about your health. A higher BMI may put you at increased risk for:
• Breathing problems
• Certain cancers (i.e. colon)
• Heart disease
• Gallstones
• Osteoarthritis
• Sleep apnea
• Type 2 diabetes
How to Calculate BMI
At first glance, BMI may seem complicated, or an indicator only your doctor can decipher. In reality, BMI is easy to calculate, especially with online tools available. Most methods require inputting common health measures you’re used to reporting, like weight, height, age, and gender.
Adult BMI
For adults, height and weight are used to calculate whether or not you fall within the “healthy weight” range. Typically, BMI is easier to calculate when using metric units (height in centimeters and weight in kilograms) rather than standard units (height in feet and inches, and weight in pounds).
When using metric units, BMI is simply weight in kilograms (kg) divided by height in meters (m) squared. If height has been measured in centimeters (cm), remember to divide the number by 100 to convert centimeters to meters before using this number in the equation.
The following equations can help you calculate BMI manually:
• Metric: weight (kg) / [height (m)]2
• Standard: 703 x weight (pounds) / [height (in)]2
Luckily, an easier method than the BMI equation exists. As long as you know your height in inches and weight in pounds, you can use a BMI table to pinpoint your BMI. These tables use the same equation as listed above to estimate body fat, but have already converted the figures from metric units to standard units).
The easiest way to find your BMI is to use an online calculator. Many professionals use the National Heart, Lung, and Blood Institute’s BMI calculator, which has options for inputting standard or metric units.
BMI categories based on height and weight are as follows:
• Underweight: less than 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25-29.9
• Obesity: 30 or greater
Children and Teen BMI
For children and teens (ages 2 to 20 years), a similar process is used. Typically, a BMI table is used to compare height and weight and growth charts are also used to track developmental trajectory.
Growth charts use the additional information of age and gender to compare the child to other children of the same gender and age. They also produce what’s called a BMI percentile, which indicates how a child measures for height and weight in relation to other kids of the same age and gender. For example, a boy measuring in the 85th percentile for weight would weigh more than 85% of boys the same age.
Percentile ranges place children and teens in weight status categories similar to adults. According to percentile, weight categories are as follows:
• Underweight: less than 5th percentile
• Normal (“healthy”) weight: 5th to less than the 85th percentile
• Overweight: 85th to less than the 95th percentile
• Obesity: equal to or greater than the 95th percentile
It’s important to keep in mind that children and teens grow at a relatively rapid rate. Weight fluctuations and height changes are normal, especially during the teen years when sex hormones also enter the picture.
BMI Chart & Classifications
BMI measurement is typically divided into four weight categories, as mentioned above (underweight, normal, overweight, and obesity).
As the name suggests, if you fall in the underweight category, you likely need to put on weight in order to be considered “healthy”. If you are categorized as underweight, you’ll likely work with a dietitian or doctor to find foods that can help move you to the normal BMI range. You may also need to address other underlying diagnoses that are causing you to be underweight, such as cancer or anorexia.
A normal BMI is considered a healthy BMI. This means you measure right along with the average person for your respective height and weight. While other measures are needed to analyze overall health, normal BMI likely means you should keep up the good work and whatever you’re doing for weight management (i.e. diet and exercise) is working.
A high BMI, meaning overweight or obese, indicates you may have too much body weight (body fat) in relation to your height. Even though it’s widely used, BMI does have its limitations.
Should You Use BMI?
While BMI can provide a good estimate, it’s not a true measure of body fat or body fatness. BMI only takes height and weight into account, which means it doesn’t accurately represent overall health, body fat percentage, or the amount of lean muscle mass.
An increase in muscle mass can cause an increase in body weight, which can easily be mistaken for an increase in the amount of body fat. This is especially common for athletes, who despite a muscular build are often mistakenly categorized in the overweight or obese category. BMI also fails to identify sedentary individuals with unhealthy habits who fall into the “normal” categories.
Special Populations
Older populations or those with certain diseases can also be placed in the wrong category, since BMI may underestimate the amount of body fat present when an individual has lost muscle.
To further complicate matters, some studies have shown that a higher BMI may be protective in some elderly people or chronic disease patients. BMI is also not particularly useful during pregnancy, although estimated pregnancy weight gain is based on pre-pregnancy BMI.
In other words, BMI fails to measure body composition, and may not be the most accurate measure for special populations or individuals in certain life stages. However, for the average, healthy patient, BMI is still a helpful tool to help predict health outcomes.
Other Methods for Measuring Body Fat
Measures like waist circumference may also be useful in screening for disease risk. Waist circumference indicates how much weight lies around the waist area. A higher amount of weight around the waist is linked with conditions like heart disease and type 2 diabetes.
You can easily conduct a measure of waist circumference at home by using a tape measure around your waist (just above your hip bones, after you exhale). For women, a waist size larger than 35 inches may be cause for concern. For men, more than 40 inches means disease risk may be increased.
Other methods may also be used to measure body fat, including:
• Skinfold thickness
• Ultrasound
• Waist-to-hip circumference
Risk Factors
If your waist circumference and BMI are unfavorable, all is not lost! Risk factors can increase your risk for weight-related disease, and addressing the following measures can be an important part of weight management:
• Hypertension (high blood pressure)
• High blood glucose (blood sugar)
• High triglycerides
• High LDL (“bad”) cholesterol
• Low HDL (“good”) cholesterol
Some predictors of weight-related conditions, like having a family history of heart disease, can’t be helped. However, many aspects of your lifestyle can be modified, such as quitting smoking, eating a healthy diet, and participating in physical activity.
Usually, if an individual has a high BMI and two or more risk factors, it’s recommended that they lose weight. Those who just have a high BMI may want to consider preventing further weight gain. A doctor or dietitian can help you decide what category you fit in and how to best approach your weight.
Non-Scale Victories
Looking for a way to manage weight without even stepping on the scale? Non-scale victories can be a helpful and positive way to reduce weight-related risk factors.
Non-scale victories (NSVs) are wins not related to the number you see on the scale. They focus on health improvements that can’t necessarily be measured, but are rather felt or sensed. For example, NSVs can include:
• A reduction in anxiety
• Feelings of self confidence
• Fitting into your favorite clothes
• Improved awareness of hunger cues
• Increased energy levels
• Noticing how happy you look in pictures
• Tracking weight only as necessary (i.e. at the doctor or once a month at home)
NSVs can capture characteristics that the scale cannot, such as discipline and drive to live a healthy lifestyle (despite what the scale says).
The Bottom Line on BMI
Despite its complex history, BMI is a relatively simple measure to use when estimating body fat. Both practitioners and patients can use BMI for a “big picture” measure, but it's recommended that risk factors and other aspects of health also be taken into account.
Even though it has limitations, BMI is still appreciated for its affordability and accessibility as a health tool.
What is a normal BMI?
A normal or “healthy” BMI is 18.5 to 24.9 for adults or between the 5th and 85th percentile for children and teens.
What is BMI and why is it important?
Body mass index (BMI) is a measure that estimates body fat and categorizes individuals as underweight, normal weight, overweight, or obese. It can help empower individuals to take charge of their health, reduce risk factors, and make meaningful changes to improve overall wellness.
References:
Academy of Nutrition and Dietetics. Defining Overweight and Obese. EatRight. Published November 2019.
Blackburn H, Jacobs D. Commentary: Origins and evolution of body mass index (BMI): continuing saga. Int J Epidemiol. 2014;43(3):665-669.
Centers for Disease Control and Prevention. About Child & Teen BMI. CDC. Published September 2022.
Centers for Disease Control and Prevention. Formula. CDC.
Centers for Disease Control and Prevention. Weight Gain During Pregnancy. CDC. Published June 2022.
Eunice Kennedy Shriver National Institute of Child Health and Human Development. How are obesity & overweight diagnosed? Nichd.nih.gov. Published July 2021.
National Heart, Lung, and Blood Institute. Assessing Your Weight and Health Risk. Nhlbi.nih.gov.
National Heart, Lung, and Blood Institute. BMI Tools. Nhlbi.nih.gov.
National Heart, Lung, and Blood Institute. Calculate Your Body Mass Index. Nhlbi.nih.gov.
