How to Calculate BMI (Body Mass Index)
& What It
Means to You
In addition to showing you how to calculate BMI, this page will cover why your score is important and where the BMI formula originated.
- The BMI formula and how it applies to you (including BMI calculator)
- Body mass index and bariatric surgery
- Why is BMI used and what are the related health problems?
- BMI for children and teenagers
- Brief history of the body mass index formula
Calculate Your BMI
Click here to calculate your body mass index now.
How to Calculate BMI: The BMI formula and how to apply it to you
According to the Agency for Healthcare Research and Quality, about 1/3 of all people have the wrong impression of their own body weight.1
After reading this section, you won’t be one of them!
This section will cover:
The BMI Formula
Before getting to the actual formula, the easiest method for calculating BMI is to input your height and weight into a BMI calculator. In the box to the right, enter your height and weight, click "Calculate" to determine your BMI, then click here to learn what your score means.
The second method is to use the he National Heart, Lung and Blood Institute’s BMI table. Simply find your height (in inches) in on the left of the table, find your weight (in pounds) in your height's row, then view your BMI in the top row above your height's weight.
Finally, you can also do the calculation yourself.
Body mass index is a measure of your weight’s relation to
your height. Here’s the BMI formula for pounds/inches and
kilograms/meters…
BMI = (weight (pounds)/height (inches)2) x 703
or
BMI = weight (kg)/height (m)2
BMI Classifications & How to Interpret Them
Your BMI score can be used to classify your weight status. But don’t take your “classification” too seriously just yet. Moderately overweight people actually live longer than those at “normal” weight.2
By the way, shouldn’t those that live longer be considered more “normal”? The naming of weight classifications (i.e. "Healthy Weight") is one of the many problems we have with how the United States treats people who are overweight. More on this can be found on our Obesity Discrimination page.
Following are the body mass index weight categories for adults…
| Classification | Body Mass Index (BMI) | |||
|---|---|---|---|---|
| How to calculate BMI and apply it to adults over the age of 19 | ||||
| Underweight | Below 18.5 | |||
| Healthy Weight | 18.5 - 24.9 | |||
| Overweight | 25.0 - 29.9 | |||
| Obesity Class I ("Obese") | 30.0 - 34.9 | |||
| Obesity Class II ("Morbidly Obese") (may be eligible for bariatric surgery) | 35.0 - 39.9 | |||
| Obesity Class III ("Extremely Obese" or "Super Obese") (may be eligible for bariatric surgery) |
Over 40.0 | |||
In addition to the questionable names applied to the above classes, the BMI rating system has other limitations…
"Despite its shortcomings, knowing your BMI is a decent first step in determining which health issues you could be at risk for.
It's also an essential component of getting insurance approval for bariatric surgery."
- It doesn’t take into account the fact that muscle weighs more than fat, so it overestimates body fat in muscular people and underestimates body fat for people with less muscle mass.
- Fat around the midsection is worse than fat that is evenly dispersed throughout the body. For example, men with a waist circumference over 40 inches and women with a waist circumference over 35 inches have a increased risk of cardiovascular disease compared to people with the same BMI but a smaller waist circumference.
- The same BMI formula is applied to both men and women despite the inherent differences in body fat between them (men’s average BMI is 27.8 while women are at 26.8). After all, women should have more body fat than men so that their bodies can deal with the demands of carrying and nursing a child. In addition, men have a taller average height than women, so on average the formula will always result in a lower average BMI for men.
Despite its shortcomings, knowing how to calculate BMI is a decent first step in determining which health issues you could be at risk for. As we'll get into below, it's also an essential component of getting insurance approval for bariatric surgery.
In general, you should be concerned about obesity health problems if your BMI is 30 or more or if your BMI is between 25 and 29.9 and you have two or more of the following...
- Cigarette smoking
- Family history of premature heart disease
- High blood glucose (blood sugar)
- High blood pressure (hypertension)
- Low HDL-cholesterol (“good” cholesterol)
- High LDL-cholesterol (“bad” cholesterol)
- High triglycerides
- Physical inactivity3
Your doctor will also know how to calculate BMI and will interpret your score along with results from several other tests to diagnose your obesity class and confirm its potential health impacts. If you doubt your BMI score and would like to cross-reference it with another at-home test, see our How to Calculate Body Fat Percentage page.
Body Mass Index and Bariatric Surgery
According to the National Institutes of Health (NIH), bariatric surgery is appropriate for people who are 100 pounds or more above an ideal body weight or have a BMI of 35 or higher.
However, most insurance companies require that…
- Your body mass index is above 40 or
- Your body mass index is between 35 and 40 and you have a serious co-morbidity (obesity health problem)
Research has shown lower-BMI patients to have similar weight loss and resolution of obesity health problems as patients who meet the above NIH requirements.7 There are rumors that the cutoff may soon drop to 30 due to the positive bariatric surgery health statistics, but for now the above applies. (We'll keep you posted on this. Sign up for the Bariatric Surgery Blog to be notified when we add this and other new content to the site.)
You may also find that weight loss surgery is difficult to get approved if your BMI is too high, as higher BMI’s carry greater risks during surgery. For example, some surgeons require that your BMI is below 50 in order to operate. Other bariatric doctors will operate if your BMI is over 50, but may require that you lose 10% of your body weight first.
See our Bariatric Treatment page to learn whether weight loss surgery could be an option for you.
Why BMI is used and the related health problems
In 1998, the National Institutes of Health (NIH) brought together an expert panel who recommended that body mass index be used to determine whether an individual is overweight or obese.
In addition to being a fast, easy and cheap measurement, the panelists felt that for most people, BMI was a good representation of body fat. They also saw that it directly correlated with the risk of death and several diseases and conditions, including…
- Adult-onset asthma
- Arthritis
- Breathing problems
- Coronary heart disease
- Hypertension
- Ovarian cancer
- Total and ischemic stroke
See our Obesity Health Problems page for a full list of weight-related conditions.
BMI for children and teenagers
Children and teenagers up to 19 years of age use a different body mass index labeling system than adults.4 They are classified as follows:
| Classification | Body Mass Index (BMI) | |||
|---|---|---|---|---|
| How to calculate BMI and apply it to children and teens from 6 to 19 years of age | ||||
| Healthy Weight | Below the 85th percentile | |||
| Overweight | 85th percentile to 94.9th percentile | |||
| Obesity Class I ("Obese") | 95th percentile and above | |||
In children and teens from 6 to 19 years, BMI is age and gender specific. In other words, a 10 year old male will have a different “normal” BMI than a 15 year old female. Being “overweight” means having a BMI at or above the 95th percentile in their specific age/gender category.
See the CDC’s individual and clinical growth charts to learn where you (or your child) fall.
Also see our Weight Loss for Kids and Weight Loss for Teenage Girls and Boys pages (coming soon) for more information about and treatments for obesity in young people.
Brief History of the body mass index formula
Believe it or not, at first body mass index had nothing to do with body fat or health. It had to do with an astronomer’s fascination with statistics.
Two men were the primary drivers of body mass index as a tool…
- Adolphe
Quetelet
(1796 – 1874), a Belgian astronomer, knew that by repeatedly observing
the heavens, statistical laws could be used to predict the movement of
the stars. In the 1830’s, he set out to determine whether the same laws
could be applied to humans.
He proceeded to compile information from various army conscripts, including the weight and height of soldiers. For every height, he found that there was a “bell curve” weight distribution around it. In other words, at each height there was an average weight range that was most common. The lighter or heavier a person was compared to the average weight range, the less common their weight.
He concluded that the top of the bell curve constituted a “normal” weight and that anyone who deviated from it was over- or under-weight.5 - Louis Dublin (1882 – 1969), a statistician at the Metropolitan Life Insurance Company, is largely responsible for the use of body mass index as a health indicator. He was familiar with the concepts that Quetelet discovered, and he wanted to find a better way to determine the risk of his company’s life insurance clients and found that…
- Thinner people lived longer, and
- The closer one’s weight to the average 25 year old, the longer they lived He then determined the weight range at which a person should live the longest.6
-
Doctors and the
government quickly took his lead and began using his findings to
determine who was considered “overweight” and how weight status could
further be used as a predictor of health risks.
For additional research, search for your topic of interest...
Also see...
References
- U.S. Department of Health and Human Services. Obesity in Adults. Available at: http://www.ahrq.gov/research/obesity2.htm. Accessed: August 30, 2009.
- Mary Anne McCaffree, MD, Chair. The clinical utility of
measuring body mass index and waist
circumference in the diagnosis and management of adult overweight and
obesity. June 2008. Available at:
http://www.ama-assn.org/ama1/pub/upload/mm/
38/a08csaphreports.pdf. Accessed: August 15, 2009. - National Heart Lung and Blood Institute - Obesity Education
Initiative. Information for Patients and the Public.
Available at: http://www.nhlbi.nih.gov/health/public/
heart/obesity/lose_wt/risk.htm. Accessed: September 2, 2009. - Centers for Disease Control and Prevention. Defining Childhood Overweight and Obesity. Available at: http://www.cdc.gov/obesity/childhood/defining.html. Accessed: September 2, 2009.
- Stigler, Stephen M. "Adolphe Quetelet." Encyclopedia of Statistical Sciences. New York: John Wiley & Sons. 1986.
- Rosen G, Mattison B. Am J Public Health Nations Health. 1969 July; 59(7): 1083–1085.
- Outcomes of bariatric surgery in patients with BMI less than 35 kg/m2. Patricio Fajnwaks, Alexander Ramirez, Pedro Martinez, Enrique Arias, Samuel Szomstein, Raul Rosenthal. Surgery for Obesity and Related Diseases - May 2008 (Vol. 4, Issue 3, Page 329, DOI: 10.1016/j.soard.2008.03.107)
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