How to Calculate BMI (Body Mass Index) & What It Means to You

Reviewed by:  

Nancy DeLuca, RD

Last Updated:  

06/15/2017

In addition to showing you how to calculate your BMI, this page will cover why your score is important and the background of the BMI formula.

BMI Calculator

how to calculate bmi

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Enter your height & weight, then click the button:

30+

Weight Loss Surgery
Required BMI

  • Below 18.5Underweight

  • 18.5 – 24.9Healthy Weight

  • 25.0 – 29.9Overweight

  • 30.0 – 34.9Obese

  • 35.0 – 39.9Severely Obese

  • 40.0 – 49.9Morbidly Obese

  • 50 or higherSuper Obese

01BMI Overview
  • How to use the formula
  • BMI classifications (Overweight, Severely Obese, Morbidly Obese, Super Obese)

If your body mass index is above 30, you may qualify for one of the following types of weight loss surgery.

See the following chart for BMI qualification requirements by procedure.

Body Mass Index
Body Mass Index
30 to 34.9
30 to 34.9
35 to 40
35 to 40
40.1 to 45
40.1 to 45
45.1 to 55
45.1 to 55
55 above
55 above
Body Mass Index
Health Problems Required to Qualify
30 to 34.9
May qualify for Gastric Balloon. This BMI range may also qualify for other procedures if the patient has poorly controlled diabetes or metabolic syndrome.
35 to 40
May qualify for Gastric Balloon. This BMI range may also qualify for other procedures if the patient has poorly controlled type 2 diabetes, a higher risk of cardiovascular disease, or suffers from another weight-related health issue.
40.1 to 45
May qualify for any weight loss procedure other than Gastric Balloon.
45.1 to 55
May qualify for any weight loss procedure other than Gastric Balloon or vBloc Therapy.
55 above
May qualify for any weight loss procedure other than Gastric Balloon, vBloc Therapy, or AspireAssist.

These qualifications apply for most countries and most procedures, including the U.S., Canada, Australia, and the U.K.

Assuming your BMI qualifies, your next step is to decide which procedures may be a fit for you.

Or you can continue to learn more about body mass index below.

02Required BMI for Weight Loss Surgery
  • 30+ with health issues
  • 40+ without health issues

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…

How to Calculate BMI Using the BMI Formula

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 (pounds/inches) = (weight / height2) x 703
or
 BMI (kg/m) = weight / height2

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).

And, by the way, we take issue with the concept of “normal” as determined by classifications like the BMI classifications. So while giving you some idea of where you are with your weight, don’t focus on it too much. Everyone has a different body, and there is not a “one-size-fits-all” body type. These weight classifications (i.e. “healthy weight”) is one example of the many discriminatory challenges the overweight face on a daily basis. More on this can be found on our Obesity Discrimination page.

Here are all the weight classifications for adults and what they mean for bariatric surgery…

Classification*
Classification*
Underweight
Underweight
Healthy Weight
Healthy Weight
Overweight
Overweight
Obesity Class I (“Obese”) (maybe eligible for bariatric surgery)
Obesity Class I (“Obese”) (maybe eligible for bariatric surgery)
Obesity Class II (“Severely Obese”) (maybe eligible for bariatric surgery)
Obesity Class II (“Severely Obese”) (maybe eligible for bariatric surgery)
Obesity Class III (“Morbidly Obese”) (maybe eligible for bariatric surgery)
Obesity Class III (“Morbidly Obese”) (maybe eligible for bariatric surgery)
Obesity Class IV (“Super Obese”) (maybe eligible for bariatric surgery)
Obesity Class IV (“Super Obese”) (maybe eligible for bariatric surgery)
Classification
Body Mass Index (BMI)
Underweight
Below 18.5
Healthy Weight
18.5 – 24.9
Overweight
25.0 – 29.9
Obesity Class I (“Obese”) (maybe eligible for bariatric surgery)
30.0 – 34.9
Obesity Class II (“Severely Obese”) (maybe eligible for bariatric surgery)
35.0 – 39.9
Obesity Class III (“Morbidly Obese”) (maybe eligible for bariatric surgery)
40.0 – 49.9
Obesity Class IV (“Super Obese”) (maybe eligible for bariatric surgery)
Over 50.0

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.”

  1. 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.
  2. 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.
  3. 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). For example, women tend to have a slightly higher percentage of body fat and men tend to be slightly heavier relative to their height because of a higher relative muscle mass. Because of this, not having a formula that takes into account gender skews the results, making them less accurate.

Despite its shortcomings, knowing how to calculate your 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 inactivity (3)

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.

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03Why is BMI used?
  • Easy to calculate
  • Calculation correlates with risk levels

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.

04BMI for Kids & Teens
  • Different than adults
  • 'Obese' defined as 95th+ percentile

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
Classification
Healthy Weight
Healthy Weight
Overweight
Overweight
Obesity Class I (“Obese”)
Obesity Class I (“Obese”)
Classification
Body Mass Index (BMI)
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 Teen Obesity & Adolescent Bariatric Surgery page for more information about obesity in young people and the appropriateness of weight loss surgery for teens.

05History of BMI Formula
  • 2 researchers' discoveries

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…

  1. 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).
  2. 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.

06Find a Top Weight Loss Surgeon
  • Ask for a free insurance check or cost quote
  • Attend a free seminar or webinar
  • Schedule a phone or in-person consultation (both often free)

Search the Weight Loss surgeon directory below to find a top surgeon by country and region:

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References for How to Calculate BMI

  1. U.S. Department of Health and Human Services.  Obesity in Adults.  Available at: http://www.ahrq.gov/research/obesity2.htm.  Accessed: August
    30, 2009.
  2. 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.
  3. 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.
  4. Centers for Disease Control and Prevention.  Defining Childhood Overweight and Obesity.  Available at: http://www.cdc.gov/obesity/childhood/defining.php.  Accessed: September 2, 2009.
  5. Stigler, Stephen M. “Adolphe Quetelet.” Encyclopedia of Statistical Sciences. New York: John Wiley & Sons. 1986.
  6. Rosen G, Mattison B. Am J Public Health Nations Health. 1969 July; 59(7): 1083–1085.
  7. 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)
  8. SFR-117 Safety and efficacy of bariatric surgery in class I obese patients. Melissa Gianos, Abraham Abdemur, Ivan Fendrich, Samuel Szomstein, Raul J. Rosenthal. Surgery for obesity and related diseases : official journal of the American Society for Bariatric Surgery 1 May 2011 (volume 7 issue 3 Page 370 DOI: 10.1016/j.soard.2011.04.172)