Body Mass Index

How to Calculate Your BMI (Body Mass Index) & What the Results Mean

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.

Click Here to See Your BMI

Click on any of the topics below to jump directly to that section

  1. BMI Overview
  2. Weight Loss Surgery
  3. Why Is BMI Used?
  4. Kids & Teens
  5. Find a Weight Loss Surgeon
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BMI Overview


  • How to use the BMI formula
  • The BMI classifications are Overweight, Severely Obese, Morbidly Obese, and Super Obese

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
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.

Tap here to see the weight classifications for adults and what they mean for bariatric surgery

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Classification*Body Mass Index (BMI)
UnderweightBelow 18.5
Healthy Weight18.5 – 24.9
Overweight25.0 – 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
*Applies to adults over the age of 19

In addition to the questionable names applied to the above classes, the BMI rating system has other limitations…

  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.


Required BMI for Weight Loss Surgery


  • You must have a BMI of 30+ and weight-related health issues; or
  • A BMI of 40+ (health issues not required) to qualify for weight loss surgery

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

Tap here to expand a chart explaining BMI qualifications for weight loss surgery.

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Body Mass IndexHealth Problems Required to Qualify
30 to 34.9May 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 40May 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 45May qualify for any weight loss procedure other than Gastric Balloon.
45.1 to 55May qualify for any weight loss procedure other than Gastric Balloon.
Above 55May qualify for any weight loss procedure other than Gastric Balloon 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.

Bariatric Surgery Source

Why is BMI Used?


  • BMI is easy to calculate
  • The BMI 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.


BMI for Kids & Teens


  • BMI for kids and teens is different than for adults
  • The classification for 'Obese' is defined as 95th+ percentile

Children and teenagers up to 19 years of age use a different body mass index labeling system than adults (4). In this system, percentiles are used that take into account the child’s age.

Tap here to to learn BMI classifications for children and teenagers up to 19 years old

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ClassificationBody Mass Index (BMI)
Healthy WeightBelow the 85th percentile
Overweight85th 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.


Find a Weight Loss Surgeon


  • You can ask a local bariatric practice for a free insurance check or cost quote
  • You can attend a free in-person seminar or an online webinar offered by a local weight loss surgeon
  • You should schedule a phone or in-person consultation (both often free), if you are interested in learning more about weight loss surgery

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

Bariatric Surgery Source

References for How to Calculate BMI

  1. U.S. Department of Health and Human Services.  Obesity in Adults.  Available at:  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:
    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:
    heart/obesity/lose_wt/risk.htm.  Accessed: September 2, 2009.
  4. Centers for Disease Control and Prevention.  Defining Childhood Overweight and Obesity.  Available at:  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)
* Disclaimers: Content: The information contained in this website is provided for general information purposes and your specific results may vary depending on a variety of circumstances. It is not intended as nor should be relied upon as medical advice. Rather, it is designed to support, not replace, the relationship that exists between a patient/site visitor and his/her existing physician(s). Before you use any of the information provided in the site, you should seek the advice of a qualified medical, dietary, fitness or other appropriate professional. Advertising: Bariatric Surgery Source, LLC has entered into referral and advertising arrangements with certain medical practices, original equipment manufacturers, and financial companies under which we receive compensation (in the form of flat fees per qualifying action) when you click on links to our partners and/or submit information. This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Read More

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