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If you commit to lifestyle changes and follow your strict weight loss surgery diet, weight loss surgery may be the tool that you have been hoping for to finally help you lose weight. To figure out if you're eligible for surgery, you'll have to know your body mass index, or BMI.

Defining Obesity: BMI

The body mass index, or BMI, is your weight in kilograms divided by the square of your height in meters.

Weight (kilograms)
[Height (meters)]2

Here's the formula in pounds and inches.

Weight (pounds) x 703
[Height (inches)]2

Experts divide BMI into the following categories:

  • BMI ≤ 18.5: Underweight
  • BMI 18.5 to 24.9: Normal Weight
  • BMI 25 to 29.9: Overweight
  • BMI 30 to 39.9: Obesity
  • BMI ≥ 40: Morbid Obesity
  • BMI ≥ 50: Superobesity
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You may be eligible for bariatric surgery if your BMI is over 40 (morbid obesity) or over 35 with a serious obesity-related health condition, such as uncontrolled type 2 diabetes, sleep apnea or severe joint pain that limits your daily activities.

Measuring Weight Loss: Pounds, EBW and EWL

The way you probably measure your weight and weight loss is in pounds. For example, you may weigh 250 pounds and have a goal of losing 100 pounds to get to a weight of 150 pounds. This is easy to understand, but using pounds makes it difficult to compare your own weight and weight loss to that of someone who has a different height. Excess body weight (EBW) and excess weight loss (EWL) are often used.

EBW is the amount of body weight you have over your "ideal" weight, which is the weight you would be at if your BMI were about 24, in the normal range.

  • If you are 5 feet 7 inches tall, your ideal body weight is about 150 pounds.
  • If you weigh 250 pounds, your excess body weight is 100 pounds (your current 250 pounds minus your ideal 150 pounds)

EWL is the percent of your EBW that you lose. Calculate it by dividing the number of pounds that you lose by the number of pounds in your EBW.

  • Let's use the above example where your EBW is 100 pounds.
  • If you lose 45 pounds, your EWL is 45%.

EBW and EWL are helpful for helping you know how much weight loss to expect. You may, for example, read about someone losing 50 pounds, but it's hard to know whether that's a lot or a little.

  • If the person who lost those 50 pounds is taller than you and has hundreds of pounds more to lose than you, 50 pounds isn't that much weight to lose.
  • However, if the person only had 50 pounds to lose, that person lost 100% EWL—that's great!