Body mass index, or BMI, is a tool doctors use to determine if a person is overweight, and to what degree. It is a calculation based on height and weight that can provide a better estimate of total body fat than weight alone.
For most adult women and men, a normal body mass index is between 18.5 to under 24.9.
Body mass index is an expression of the relationship between a person’s height and weight as a single number that is not dependent on frame size. BMI is calculated from the weight and square of the height: BMI = body weight (in kg) ÷ height (in meters) squared.
|BMI less than 18.5||Below normal weight|
|BMI 18.5 to under 24.9||
|BMI 25 to under 29.9||
|BMI 30 to under 34.9||Class I Obesity|
|BMI 35 to under 39.9||
Class II Obesity
|BMI 40 or greater||Class III Obesity|
|Pre-Pregnancy BMI||Recommended Pregnancy Weight Gain|
|Underweight (below 18.5)||28 to 40 pounds|
|Normal weight (18.5 to 24.9)||25 to 35 pounds|
|Overweight (25.0 to 29.9)||
15 to 25 pounds
|Obese (30.0 or greater)||11 to 20 pounds|
Are There Drawbacks of Using Body Mass Index?
Body mass index (BMI) is simply one measure healthcare professionals can use to assess whether a person is a healthy weight, but other factors must also be taken into account.
- Body mass index should not be used as a measurement for pregnant women.
- Women who are muscular, such as bodybuilders, boxers, and other professional athletes, may have their body fat overestimated using BMI because muscle is denser than fat. This means an athletic person may be categorized as obese based solely on body mass index, but they are actually a healthy weight.
- On the other hand, older women who have a loss of muscle mass due to aging may have their body fat percentage underestimated.
- In addition, people with physical disabilities who are unable to walk may have muscle wasting. This may result in a lower BMI, but the person may not be underweight.
- BMI often overestimates obesity in shorter people (under 4’1”) and underestimates it among taller people (over 6’2”). It is far less useful for those who are very short or very tall.
- Finally, BMI may not be as accurate in providing a full picture of body weight in people who are of any ethnicity other than white. Asian, Indian, and Mexican women tend to have more body fat at a given BMI than people of European descent, while Black women tend to have more muscle mass than white women or those of Mexican descent. Even with the same BMI, the risk of obesity-related illness may be different depending on a person’s ethnic background.
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