Body Mass Index is a calculation that is used to determine the degree to which someone is overweight or obese (or underweight) in a way that relates to overall health risk. This number takes into account different body shapes in relation to overall body mass (‘unhealthy’ or fat tissue vs. ‘healthy’ or lean tissue). Now it is not perfect. You will hear that for people who are very lean and muscular – it suggests they are overweight or obese when in fact they are not. That is true. But for most people, it’s a good estimate of whether someone's body weight is healthy or whether it puts them at risk for weight-related health problems. Another important thing to consider in tandem with BMI that helps improve the information you get is waist size. For men a waist that measures above 40 inches and for women 35 inches suggests a substantially higher risk of weight-related health issues.
BMI is accepted by most health organizations (National Institutes of Health and the World Health Organization) as a good initial method for categorizing and defining the health risks associated with overweight and obesity. Why? Well its based on large scientific studies showing that people in these higher BMI categories tend to suffer more from a range of health conditions and are more likely to experience premature death. Other factors, such as waist circumference, weight gain since young adulthood, fitness level, and ethnic or racial background, also influence the relationship between BMI and overall disease risk.
BMI is the ratio of your body weight to your height. The number you get when you calculate BMI is NOT percent body fat. Often people confuse these since we often talk about how BMI “corresponds fairly well to the degree of excess fatty tissue” — what we mean is that people with higher BMI tend to have a higher amount of body fat overall. It is excess body fat that leads to the health issues we have outlined in the table below.
We have provided on this site a handy BMI calculator. Just enter your height and weight then take the number you get and match it to the table below.
What It Means
|Less than 18.5||Underweight||Higher risk of malnourishment-related issues|
|18.5–24.9||Normal||Lowest risk for weight contributing to illness|
|25–29.9||Overweight||Sub-optimal weight – increased risk of weight-related health issues.|
|30–34.9||Obesity I||Heightened risk of medical problems commonly linked to obesity such as type 2 diabetes, high blood pressure, abnormal cholesterol, shortness of breath or joint pain (30–40 pounds overweight).|
|35–39.9||Obesity II||Very high risk of common medical problems associated with obesity, such as type 2 diabetes, high blood pressure, abnormal cholesterol, shortness of breath or joint pain|
|40 or greater||Obesity III (Severe obesity)||Extremely high risk of associated medical problems and greater severity of illness. (~100 lbs overweight)|