Calculate Your Body Mass Index

Finding out your body mass index (BMI) is a quick way to figure out if your weight is healthy for your height. Your BMI value is more useful for predicting your health risks than your weight alone, but it's even more useful to treat it as one of many factors that influences your health, including total body fat, waist circumference and waist-to-hip ratio.

How is BMI calculated?

Body mass index is calculated by this formula, developed by the Belgian statistician Adolphe Quetelet in the nineteenth century:

Weight in kilograms = BMI
(Height in meters)²

Since Americans measure weight and height in pounds and feet instead of kilograms and meters, the converted formula looks like this:

Weight in pounds x 704.5 = BMI
(Height in inches) ²

Quetelet decided that if the result of the calculation was greater than 30, it signaled obesity. This is still a good rule of thumb, but over the years nutritionists have developed more refined ways to interpret BMI values. For example, different BMI values can mean you are underweight, ideal weight, slightly overweight or obese, and these BMI ranges are slightly different for men and women.

(Source: Understanding Nutrition by Whitney and Rolfes)

In June, 1998, the National Heart, Lung, and Blood Institute and the National Institute of Diabetes and Digestive and Kidney Diseases issued controversial new guidelines for clinicians to identify overweight people. These guidelines use the same ranges of numbers for men and women, and place many more people in the "overweight" category than previous estimates.

(Source: NIH Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, June 1998)

Quick Reference Table

Using BMI to predict overweight still has its limitations, though. It doesn't take frame size into account, so people with stockier builds may be considered overweight even if they don't have a lot of body fat.

Tests that physically measure body fat and distribution are better than BMI at telling if you are overweight. A skinfold measure, for example, uses a instrument called calipers to measure the thickness of the fat layer on your arm or stomach. A bio electrical impedance test how easily electricity travels through your body (fatty tissue slows down the current) to estimate what percentage of the body is fat. These tests involve a physical measurement by a trained technician at a gym or doctor's office.

Also, BMI is not a good predictor of overweight for these groups of people: