How to Calculate BMI Z Score

Overview

BMI, or body mass index, was created to quickly generate a chart of overweight and obese populations. Though it can be problematic when observing certain populations (e.g. bodybuilders and athletes), it is quite accurate when looking at the general population. BMI is calculated using height and weight measurements, and it applies to both men and women between the ages of 18 and 65. A Z score (the number that comes up at the end of the calculation) of below 18.5 is underweight; a Z score between 18.5 and 24.9 is normal; between 25 and 29.9 is overweight; between 30 and 34.9 is considered obese class 1; and between 35 and 39.9 is considered obese class 2. Anything over 40 is considered morbidly obese.

Step 1

Convert your height into inches. If you are 5'9" tall, your height in inches would be 69 inches. Square this number (multiply it by itself). In this case, the result would be 4,761.

Step 2

Divide your weight in pounds by your height in inches squared. If you're 5'9" and weigh 190 pounds, your calculation would look like this: 190/4,761 = .04

Step 3

Multiply this number by 703 to obtain the Z score. In the example used above, the Z score would be 28.06. Locate your place on the chart to determine your standings. The person in this example would be considered overweight.