Calculating BMI and the waist-to-hip ratio

In a nutshell

An overweight individual with a lot of body fat is said to be obese. Obesity can be measured using the BMI calculation or the waist-to-hip ratio.

Body Mass Index

Body Mass Index (BMI) gives an indication of whether someone is overweight, underweight or at a normal weight using their height and body mass. It is calculated using the following formula.

​$$BMI = \dfrac{mass (kg)}{(height (m))^2}$$

The value you calculate can be compared to the table below to find the description of the BMI value.

Example

Calculate the BMI of a person who is $$1.68\ m$$​ tall and has a mass of $$70\ kg$$​. State their weight classification.

$$\begin{aligned}BMI &= \dfrac{mass (kg)}{(height (m))^2}\\\\&=\dfrac{70}{1.68^2}\\\\BMI\ &= \underline{24.8}\end{aligned}$$

​​​​​$$\underline{This\ person's\ BMI\ is\ 24.8\ therefore,\ this\ person\ is\ in\ the\ normal\ weight\ range.}$$​​

BMI will increase if you have a high fat or high sugar diet and don't do enough exercise. This means you consume more energy than you use. This excess energy is stored as fat.

BMI doesn't take into account muscle mass so it is not a reliable measure for everyone, especially athletes. They may have a higher mass because of their muscle, but that does not make them obese.

Waist-to-hip ratio

The waist-to-hip ratio uses the idea that the higher your ratio, the more weight you are likely to be carrying around your middle. It is calculated using the following formula.

​$$waist-to-hip\ ratio = \dfrac{waist\ circumference}{hip\ circumference}$$​​

A ratio above $$1.0$$​ for males and above $$0.85$$​ for females indicates abdominal obesity. This means individuals in this category are at higher risk of developing obesity-related health problems like cardiovascular diseases and type $$2$$​ diabetes.

Example

Calculate the waist-to-hip ratio of a man with a waist measurement of $$40\ cm$$​ and a hip measurement of $$50\ cm$$​. State whether or not this man is obese.

​$$\begin{aligned}waist-to-hip\ ratio &= \dfrac{waist\ circumference}{hip\ circumference}\\\\&=\dfrac{40}{50}\\\\&=\underline{0.8}\end{aligned}$$

$$\underline{ Therefore,\ the\ man\ is\ not\ obese\ as\ his\ waist-to-hip\ ratio\ is\ less\ than\ 1.0.}$$​