Nutritionist
Kara Vogt

How To Work Out Your BMI And What It Means

Written by
Kara Vogt

BMI, or Body Mass Index, is a number used to indicate whether your weight is in the healthy range for your height. BMI is calculated using your weight and height.

Below 18.5 – you are underweight and possibly malnourished

18.5 – 24.9 – your weight is within the healthy range for your height

25.0 – 29.9 – you are overweight/your weight is above the healthy range for your height

Over 30 – your weight is classed as being in the obese range, and above the healthy range for your height

Calculate your BMI here (BMI calculator widget)

You can also calculate it manually using a calculator and the equation below:

BMI = weight (in kilograms) divided by your height (in metres) squared.

Example: if weight is 74kg and height is 168cm (1.68m)

First square your height, so calculate 1.68 x 1.68. This equals 2.8

Then divide your weight by that number; 74 ÷ 2.8

This equals 26.4, therefore the BMI is 26.4.

Exceptions to BMI

The calculation of BMI does not take into account weight from fat versus weight from muscle. Therefore it can be less accurate in people with higher muscle mass, and categorise the BMI as higher or unhealthy when this may not be the case.

BMI is not a suitable tool for use with children (under 18 years of age) or pregnant women. For people over the age of 65 years, studies have shown a higher BMI to be protective, with the lowest disease risks at a BMI of 24-31.

BMI is widely used in health care because being overweight or obese is strongly linked with several diseases including some cancers, heart disease and type 2 diabetes. Having a BMI that is too low/in the underweight category, is also linked with problems such as malnutrition, nutritional deficiencies,osteoporosis and fertility issues.

What is BMR?

BMR (Basal Metabolic Rate) is the amount of energy (in calories or kilojoules) that your body needs at rest. This is to fuel all bodily functions like keeping your heart beating, breathing, digestion and keeping your blood circulating. It is the amount of energy you need to function, before any exercise you do.

BMR is largely dependent on the amount of muscle mass you have, because muscles use a lot of energy to be maintained. This is why increasing your muscle mass is often said to increase your metabolism, as it increases the amount of energy your body uses. Anything that reduces your muscle mass such as a significant reduction in exercise or rapid weight loss and undereating, will reduce your BMR. This can put your body into a ‘starvation’ type response, making weight loss more difficult.

There are different ways to calculate BMR and several complex equations, created from studies of large groups of people. Your BMR calculation is done for you when your personalised Strictly You plan is formulated. BMR is useful when planning to lose weight, to ensure you get the right balance of calories to achieve healthy weight loss.

Prepared by Kara Vogt –Accredited Practicing Dietitian