What is BMI
BMI is an old-school method of deciding whether you are at a healthy weight for your height. It uses your height and weight in a simple formula. Although old-school, it can still be a valuable metric to track over time.
The formula is BMI = kg/m2 where kg is a person's weight in kilograms and m2 is their height in metres squared.
What does your BMI mean?
Underweight = <18.5
This result means that you may be underweight. Being underweight can be associated with a range of health issues. If you're concerned about your weight, we recommend discussing your result with your GP, practice nurse or dietitian.
Normal weight = 18.5–24.9
This result means that you are a healthy body weight which is generally good for your health. Keep up the great work!
Overweight = 25–29.9
This result means that you may be overweight. Carrying extra weight is associated with a range of health concerns, including being at an increased risk of heart disease. If you're concerned about your weight, we recommend discussing your result with your GP, practice nurse or dietitian.
Obesity = BMI of 30 or greater
This result means that you may be obese. Obesity is associated with a range of health concerns, including being at an increased risk of heart disease.
How accurate is BMI?
BMI is accurate. The better question is 'how useful is BMI?'
BMI was initially developed as an easy way to assess large groups/populations over time e.g. are New Zealanders getting bigger or smaller over the past 30 years (the answer is our BMI's are getting larger by the way). BMI has and continues to be a useful tool for comparing populations (e.g. what percentage of New Zealanders are a healthy weight for their height compared to Australians, Americans etc).
One of the benefits of BMI is that you only need your height and weight to work it out, it's quick, and you need little equipment.
BMI was NOT designed to track individual body composition over time. One of the limitations of BMI is that it does not tell you how weight is distributed throughout your body, what percentage of your body is body fat/muscle etc.
BMI is generally less accurate in adults with higher levels of lean body tissue/muscle mass. A good example of this is that All Black Rugby Props are told that they are overweight for their height, when really they are just carrying a lot of muscle (not fat). BMI is unable differentiate muscle from fat.
BMI may also be a less accurate indicator of being overweight in different populations e.g. BMI may not pick up someone being overweight in certain Asian populations.
When BMI was first developed, technology such as Fit3d Body Scanners hadn't been invented however Fit3d Body Scanner are now the recommended way to track your individual body composition
BMI, waist circumference and diabetes risk
Obese men (BMI > 30) with a large waist >102cm were 22 times more likely to develop diabetes than men with a low-normal BMI (18.5-22.4) and a smaller waist (less than 94cm).
Obese women (BMI >30) with a large waist (>88cm) were nearly 32 times as likely to get diabetes than women of low-normal BMI (18.5-22.4) and a smaller waist (less 78cm).
The reference for the study above can be found here
How can I improve my BMI
You cannot change your height however you can change your weight. Read more here on how to improve your weight:
Additional Resources
Food portion sizes for weight loss