How accurate is a BMI calculator?

A BMI chart alone may not be the best measure of weight-related health risks. But, it does provide a good starting point for most people.
Published 2 April, 2019 | Updated 31 January, 2023

What is BMI?

Body Mass Index, or BMI, is a simple measure of weight in relation to height and is used to broadly measure weight-related health risk.

Use our BMI calculator below to find your BMI, which represents the estimated amount of fat in the body.1

What’s your BMI? Find out in seconds

Your results will fall into the following categories: underweight, healthy weight, overweight, obese and morbidly obese. If you are classed as overweight or obese, you may be at higher risk for certain diseases including type 2 diabetes, some cancers and sleep apnea.2

A higher percentage of body fat is proven to be associated with increased risk for developing certain diseases such as heart disease, high blood pressure, type 2 diabetes, breathing problems, certain cancers and a higher risk of early death.3, 4, 5

Underweight is the range of weight that is lower than what is generally considered healthy for a given height, and may indicate underlying medical conditions or an eating disorder and higher risk for conditions such as osteoporosis.

But can we really rely on BMI?

Some research suggests BMI has certain limitations, with Dr. Frank Q. Nuttall, author of a 2015 study published in Nutrition Today saying it is “increasingly clear that BMI is a rather poor indicator of percent of body fat”.6

BMI and fat distribution

Nuttall noted that BMI “does not capture information on the mass of fat in different body sites”. 

This is an important variable when analysing the metabolic and mortality consequences of excessive fat accumulation. Why? Accumulation of fat in the upper part of the body versus the lower part of the body is associated with an increased risk for coronary heart disease and type 2 diabetes.

READ: 3 myths about type 2 diabetes

More specifically, visceral fat - deep abdominal fat that surrounds your organs - accumulation is linked to development of insulin resistance and type 2 diabetes, plus increased risk for coronary heart disease and hypertension.7

For example, one person may carry a lot of fat around their middle but not in their bottom or legs, while another could have the same amount of fat but distributed more evenly around their body. Both may have the same BMI, but the first person may be a higher health risk.  

It’s also possible to carry too much visceral fat but have a BMI in the healthy range.8

BMI and risk for disease

People with a similar BMI can have very different health risks due to genetics, lifestyle and their environment. This is one of the reasons why the use of BMI is debated so much.9, 10, 11

For a full analysis of health risk, a clinical evaluation including BMI, the distribution of body fat, waist/hip ratio, clinical status and family history should be taken into account.

Similarly, history of smoking, alcohol abuse, serious mental disorders, duration of obesity, when obesity became a problem and whether the problem is stable or progressive should also be taken into account.12

BMI and muscle mass

People who carry a lot of muscle but little fat (such as bodybuilders and athletes such as boxers and rugby players) may fall into the ‘obese’ category when calculating their BMI - despite the fact that they may be fit and healthy.

Take two females of the same age, weight and height. One may be very active with a low percentage of body fat and a high percentage of muscle mass, while the other might be sedentary with a high percentage of body fat and a low percentage of muscle mass. It’s possible for both women to have the same BMI, despite having dramatically different body compositions.13

Men tend to have a lower body fat percentage and higher muscle mass than women. And, and since people tend to lose muscle mass as they age (as a function largely of reduced activity levels) , they may start to fall into the “healthy weight” range even if they’re carrying excess fat.14 

This suggests that BMI may not be a ‘one size fits all’ measurement of weight-related health risk.

BMI continues to be the best estimation of weight-related health risk

All this being true, BMI continues to be the best estimation of weight-related health risk at a population level because of its scalability and accessibility. It is a simple calculation based on easily measured values (height and weight) that is relatively easy to understand.15, 16

Other measures of body composition such as bioelectrical impedance, dual energy x-ray absorptiometry, skinfold thickness measurements and underwater weighing tend to be expensive, invasive, difficult to obtain and require highly trained clinicians.17

Tim Cole, professor of medical statistics at University College London Great Ormond Street Institute of Child Health, said BMI was “still extremely relevant”.

And Prof Naveed Sattar, from the University of Glasgow, said: “It works in the vast majority of people, the vast majority of time. If two people are the same height and one has a BMI of 25 and the other a BMI of 40, then excess body fat is the reason.”18

In essence, BMI provides a good starting point for the majority of people. Calculate your BMI, then scroll down for more content to motivate you to get started on your weight loss and wellness journey with WW.

18. BBC (2018).