Body Mass Index (BMI) is used to place a person into one of several categories according to their height and weight. A person can be classified as very severely underweight, severely underweight, underweight, healthy, overweight, moderately obese, severely obese or very severely obese.
Developed in the early 19th century, BMI has been widely adopted as a quick and simple measure of determining a person's ideal weight range.
While it can be a useful tool, it's worth noting that BMI has come under criticism for several reasons:
- It does not cater for the fact that body dimensions tend to be different for taller and shorter people and incorrectly works on the basis that mass is inversely proportional to the square of height. Because of this taller people may find themselves placed into a higher BMI category than would be expected.
- It doesn't take into account the ratio of lean body tissue to fat. As such, those with muscular builds may find themselves placed in the wrong category.
- It can shift the focus from a healthy lifestyle to a focus on weight, which it is argued is a less important - and possibly unhealthy - approach.
- The boundaries are somewhat arbitrary, even differing between countries.
- It's been argued that while BMI is useful for considering trends in populations, it is much less useful for individuals.
Highlighting the problems with BMI as a measure of health, research in 2016 from Copenhagen University published in the Journal of the American Medical Association showed that a BMI of 27 - which puts a person in the "overweight" category - is associated with the lowest risk of death. The researchers found that optimal BMI (i.e. that associated with lowest mortality) has increased by 3.3 since the 1970s, but the reason for this is unclear.
BMI as a measure of ideal weight range tends to prove even more problematic for athletes, who often have different requirements than the general population. It's often noted, for example, that many bodybuilders are classified as overweight or obese according to BMI categories despite having very little body fat.
Given its flaws and limited utility for runners, it's probably best to use BMI as a rough guide - along with other available information - when assessing your healthy running weight.
If you're keen to work out your ideal running weight range, then we recommend the book Racing Weight as a good starting point.
Using the calculator
To use the tool simply enter your height and weight and hit "Calculate". You can enter height in centimetre or feet and inches and you can enter weight in kilograms, pounds, or stones and pounds.
How It Works
The BMI calculation itself is straightforward. Using metric values (kilograms and metres):
BMI = masskg / heightm2
An imperial measures equivalent (using pounds and inches) is available by applying a multiplication factor of 703:
BMI = masslb / heightin2* 703