Body Mass Index (BMI) is a scale used to measure if you are a healthy weight for your height. Your BMI is found by dividing your weight by your height squared, for example, so if you weigh 60kg and you're 1.65m high then your BMI is:
60 ÷ (1.65 x 1.65) = 22.03.
You can find a BMI calculator here.
A healthy BMI for adult men and women is between 19 and 25; below 19 is considered under-weight and above 25 over weight.
What's a healthy BMI for a cyclist?
The four cyclists below are all in the healthy range – just – but a very powerful, muscular rider like Robert Förstemann is almost in the overweight category, whereas Chris Froome is verging on being underweight. Conor Dunne is very tall and Nairo Quintana short, but both have exactly the same BMI.
BMI is widely used by health professionals and is a useful tool, but it doesn’t tell the whole story. It was designed by a Belgian statistician in 1832 for examining normal growth rates in a large population, but it was not designed to monitor obesity or health in individuals. As a means of looking at large scale trends it is still very useful but if you happen to be very tall, or very short, or (as is the case with many cyclists) very muscular and very lean, then it can be misleading.
BMI doesn’t take into account what your body weight is made up of, how much of it is lean muscle, and how much is fat. Even more importantly, it doesn’t give you any information on where that body fat is stored. For the full picture you need to find out your lean body mass.
Watch: six weight-loss tips for cyclists
Calculating Lean Body Mass
Lean body mass is the amount of weight that you carry which isn’t fat. This is far more important to a cyclist than BMI. Fat is metabolically inert, it contributes nothing to your cycling performance, and if you are dragging extra weight uphill it is just going to slow you down.
A healthy body fat percentage is 14-17% for men and 21-24% for women, but professional athletes have lean body fat percentages as low as 6-13% for men and 14-20% for women. Anything below those lower limits is damaging to health, as a small amount of fat is essential. Someone like Förstemann, as per our earlier example, is at the upper limit of the healthy range for BMI but has a low body fat percentage – which is a good example of why looking at BMI can be misleading.
If you want to improve as a cyclist, but remain healthy, it is your lean body mass you need to know. This can be found at home by using body mass scales such as these from Tanita.
Other methods for finding lean body mass include body fat calipers, which measure the fat in a skin-fold, or the sophisticated method of underwater weighing, which can only be done in a laboratory. For most of us, though, a set of scales will work perfectly for measuring lean body mass.