For those who are of average height and inactive, BMI offers a reasonable indication. However, people’s body types vary a lot. For many, BMI does not accurately represent their body fat percentage. For example, tall people are often more slender in proportion to their height than shorter people. Therefore, their BMI is higher than their actual body fat percentage would warrant. Based on their BMI, they may be seen as overweight, even though they are not. On the other end of the spectrum, shorter people often have a lower BMI than they should have, based on their body fat percentage.
The Body Mass Index also does not take muscle into account. Muscle is heavier than fat. Therefore, a bodybuilder can be classified as obese, even though he has a very low fat percentage. This goes both ways. Someone who is slender and has very little muscle might think, based on BMI, that he is not overweight. However, they can still have an unhealthy amount of body fat.
Imagine you decide to start going to the gym. You might be losing fat, but not see any change to your BMI. What your BMI is not telling you, is that you are gaining muscle at the same time. Getting the impression that you are not improving can be very demotivating.
Finally, BMI does not differentiate between types of body fat. Visceral fat is more of a health issue than the fat under your skin. When assessing your health, it is valuable to be aware what percentage of your fat is visceral.