BMI is a one of the standard measures which helps people determine whether they are overweight. BMI means Body Mass Index and it is measured using a combination of height and then weight measurement.
The BMI chart determines that a person who has a BMI of < 19.5 is underweight, 18.5 to 24.9 a person is determined to be of a normal weight range, 25 to 29.9 means you are overweight and then a BMI of over 30 means you are obese.
BMI is OK for a general measure but in certain circumstances it can be misleading. Take for instance a body builder who is in competition form. They have very little body fat on their body and they have a lot of muscle.
Of course this muscle weighs a lot more than fat and so even though they have little fat on their body, if you take a BMI reading it will show a value which does not truly indicate the state of their health.
Say this body builder man weighs 115kg (of almost pure muscle) and they are 193cm tall. If you plug their height and weight into a BMI tool, they will have a BMI reading of 30.9. Wow, that BMI is totally wrong when describing this person. It indicated that they are obese, but of course we all know that a body builder is far from obese.
They are actually some of the healthiest people with a great amount of muscle, lean mass and low body fat.
In my experience BMI measurements are really only a general indicator of health and can be misleading in certain circumstances. A much better indicator of your health is to actually measure your body fat percentage rather than use BMI. Body fat percentage is much more accurate and is a true measure of the amount of fat on your body.