Body Mass Index is a calculation of your weight in relation to your height. When it comes to determining whether you are healthy or not, it is a more accurate gauge than weight alone. A taller person might be heavier than a shorter person because of his larger bones and muscles, and BMI takes this into account. However, BMI is a means to estimate -- while it does compensate for height, it can't compensate for frame size or the amount of muscle you have. It is possible to have a high BMI and low body fat by being extremely muscular or a low BMI and be extremely unhealthy by being light but not adequately muscled.
Establish whether you have a large frame or not by measuring your wrist's diameter. If you are a woman less than 5 feet 2 inches tall, you have a large frame if your wrist diameter is over 5.75 inches. If you are between 5 feet 2 inches tall and 5 feet 5 inches tall, then your wrist needs to be more than 6.5 inches in diameter to be considered large framed. If you are more than 5 feet 5 inches tall, then your wrist needs to be larger around than 6.5 inches. For men more than 5 feet 5 inches tall, wrist diameter needs to be at least 7.5 inches to be considered large framed.
Square your height in inches. For example, if you are 6 feet tall, you should multiply the number 72 by itself. This result would be 5,184.
Divide your weight by the number you arrived at in Step 2. If you weigh 90.7 Kilogram, your result will be 200 divided by 5184, or 0.038580247.
Multiply the result of Step 3 by 703. The result in this case is just more than 27. The result is your BMI.
Compare your frame size to your BMI. For a normal-frame person, below 18.5 is underweight, while 18.5 to 24.9 is normal and more than 25 is considered overweight. So, the person in our example here is officially overweight -- but not by much. Since 27 is only 2 more than 25, it is safe to assume that this person is a healthy weight due to his large frame. However, if his BMI was 40 this would be past the margin allowed for a large frame.
Remember that these are estimates that work on an aggregate basis -- they may not apply to individuals.