The Body Mass Index is a useful indication of a person's weight in relation to their height. It does not actually measure body fat and gives only a rough indication in this regard.
To calculate the BMI all that is necessary is to measure weight and height:
Then BMI equals: Weight (in stone) multiplied by 9,840 divided by height (in inches) squared.
Or more simply: Weight (in kg) divided by height (in m) squared.
The US dept of Health defines normal weight as between 18.5 and 25 BMI. Anything below this is consider underweight and anything above is considered overweight.
To read the chart below find your height on the left hand axis. Go across and locate the point where this meets the horizontal axis for weight. Then read your BMI from the blue or red lines that run from left to right. So if you are 5'6'' and your weight is 10 stone your BMI is just under 23.
The green section above shows the desired range for BMI. However the are certain limitations and even flaws with this system. For example it is possible to be outside these indices and be perfectly healthy. This could be because of age, gender, or because the person has a large (or small) frame and be powerfully built, or visa versa.
Take a man of 35 years old and of average height, 5ft 10 inches. He weighs 12 stone 10 lb and is perfectly healthy. He gets a regular check up and his blood pressure is fine, his cholesterol is good and he appears fit and well. But his BMI is 25.5 - outside the 'normal' range. He is considered to be overweight. Yet he is at the 46th percentile, which means he is below average weight for a man his age.
In other words it is not possible to be in the 'normal' BMI range without being below average weight!
Nevertheless the BMI is a useful tool, and the reality is that most people could do with losing a few pounds. For sure if your BMI is within the desired range - 18.5 to 25 - you do not have a weight problem!!
Here's a link to a BMI Calculator provided by the US National Heart, Lung and Blood Institute.
The chart below shows the average weight for a 35 year old American woman. The black line shows this against height.
As can be seen from above diagram most women are above the recommended body mass index range, particularly at the lower average height range.