Knowing your BMI (Body Mass Index) is important for your health and setting your fitness goals. This number is calculated from your height and weight and is used as an indicator for certain diseases related to having high body fat.
But I like to know mine in order to track my progress on lowering this number. As an athlete, I want to know and keep track to see if I am losing muscle (which I don't want to do) or fat or both. Knowing my body's composition, what it is made up of and in what percentages, helps me to track this.
There are several ways to take measurements:
1. Use a skin fold caliper. With this method, you pull your fat with your fingers in several different areas of your body and measure in millimeters with this tong-like apparatus. A book with ages and gender chart is used to decipher what your percentage is.
This method is pretty accurate, able to be repeated and pretty dependable as long as you measure the same way each time and in the exact same areas. If you are over 30 pounds overweight, the fat may not fit in the calipers and so this method may not work for you.
2. Hydrostatic Weighing. The idea behind being weighed underwater is that muscle sinks and fat floats. The more body fat you have, the more bouyancy you will have. You are fist weighed dry and then get into a large tank of water, sit on a special kind of scale, and the measurement taken - this is repeated 3 times and an average taken.
This method costs over $100 a test (maybe more now) is very accurate but expensive and impractical, especially when you can spend $10-15 on skin calipers and get the same results:)
3. Bioelectric Impedance Analysis (BIA). This is a scale-type of device that basically it measures the opposition to an electric current that is ran through the body that calculate an estimate of the amount of water in your body. Muscle has higher water content than fat and this device looks at height, weight and the strength of the opposition to estimate the percentage of fat you have.
The scale costs around $50 and its accuracy is questionable because of your hydration level.
The actual mathematical formula used to calculate BMI is to divide your weight (in pounds) by height in inches (in) squared and multiply by a conversion factor of 703.
The formula to use for calculating in kilograms is weight in kg divided by height in meters squared. (Divide height by 100 if measured in centimeters to get meters measurement).
You can also use the handy dandy free fitness calculators on my site or the one below!
Here is also a video demonstrating how skin fold calipers are used to measure a woman's body fat: