Determining Muscle Mass (Lean Body Mass)
It is not always the body fat that is of interest. There are several methods to measure the amount of muscle mass or lean body tissue.

One of the easiest ways is to use the two-compartment model for assessing the lean body mass. Girth measurements could be another solution, but the problem with this method is that a girth always involves the fat under the skin as well, therefore if someone gains fat, it will also increase his girth circumference. Although the methods already mentioned at Body Fat Percentage calculation, such as DEXA (Dual-Energy X-Ray Absorptiometry), CT (Computed Tomography), MRI (Magnetic Resonance Imaging), TBK (Total Body Potassium, or Total Body Electrical Conductivity (TOBEC) could be used, these are too expensive and sophisticated methods.

What can you do at home?

Martin et al. (1990) developed a simple equation based on some girth and skinfold measurement for estimating the body muscle mass. If you have skinfold caliper (or at least a caliper) at home, use Calculation 2, if not, use Calculation 1 which will help you to calculate your lean body mass of your body fat percentage.
Calculation 1
Based on the Siri equation and the two-compartment model, your lean body mass is:
Calculation 2
You only need to do six anthropometric measurements to get the result out of this equation:
Your height: cm
Your mid-thigh girth: cm
Your calf girth: cm
Your forearm girth: cm
Your mid-thigh skinfold: cm
Your calf skinfold: cm
According to the formula of Martin et al. your muscle mass is: