The 'classic' method of determining your hair density is the simplest of the measurements, as all it requires is hair long enough to put in a ponytail, and a measuring tape. You simply have to measure the circumference of the ponytail. If you have bangs and/or you can't get all of your hair in there adjust according to how much of your hair you have measured.
To measure the circumference - take a piece of string, or a soft tape measure (the kind used by dressmakers). Wrap the string or tape measure around your ponytail - just below the elastic you have in to hold the ponytail together. Read the corresponding number from the tape measure, or hold the string at the length you've measured and use a ruler to determine how much it is. The number given is the circumference of your ponytail.
i - thin (less than 2 inches/5 centimeters)
ii - normal (between 2-4 inches or 5-10 centimeters)
iii - thick (more than 4 inches/10 centimeters)