Hair volume is a matter of the number of hairs per square inch
Here's an easy way to determine the volume of your hair: Pull your hair back into a ponytail. If the base of the ponytail is very slim—about the diameter of a dime— the volume of your hair is thin. Another clue for thin volume: can you see your scalp when your hair is wet? If so, your hair is probably a thin hair volume type.
If your ponytail is "two-fisted"—and looks almost like a true horse's tail—your hair a thick hair volume type. When your hair is wet, it covers your scalp very well, without any of the scalp showing through.
If you fall between these extremes, you have a medium hair volume type—this is the most common hair volume type.
Thin hair volume type
Thin hair volume type has strands that are very, very small in diameter. Thin hair tends to be weightless and may be flyaway. Frequently, thin hair is straight, and won't hold a curl easily. Your hair's volume can be maximized with blunt hair cuts that capitalize on its often silky hair texture. Styling techniques, including combing and drying from the roots are helpful for adding volume. Hair care products that reinforce thin hair—proteinized conditioners, light gels, and mousses—can also boost thin hair's "wimpy" looks. Avoid heavier products, which will weight the hair down. Thin hair has to be permed with extreme care to prevent damage.