While hair width measures the width of individual strands of hair, density refers to how closely those strands are packed together on your head. Your hair's density can also be affected by your hair texture, porosity and width. Knowing your hair density, along with your curl pattern, porosity and width, will help you choose the right products, styles and cuts to maintain the volume of your hair.
The most accurate way to determine hair density is to have someone count the number of strands growing in a 1"x1" area of your scalp. But since the average person has about 2,200 strands of hair per square inch, it can take some time to count them all. Fortunately, there are other ways you can determine your hair density.
Start with dry hair, because hair that's wet will often look thinner than it really is. Let your hair hang loose in its natural, unparted shape and look at it closely from all different angles. If you can see your scalp very easily, then you have low hair density. If you can see some of your scalp, you have medium hair density. If your scalp is difficult or impossible to see, then you have high hair density.
If you have low hair density, use light products that won't weigh your hair down and reduce its volume. Use a mousse to make your hair look fuller, and consider using dry shampoos, volumizing shampoos and volumizing conditioners with thickening agents. Simple, rounded hairstyles will also allow your curls, waves or coils to fall into place and give the appearance of more volume.
If you have medium density hair, you can use a variety of products, styles and cuts to enhance your texture. Try mousse and dry shampoo to play up volume, or use heavier creams and butters to give your hair more weight and hang. Wash'n'gos will allow your existing texture to shine through, while twists and braids will work well to alter your curl pattern and give it more structure.