Hair porosity is generally divided into three categories: low, medium, and high.
Low porosity hair tends to take longer to absorb water and longer to dry, and often times hair products will build up on the surface rather than be absorbed. People with this hair type should usually avoid protein-heavy shampoos and products as they tend to weigh the hair down.
Medium porosity hair absorbs and holds water and hair products generally well. Medium porosity, sometimes referred to as "normal porosity" is typically the basis on which conventional hair products and treatments are made for. Your hair will be generally shiny and easy to style.
High porosity hair absorbs water and hair products very easily and dries just as quickly. Your hair tends to break very easily and can be characterized as dry and frizzy.
To find your hair porosity: fill a glass with room temperature water. Run your fingers through clean hair, collect a few loose hairs, and place them in the glass of water. Leave them for a few minutes.
If hair floats on the surface you have low porosity hair.
If hair remains suspended in the middle you have medium porosity hair.
If hair sinks to the bottom you have high porosity hair.