Depending on how fine or thick your hair is, as well as how dehydrated your hair is, will determine how much hair serum should be used. As a rule of thumb, start with one pump for fine hair and work your application up from there. Remember it is always easier to add more oil than to take it away. If you are unsure, start with one pump, rub your palms together and then work the product through your ends. If you feel your hair feels near the same as before, double the application.