How much tea tree oil per 100ml of shampoo?

Tea tree oil is a potent essential oil that should be used in low concentrations. For a 100ml bottle of shampoo, you should typically add no more than 5 drops of tea tree oil. This will give you a 0.5% concentration of tea tree oil, which is generally safe for use. If you have sensitive skin or scalp, you may want to start with a lower concentration, such as 2-3 drops of tea tree oil per 100ml of shampoo.