A standard diffuser can hold up to 100 ml of water, to which it is recommended to add 3 to 4 drops of oil. However, this depends on the size of your room and your personal preference. If you're a first-time user, I recommend you start small and increase the amount of oil gradually (I learned this the hard way - post-trial, error & almost giving myself a migraine). Let's not forget, when we use essential oils, that they are highly concentrated, and only a few drops are enough to establish a calming & fresh atmosphere in the room. Remember, less is more!