There's 29.54 ml in an oz. I use a graduated cylinder and measure my eo's and fo's by mls. I work with chemicals so it seems like a natural way. Also, it's pretty easy because if you are using eo's at .5 oz. ppo, you will get close enough for government work by using 1 ml. per oz. oil. So, 100 oz. oil = 100 ml. eo. easy squeezy.