You can get a one-click free conversion of drops into milliliters using
www.mycalcu.com. But in the case of an elaborated version of the conversions here is a brief context.
It's accurate to say that one fluid ounce equals 29.57 milliliters. When dealing with recipes that call for a total amount of up to four ounces, we usually round this figure down to 30 mL. (beyond this scale, those missing 0.43 mL start to add up and impact your results). Measure using a sensible unit, such as drops when necessary or a teaspoon when proper.
It takes around 20 drops to equal 1 milliliter. These numbers are only approximations, however. Each drop of essential oil has a different volume because of its unique viscosity.