20 drops per ml or 15 drops per ml?

i have always been told 20 drops per ml is rule of thumb, i am a certified pharmacy technician, and that is what we go by when dispensing medication

It, of course, depends on factors like type of solution, temperature, viscosity, etc. (All of which are, yes, taught to all students in pharmaceutical classes) The typical "rule of thumb" is 20 drops/ ml for low viscosity liquids, such as water.