How big is a drop of essential oil? And how many drops are there in 1ml? When calculating percentages, this is important to know!
In three empirical investigations dating from 1999, 2000 and 2016, 1, 25 and 23 different essential oils were assessed using 3 – 7 styles of drop dispenser and a variety of different brands. Interestingly, in all three instances, the average weight of 20 drops of essential oil was the same – 0.66g. Using simple math we can deduce that 30 average drops would weigh 0.99g, a close equivalent to 1ml.
Of course, an average is just an average, and the number of drops per ml in these studies varied from about 20 to 40, reminding us that a drop is never a precise measure. However, unless we are working in a clinical environment requiring strict controls, using the average measurement is totally safe.
(When measuring quantities we always have a choice between weight and volume measurement, and for any given formulation, we should use either one or the other. For most of us volume is easier, but if you happen to possess a balance that measures down to 0.01 g, by all means use it.)
The Tisserand Institute dilution chart http://tisserandinstitute.org/essential-oil-dilution-chart/works on the assumption that there are 30 drops per ml and has proved to be a useful tool for home blending.