Using the number of drops per milliliter when figuring how many drops are required to make a particular
% concentration in a carrier oil:
When creating blends, you may come across notations of % concentrations. Such as, "use Tea Tree essential oil at 5% concentration in Hazelnut for..." or "do not exceed 5% concentration total of essential oil..." How do you measure the percent concentration of an essential oil in one or more ounces of a blend in a carrier oil?
One way to measure the % concentration of an essential oil in each ounce of blend with a carrier oil is to use the number of milliliters in each ounce (30), then figure the number of milliliters of essential oil needed, and then how many drops of essential oil that is. The calculation goes like this:
- There are 30 milliliters (ml) in one fluid ounce. (Actually 29.57ml/floz, but 30 really is close enough).
- You can figure the number of milliliters of essential oil required in each ounce by multiplying the % desired concentration by 30.
- For example, 10% is 0.10 x 30 = 3ml
- 1% is 0.01 x 30 = 0.3ml, or about 1/3rd of one milliliter
- 0.5% is 0.005 x 30 = 0.15ml...and so on.
So how do we measure 0.15ml of essential oil? If the oil is High Elevation Lavender, then from the table above we see there are 40 drops per milliliter. So the number of drops in 0.15ml is 0.15 x 40 = 6 drops. This is the number of drops for a 0.5% or 1/2% concentration in one ounce of blend. 1% is, as above, 0.3 x 30 = 12 drops. To be accurate, you would add this to 1 ounce of carrier minus 12 drops, or put the 12 drops in an empty one ounce bottle, then fill the rest with your carrier (this becomes more critical with higher % concentrations -- this is instead of just adding the drops to an already measured one ounce of carrier).
Typically we're working with pipettes that are possibly marked at each 1/2 ml, or eye droppers without any measurement markings at all. Here's where we make use of the number of drops of essential oil per milliliter. It is commonly thought that there are 20 drops per milliliter, and this is true of water when using 'standard' measuring tools -- the 3ml graduated pipettes or glass eye droppers that come with certain essential oil sizes from Ananda.
However, the number of drops per milliliter of a liquid is not only affected by the dropper, but by the properties of the liquid itself. There is a property called "cohesion", which causes the liquid to stick together more or less. Water has high cohesion, therefore the drop sizes are larger and there's fewer drops per milliliter. The high cohesion requires that the drop weigh more before it separates from the rest of the liquid. Each essential oil has a different number of drops per milliliter!
So when creating your own blends and want to use a particular % concentration of essential oils, or when you need to measure an essential oil in milliliters, but do not have a measuring device, you can use the above table and simple calculations to find the number of drops you require.
Again, to find the number of drops of essential oil in each ounce of blend with a carrier, use this formula:
(The desired % concentration) x 30ml/oz x (The number of drops per milliliter from the table above). Ex: 5% of Tea Tree in one ounce is 0.05 x 30 x 41 = 61.5 drops (it's up to you to put in the extra "half" drop or not!)