**Five Ways Your Dilution Ratios May Be Wrong**

- Assuming 1 oz = 30 mL
- Using the Wrong Divisor
- Variable Dropper Size – Orifice Size
- Variable “Drop” Size
- Mixing Up 20 & 30 “drop” charts

*Bonus: **Not Calculating Individual Oils in the Blend*

**Part I – Calculation Errors**

These first two items are based solely on mathematical principles misapplied when calculating essential oil blend ratios. The first typically causes your actual ratio to be less than you have wrongly calculated (think reduced potency); but on rare occasion will increase the ratio, as explained. The second will always cause your actual ratio to be less than your wrongly calculated ratio.

**Assuming 1 oz = 30 mL**

Time and again, essential oil literature, books and websites will portray that ** 1 fluid ounce = 30 milliliters**. This is simply not true.

According to the NIST^{(1)}, the governing body in the U.S. for weights and measures, ** 1 fluid ounce = 29.57353 milliliters**. This error only applies with calculations that use a mixture of metric and U.S. (customary) measurements, such as ounce, teaspoon, tablespoon, cup, etc. This also may apply when “drop” charts are used – which will be described in #3, below.

In fairness to much of the literature and websites, there is often a disclosure of this difference, but excuses the variance in some form of expression, such as “close enough for ease of remembrance purposes”.

What is the impact of this assumption? If your ratio uses essential oils in milliliters or “drops” and carrier oil in ounces, teaspoons, tablespoons or cups, then your dilution percentage will be LESS than you calculated – this is the most common. But, if your ratio is based on your essential oil being in ounces, teaspoons, tablespoons or cups and your carrier oil is in milliliters, then your ratio will be MORE than you calculated.

Here is a chart showing the typical calculation of essential oil in milliliters or drops and carrier oil in ounces, teaspoons, tablespoons or cups:

Published Actual

1.00% 0.99%

2.00% 1.97%

3.00% 2.96%

4.00% 3.94%

5.00% 4.93%

6.00% 5.91%

7.00% 6.90%

8.00% 7.89%

9.00% 8.87%

10.00% 9.86%

How to overcome this error: Use ** 1 fluid ounce = 29.57353 milliliters** for calculations.

** **

**Using the Wrong Divisor**

The second common error in essential oil “recipes, blends and drop charts” is a basic math error. Often a blend is presented as a ratio of the essential oil (*EO*) divided by the carrier oil (*CO*). i.e. “divide your 1mL of *EO* by your 10mL of *CO* to get your 10% blend” – a common ratio used for roller ball blends.

On paper this would appear as

When, the actual calculation is

This chart follows the error in the calculations of 1% – 10% blends:

Published Actual

1.00% 0.99%

2.00% 1.96%

3.00% 2.91%

4.00% 3.85%

5.00% 4.76%

6.00% 5.66%

7.00% 6.54%

8.00% 7.41%

9.00% 8.26%

10.00% 9.09%

How to overcome this error: Use ** the Total of both Essential Oils and Carrier Oils as the Divisor** in calculations.

**Cumulative Effect of #1 & #2**

Errors #1 & #2 regularly coexist in the same blend ratio presentations; therefore, this chart tracks the cumulative deviation of them together:

Published Actual

1.00% 0.98%

2.00% 1.93%

3.00% 2.87%

4.00% 3.79%

5.00% 4.70%

6.00% 5.58%

7.00% 6.46%

8.00% 7.31%

9.00% 8.15%

10.00% 8.97%

**Part II – The Problem with Drops**

Understand that there is no regulated, standardized definition of what a “*drop*” is, as a quantity of measurement. You will not find it in NIST’s *Handbook 44*^{(1)}, or that of any other governing authority. The reason is the forces that creates a drop of liquid and its size from a pippet, eyedropper, dropper-insert, or euro-dropper are dependent upon two primary factors – 1) the orifice size of the dropper, and 2) the cohesion/surface tension of the essential oil.

In a 2001 article in the *International Journal of Aromatherapy*^{(2)}, K.P Svoboda and her collaborators began with the opening summary:

Generally, aromatherapists dispense their essential oils using the dropper-insert, adhering to the theory that 20 drops is equivalent to 1 ml. This article determines that this is very seldom, if ever, the case.

In Robert Tisserand’s *Essential Oil Safety*, ^{(3)} there is a footnote to Table 4.6 *Calculating essential oil concentrations*, citing Svoboda, he states in part (referring to how table 4.6) *“These figures are averages, as the number of drops per ml can vary from 20 to 40, according to the type of dropper used.”* This range is borne by the table in Svoboda’s article showing measured milliliters per 20 drops, in a laboratory test using seven different supplier’s dropper inserts and 12 different essential oils, along with distilled water as a control test. The full range listed here is accounted for by both the orifice size of the dropper AND the variety of the different oils.

* *

**Variable Dropper Size – Orifice Size**

The orifice, or opening, in the dropper has a great impact on the volume of a drop of liquid. The larger the opening, the bigger the drop and therefore more volume.

The third test of the Svoboda article was to “*assess the volume of twenty drops of various oils using a range of seven dropper-types*”. The oils used were Basil, Benzoin Resin, Cardamom, Frankincense, Laurel Leaf, Lavender, Rose (Harrop), Rosemary, Sweet Orange, Thyme, Vetiver; as well as distilled water for a control. Seven different droppers, all processing 11 different oils.

Related to Dropper Size, the control test of distilled water, the seven droppers ranged between 14.49 – 22.73 drops per milliliter.

**That is a 22% variance.**

The variances, within the tests for individual oils, between the seven droppers ranged from 9% – 40%.

How to overcome this error: ** Use Only Standardized Measurements and Equipment** for calculations – milliliters, ounces, teaspoons, tablespoons, cups, etc. Avoid using “drops”.

**Variable Drop Size**

The other outcome of the Svoboda tests is to recognize that different essential oils have different characteristics that impact drop size. These are cohesion, surface tension, viscosity, and others. While a deeper study of chemistry is necessary to understand these characteristics, the net effect is evident in that with the exact same size orifice, different oils will produce a differing amount of volume (as measured in milliliters, etc) per the same number of drops. Literally, the “drops” are different sizes.

In the Svoboda tests, 20 drops of essential oils measured from 0.52mL (Cardamom) to 0.77mL (Basil), demonstrating that a drop of Basil is more volume of oil than a drop of Cardamom. Notice that NONE of the sample oils approached the 20 drops per milliliter “standard”. Of the 77 tests (11 oils x 7 droppers), only 6 reached the level of 20 drops per milliliter.

Another example of acknowledging differing drop sizes per milliliter can be found at the website for The Ananda Apothecary^{(4)}. While not knowing specifics about their dropper used, the 123 oils, range from 25 drops/ml for Rosemary to 52 drops/ml for Fir Needle.

**This is a 48% – 208% variance.**

How to overcome this error: ** Use Only Standardized Measurements and Equipment** for calculations – milliliters, ounces, teaspoons, tablespoons, cups, etc. Avoid using “drops”.

**Mixing Up 20- & 30- “drop” charts**

After having read this far, and gaining an understanding of why using drops in calculations is not recommended, you may still choose to use drops charts, as in the illustration. Just one simple word of warning. Approximately 2/3 of drop charts in books and on the internet, are provided in 20 drops = 1 milliliter. The other 1/3 are 30 drops = 1 milliliter.

Be certain you are using the correct chart for your “drop” size.

As you can see from the illustration, if you look at a 30-drop dilution chart and then use a dropper providing 20 drops, you will have a result with a 50% increase – your expected 2% blend will be a 3%. Not so bad, but what about your 10% blend becoming a 15% blend.

And the inverse is true. If you use a 20-drop chart dilution chart and then use a dropper providing 30 drops, you will have a result with a -33% decrease – your expected 3% blend will be a 2%. That 10% example actually computes to 6.67%. The blend is -33% less effective.

**Bonus: Not Calculating Individual Oils in the Blend**

In addition to recommended safety levels for the dilution percentage of your overall blend, certain oils are recommended to be used at even lower dilution rates. For example, Cinnamon Bark, is generally recommended by Tisserand in his book^{(3)} and in a survey^{(5)} of several essential oil suppliers to have a max dilution of 0.07%.

Using this information, we see that a straight blend of Cinnamon Bark and a Carrier Oil would look something like this:

But, how do you compute when the blend looks like this?

1 drop Cinnamon Bark

6 drops “another oil”

6 drops “another oil”

6 drops “another oil”

9 drops “another oil”

9 drops “another oil”

2 ounces Carrier Oil

This gives a nice tidy ~3% blend, overall. But, the Cinnamon Bark component is 0.081% – over the recommended 0.07% maximum.

1 drop Cinnamon Bark 0.081%

6 drops “another oil” 0.485%

6 drops “another oil” 0.485%

6 drops “another oil” 0.485%

9 drops “another oil” 0.728%

9 drops “another oil” __0.728%
__Total Essential Oil 2.991%

2 ounces Carrier Oil 97.009%

Unless you take the time to compute the individual oils with lower dilution recommendations, you have made a blend with too high a dilution rate.

A simple adjustment will give a safer dilution, when we substitute milliliters.

2 ounces carrier oil = 59.1471 mL; now increase that to 70 mL

Here are the results:

1 drop Cinnamon Bark 0.070%

6 drops “another oil” 0.418%

6 drops “another oil” 0.418%

6 drops “another oil” 0.418%

9 drops “another oil” 0.626%

9 drops “another oil” __0.__ 626__%
__Total Essential Oil 2.575%

70 milliliters Carrier Oil 97.425%

In this example, the Cinnamon Bark is lowered to the safe level; the essential oils remain in the same proportions; and, the overall blend dilution is only lowered from ~3.0% to ~2.5%.

**In Summary**

There are a few simple steps to correct all these issues, and get precise calculations and reproducible results.

- Do not use
*drops*as a measurement, or the corresponding drop charts.- If the blend information/recipe you have available is listed in drops
- Confirm which equivalent of drops per milliliter the author uses.
- If based on 20 drops = 1mL; substitute 0.050mL for each drop
- If based on 30 drops = 1mL; substitute 0.033mL for each drop

- Confirm which equivalent of drops per milliliter the author uses.

- If the blend information/recipe you have available is listed in drops

- If the recipe is in mixed metric (mL) and customary (oz, tsp, tbsp, cup, etc.)
- Convert at a ratio of 1 oz = 29.57353 mL

- Use precision measuring utensils
- Ensure your ratio calculation is based on the correct divisor
- Double check the dilution of individual oils in the recipe for safe usage levels
- As these calculations are precise and difficult to compute by hand, use a tool
- A spreadsheet that you create yourself to do the calculations precisely
- Or, the online
*Essential Oil Calculator*and*MasterBlend Calculator*available at DiluteOil.com

Footnotes:

(1) NIST Handbook 44 – 2017, *Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices
*(2) K.P Svoboda,

(3) Robert Tisserand,

(4) The Ananda Apothecary

(5) Robert Tisserand,