Why You Should Never Measure Essential Oils in Drops

Updated: 20.02.24

Formula Botanica teaches its students how to work safely with organic cosmetic ingredients when formulating new skincare and haircare products. Many of our students start out following fun DIY recipes that they’ve found on websites, blogs or social media. Many of these DIY formulations tell you to measure essential oils in drops, but in our courses, we instruct you to measure all your formulation ingredients strictly by weight.

We love a good DIY recipe as well, but it’s important to remember that as soon as you start to learn how to formulate professional organic cosmetics, you must start to follow professional formulation techniques. In other words, you must always measure essential oils by weight and not in drops.

In this blog post, we are going to explain why this is necessary and address some of the key FAQs we’ve come across about weights and measures in cosmetic formulation.

Why you should never measure Essential Oils in Drops

Let’s run through our top 5 reasons why you should never measure essential oils in drops.

1. You cannot get accuracy with drops

The important point to note here is that the weight of each drop will vary depending on what oil you use, where it’s from, how it’s decanted, its viscosity, the mechanism and size of the dropper nozzle, human error, the amount of air in the bottle, temperature, and more. So, we need to weigh essential oils, and not rely on drops at all in professional formulating.

Tip on weighing small amounts: If you have very small amounts of essential oil and are using jewelry scales rather than precision lab scales, we advise making a scaled up, proportionate blend of the essential oils in your formulation (5g or 10g for example). Then, add the oil blend rather than weigh out tiny amounts of each individual oil.

2. Different essential oils have a different density

A drop will be a different size and weight, depending on the oil you use. Some oils will yield large drops, others will be thin and runny and pour out of the bottle in tiny droplets which don’t weigh much. So your formulation could call for 20 drops and for one oil that might be 0.3 grams, whereas for another that might be 0.9 grams. It’s simply not an accurate measurement tool. Never assume that X number of drops will be the same weight for different oils.

While we’re on the topic of density, it’s worth talking about why you can’t use volume measurements in formulations, and why you shouldn’t mix volume and weight. Volume measures the space a material takes up. Clearly, liquids expand and contract with fluctuations in temperature. Weight is not affected in this way. Plastic pipettes with volume (ml) graduations on the side are suited to transferring oils from one vessel to another, but cannot be used to measure essential oils when formulating.

3. Skin sensitisation risks

Human error is always possible when you’re counting drops. So, if you make a mistake in your counting and accidentally add too much of a certain essential oil, you run the risk of causing skin sensitisation. Many essential oils, or the chemical compounds within them, have dermal limits (we teach this in our Diploma in Organic Skincare Formulation) and these limits need to be respected in your formulation. If you make a mistake with the amount of essential oil in a formulation, your customer might have a skin reaction to your product. It is almost impossible to calculate allergens if you measure essential oils in drops.

4. You cannot replicate your exact formulation

So now you’ve created your perfect skincare formulation and you want to replicate it again and again for your customers. How will you do that when your essential oil measurement is not exact? As you’ll see from our  experiment below, even the same number of drops from the same bottle can provide a different weight measurement if you are not using high-precision lab scales.

5. You cannot scale up production

If you want to scale up your production to start manufacturing larger batch sizes, then you obviously cannot measure essential oils in drops. Imagine that your first batch is 500 grams, but within a year your orders have increased so much that you’re now working in batches of 5 kilograms. You clearly can’t just multiple the essential oil drops by a factor of 10 as this is highly inaccurate. Clearly, you also can’t count the tens of drops needed to manufacture kilos of a cosmetic product. Human error will come into play. You need to have an exact weight and percentage of essential oils in your formulation and then you can scale up production accordingly.

Our Essential Oil Drop Experiment

In our experiment, we weighed 20 drops of some common essential oils. This was our methodology:

• We chose 23 oils which are frequently used in skincare.
• We duplicated two of these oils (helichrysum and yarrow) by using bottles from different brands to test the difference between suppliers and averaged the results recorded for each brand.
• We set up a beaker with some small jeweller’s scales and counted 20 drops of each oil into the beaker.
• After measuring the first round of 20 droplets, we used the exact same bottle from the exact same supplier to measure another 20 drops of the same oil.
• We then compared the weight between oils as well as the weight between the two measurements of the same oil.

Note: It is almost impossible to gain total accuracy in the weight of essential oil drops if you are not using high-precision laboratory scales that weigh to 1/1000th of a gram. Most home formulators when starting out will be purchasing less expensive scales readily available from online stores. These can weigh 1/100th of a gram (0.01), but are far more likely to produce the varying results in the weight of drops that our experiment below shows.

What can we learn from this information?

Let’s examine these results together in detail:

• The average weight of 20 drops in our experiment is about 0.6 – 0.65 grams. This does not mean that you should now assume that 20 drops = 0.6 grams, as obviously this is highly inaccurate.
• Only 20 drops of one oil (yarrow) measured exactly 1 gram. But, as you can see, the second time we weighed 20 drops from the same bottle, it weighed 0.55 grams.
• Only two oils (ginger and palmarosa) weighed exactly the same for both measurements. This is possible even with jeweller’s scales, but could be a random coincidence.  It does not tell us anything about these two oils.

What else can we learn from these results?

Let’s examine the results together:

1. 20 drops of essential oil does not equal 1 gram.

Or 1 millilitre. In fact, you’ll see that 20 drops of essential oil sometimes weighs as little as 0.30 grams or as much as 1.05 grams. There is no rule of thumb here. Even if you do your own experiment, you can not rely on your results to provide you with a quick, essential oil calculator converting drops to weight that you can use in the future in place of  accurate weighing.

2. You probably won’t get the same result twice.

Drops are so inaccurate in terms of weight that you can weigh the same number of drops from the same bottle of essential oil several times and you are likely to get a different result unless using lab-precision scales. We challenge you to try this for yourself at home too.

3.  Measurements will also vary between suppliers.

Essential oils will vary by batch, by supplier, by geographical region, by plant origin, by growing conditions, and so on. In other words, you can buy three different batches of essential oil from three different suppliers and it’s likely that 20 drops of all three essential oils will weigh different to each other. Don’t forget, suppliers use different bottle and dropper sizes.

Why are all these measurements so different?

Here are the likely theories:

Human error

The opportunity for human error is enormous. You can easily get distracted by something happening around you and you can lose count. You don’t have equipment measuring the drops for you, so you need to concentrate and keep focused on those minute drops which in the case of transparent oils, are often difficult to see.

Furthermore, the viscosity of the essential oil can cause error in counting too. If the liquid has a low viscosity and drops out of the bottle quickly then you might not be able to keep up. If the liquid has a high viscosity and takes forever to drop out of the bottle, then you might become distracted and lose count.

The dropper mechanism

Having tested out many different types and brands of essential oils over the years, we have seen many different dropper mechanisms on the caps. Some of them are great. Some of them are awful. It varies from supplier to supplier.

A few of the essential oil bottles we tested in our experiment created air bubbles when we first tipped them up to drop out the liquid. The air bubble would then take along more of the liquid than normal, effectively causing 2-3 times more essential oil to drop out of the bottle. So what looks like 1 drop, may actually deceptively be several ‘drops’. Finally, the size of the droplet depends on the size of the dropper nozzle on your bottle. So you might have a large droplet if your nozzle is wide, which would then influence the weight of the drop itself.

Quality of your scales

We mentioned this earlier, but it’s an important point. We used a small set of jeweller’s scales for this experiment. It measures down to 0.05 gram, so should be able to handle this type of experiment. Nonetheless, we weren’t using expensive precision equipment, so there were undoubtedly discrepancies between the measurement readings and the actual weight of the essential oil.  When measuring very small quantities it’s not surprising that occasionally there will be a slight discrepancy. This is a good reason to invest in good quality scales when advancing in your professional formulation journey.

We cover what to look for in purchasing scales for cosmetic formulation in our post:

The right scales for making skincare

Splashback

You’re standing over your beaker with your essential oil bottle, counting the drops as they splash into the formulation below. Because you’re dropping in the essential oil and not using a pipette or other tool to dispense the liquid, it has the potential to splash back out of the watch glass, beaker or other vessel that you’re using. This will ultimately effect the overall weight of the essential oil in the formulation.

Our conclusion on why you should never measure in drops

The conclusion is that you must never rely on essential oil drops if you are formulating skincare. If you follow a DIY recipe, you might still want to weigh out your essential oils to make sure you aren’t causing skin sensitisation and that you are meeting recommended dermal limits for your oils or the sum total of any allergens within them. Buy yourself a good set of jeweller’s scales (here’s an example) and stay safe in your formulations.

If you come across essential oil calculators offering drop to weight conversions for popular essential oils, you will know from our experiments above that they may be inaccurate and won’t reflect the specific gravity of your particular oil. Just to reiterate, do not be tempted to create your own essential oil calculator on the basis you will always use the same oil from the same supplier.

There are some drop calculators online aimed at helping aromatherapists create competent, useful blends for their private practice clients.  Aromatherapists’ needs are not the same as those of skincare formulators who are creating cosmetics for sale and are therefore required to have percentage formulas so their products can be safety and stability tested and, depending on the formulation, also challenge tested.

Have you ever tried to weigh 20 drops of essential oil? What weight did you measure? Share your results in our comments section below!