The formula is one line
Diluting an essential oil comes down to a single equation.
Drops needed = carrier oil (mL) × concentration (%) ÷ 100 × drops per mL
If you treat 1 mL as 20 drops, it looks like this:
- 10 mL of carrier at 2% → 10 × 0.02 × 20 = 4 drops
- 30 mL of carrier at 1% → 30 × 0.01 × 20 = 6 drops
- 5 mL roll-on at 1% → 5 × 0.01 × 20 = 1 drop
Anyone can do that with a calculator. The interesting part is what happens next.
When it does not divide evenly, what is the real concentration?
That formula usually returns a fractional number of drops. 10 mL at 1.5% happens to give exactly 3 drops, but 15 mL at 1.5% gives 4.5 drops — and you cannot dispense half a drop.
Round down to 4 drops and the real concentration becomes 4 ÷ 20 ÷ 15 × 100 = about 1.33%. Round up to 5 and it is about 1.67%. There is no way to land on 1.5% exactly.
Tisserand and Young's Essential Oil Safety, the reference most safety guidance leans on, is explicit that drop counts are always an approximation. So there are really three numbers worth knowing:
- the target concentration you are aiming for;
- the whole number of drops you will actually dispense;
- the real concentration after rounding — the one that ends up on skin.
Skip the third and "the limit is 2%, so 4 drops" quietly goes over the limit — and the smaller the bottle, the worse it gets. In a 5 mL roll-on, one extra drop moves the concentration by a full percentage point. That single fact is the most commonly missed part of dilution math.
How much is "one drop"?
The drops per mL figure the formula depends on is conventionally taken as 20 drops. It is not a fixed physical quantity.
- It changes with the viscosity of the oil. A thin citrus oil and a thick vetiver or sandalwood do not form the same size drop.
- It changes with the orifice of the dropper, which varies between brands and bottles.
- In practice a range of 15–30 drops per mL is sometimes used.
When precision matters, measure a few drops from your own bottle. Treat "20 drops = 1 mL" as a starting point, not a constant.
What percentage should you use? It depends on whose skin
The usual guidance is not about what you are putting on skin so much as whose skin it is going on. The figures below follow the general guidance published by NAHA and by Tisserand and Young's Essential Oil Safety.
| Who it is for | Commonly cited maximum |
|---|---|
| Adults (body) | 2% |
| Adults (face) | 1% |
| Elderly | 1% |
| Pregnancy | 1% (professional advice recommended) |
| Ages 6–15 | 2% |
| Ages 3–6 | 1% |
| 3 months – 2 years | 0.25% (professional advice recommended) |
| Under 3 months | Avoid topical use |
Guidance for pregnancy and for children differs between schools of practice. What is above is widely cited, not the single correct answer. When in doubt, err toward the lower figure.
The citrus trap: phototoxicity has a ceiling of its own
This is where looking only at the table above will catch you out. Expressed (cold-pressed) citrus oils can be phototoxic: left on skin and exposed to sunlight, they can react. For that reason each oil has its own maximum for leave-on products.
| Essential oil | Maximum in leave-on products |
|---|---|
| Bergamot (expressed) | 0.4% |
| Lime (expressed) | 0.7% |
| Bitter orange (expressed) | 1.25% |
| Lemon (expressed) | 2% |
| Grapefruit (expressed) | 4% |
| Bergamot FCF / sweet orange / mandarin | Not considered phototoxic |
Look at bergamot at 0.4%. Blend it thinking "adults, so 2% is fine" and that one oil alone puts you five times over. The limits differ per oil, and there is no reasonable way to hold them all in your head.
Two exemptions are worth knowing: rinse-off products (soap, shampoo) sit outside these limits, and oils that are steam distilled, or FCF grades with the bergapten removed, are not treated as phototoxic. The same word "bergamot" behaves differently depending on how it was produced.
Do not invent numbers the sources do not fix
Some oils are known irritants without a settled maximum percentage in the literature — the "hot" oils such as cinnamon and clove.
When that happens, the most dangerous thing to do is put a plausible-looking number in the table just to fill the cell, because people believe numbers. Where a figure is not established, say that the oil needs care and give no figure. That is how I built my own app, and this article follows the same rule.
Let a tool carry the arithmetic
Put together, every blend needs the same four steps:
- work out the drops from carrier volume and target concentration;
- round to a whole number and calculate backwards to the real concentration;
- check that concentration against the limit for the person using it;
- check whether the oils involved carry a phototoxicity limit.
Doing the fourth step every single time is genuinely tiring. To hand that to a machine I built a free app called Dilutoria (Oil Dilution Calculator) — it is made here at GRAMSHIFT.
- 🧮 Enter the volume and the concentration and it shows the drops, the real concentration after rounding, and the essential oil volume together.
- 🧴 It also checks a blend you have already made: enter the drops and the carrier volume and it reports the actual concentration, whether a limit is exceeded, and how to correct it — fewer drops, or more carrier.
- ⚠️ Limits switch by who it is for — adult, face, elderly, pregnancy, children by age, infants — and phototoxic oils are flagged against their own per-oil limit.
- 📴 It works offline and needs no account.
The app and this article are for calculation and general reference. They are not a medical device and do not diagnose, treat or prevent anything. The figures are commonly cited guidance, not a guarantee of safety.
Summary
- Drops = volume × % ÷ 100 × drops per mL. 1 mL = 20 drops is the starting point, not a constant.
- You can only dispense whole drops, so always look at the real concentration after rounding.
- The smaller the bottle, the heavier one drop weighs. At 5 mL, one drop moves the result by about a percentage point.
- The limit depends on who it is for. When unsure, go lower.
- Expressed citrus is a separate rule. Bergamot sits at 0.4%, so a "2% blend" can be five times over.
- Where the sources do not fix a number, do not invent one.
FAQ
How many mL is one drop of essential oil?
It is generally calculated as 20 drops per mL. Drop size is not fixed, though: it varies with the viscosity of the oil and the orifice of the dropper, and thicker oils form larger drops. Where precision matters, measure a few drops from your own bottle to find what one drop really is.
What concentration should I dilute to?
As commonly cited guidance: up to 2% for adult body use, up to 1% around the face, and up to 1% for elderly users or during pregnancy. For children the figure drops with age — around 1% for ages 3 to 6, around 0.25% for 3 months to 2 years, and topical use is generally avoided under 3 months. These are guides, not guarantees, and the right figure varies with the person and the area of skin.
Why do citrus essential oils need extra care?
Expressed (cold-pressed) citrus oils can be phototoxic: if they remain on skin that is then exposed to sunlight, a reaction can occur. Because of this, leave-on products have a maximum concentration set per oil — bergamot is 0.4%, for example. Rinse-off products are outside these limits, and steam-distilled oils and FCF grades with the bergapten removed are not treated as phototoxic.
Also in Japanese
This guide is also available in Japanese: アロマの希釈計算のやり方（日本語版）.