Every diffuser tank holds a different amount of water, and the right number of drops changes with it. Too few and you'll barely notice the scent; too many and it can fill a room faster than you'd like. Here's a simple table by tank size, followed by a few practical notes on ratios, run time, and switching between oils.
Drops by diffuser tank size
| Diffuser tank size | Water | Essential oil (starting point) |
|---|---|---|
| 100ml (e.g. doTERRA Roam Diffuser) | Fill to the marked line | 3–5 drops |
| 150ml | Fill to the marked line | 5–7 drops |
| 200ml | Fill to the marked line | 6–9 drops |
| 300ml | Fill to the marked line | 9–13 drops |
| 500ml | Fill to the marked line | 15–20 drops |
Use this as a starting point rather than a fixed rule. doTERRA's own instructions are deliberately loose on this — fill to the marked water line, add oil, and adjust to your own preference — because room size, ventilation and the particular oil you're using all change how strong it feels in practice. Start at the lower end of the range for your tank size and add a drop or two next time if you'd like more.
Water-to-oil ratio, in plain terms
Across most ultrasonic diffusers, the common starting ratio is roughly 3–5 drops per 100ml of water. That doesn't scale up in a straight line as tanks get bigger — a 500ml tank rarely needs five times the oil a 100ml one does, because the room around it isn't five times bigger either. Individual oils vary too: sharper, more concentrated oils such as Peppermint or Clove go further on fewer drops than a lighter citrus oil, so it's worth using the bottom of the range the first time you try one.
Does more oil mean a stronger scent, or a longer-lasting one?
Stronger, not longer. The number of drops changes how strong the aroma feels, not how long the diffuser runs before it needs topping up — that's governed by the water volume and whichever mist setting you've chosen, not the oil. The doTERRA Roam Diffuser, for example, runs up to 5 hours on continuous mist or 10 hours on its intermittent setting per fill (on the adapter), whether you've used 3 drops or 8. If you want a session to last longer, switching to an intermittent setting stretches a single fill much further than adding extra oil will.
Switching oils and keeping the tank clean
If you're moving from one oil to another during the day — something citrus-led earlier on, a heavier or woodier oil later — it's worth a quick wipe of the tank before refilling, so the new oil isn't mixing with residue from the last one. This matters even more if you're in a hard water area, where mineral scale builds up in the tank alongside any oil residue. We've covered the full cleaning and descaling routine, including the UK-specific hard water angle, in our guide to cleaning an essential oil diffuser.
FAQs
Do I need fewer drops in a small room?
Yes. In a small or enclosed room, start at the bottom of the range for your tank size — or even below it — and only add more if the scent still feels faint after a few minutes.
Can I mix two or three essential oils in the same diffuser session?
Yes. Read the table above as a total drop count across whichever oils you're blending, rather than per oil — so a 200ml tank might take 3 drops of one oil and 4 of another, rather than 6–9 of each.
How do I know if I've used too much oil?
The scent will feel overpowering rather than pleasant, and you may notice an oily sheen sitting on the surface of the water. If either happens, empty the tank, rinse it out, and start again with fewer drops next time.
Does the type of diffuser change how many drops I should use?
This table is written for ultrasonic diffusers, which use water and are by far the most common type sold in the UK, including all of doTERRA's own diffusers. Nebulising diffusers work differently and don't use water at all, so they're a separate case.