How we compare

This page sets out how the recommendation on the front page is reached, so that you can argue with the substance rather than the tone. Version of 9 September 2026.

First the bottle, then the machine

Every other guide we read weighs machines against each other as though they were interchangeable. They are not, because they do not take the same things. So the first step here is not a score at all: it is a question with two answers.

If you want to choose the oil yourself — a named plant, your own bottle, a different one next month — the answer is an ultrasonic machine, and no waterless machine of any brand can be recommended to you at any price. If you want a room to smell the same every evening without measuring anything, the answer is a sealed blend and a closed system. Only after that does a comparison of machines mean anything, because only then are you comparing like with like.

Then four figures, in this order

Stated coverage in m² — roughly a third of the judgement. A machine that cannot reach the far end of the room is not quiet, cheap or handsome enough to make up for it. We use the maker's own figure and record when there is none, which in Britain is nearly always.

Refill cost, by the millilitre and by the month — roughly a quarter. Price per millilitre multiplied by the stated life of a refill, at a fixed assumption of 12 hours a day so the rows can be lined up. Where a seller states no life, the monthly column stays empty rather than being estimated.

Stated noise in dB — roughly a fifth. A machine at 40 dB and a machine at 60 dB are two different objects to sleep next to.

Control — the remainder. A timer is not a schedule. We look for a weekly schedule, an app that pairs over Bluetooth rather than depending on the broadband, and a control on the machine itself for when the phone is upstairs.

We publish no marks out of ten. A guide with one recommendation does not need a league table, and a decimal point would imply a precision we do not have.

Where the facts come from

• Manufacturer specification tables, read from the maker's own product page.
• Retailer product pages: John Lewis, tisserand.com, homedics.co.uk, neomwellbeing.com, hollandandbarrett.com, yankeecandle.co.uk.
• Public review platforms: Trustpilot for Scentech, the retailer's or brand's own review widget for the rest, always with score, count, platform and date.
• Published guidance from authorities, such as the US Environmental Protection Agency on humidifier tanks.

Where a figure is our own arithmetic — pounds per millilitre, pence per hour, pounds per month — the sum is printed beside it so you can check the working.

What we did not do

Nothing on this site was measured. No sound meter, no test room, no two machines running side by side on a bench, and we never imply otherwise. A specification is a claim made by a seller; a review score is an aggregate of strangers' experiences. Both are useful and neither is a measurement.

We also publish no safety guidance about particular oils, around pets, children or anyone else. We found no sourced British list, and inventing one would be worse than leaving the gap visible.

Some of the market is missing here rather than guessed at. Amazon.co.uk would not show us British prices from where we work, and Argos, Dunelm, Boots and The Range would not serve us their pages on the day, so those retailers are named without figures. No consumer body had published a comparison of these machines as of 8 September 2026: not Stiftung Warentest, not the Consumentenbond, and we found no British equivalent.

The shipping caveat

We have not verified that a Scentech order can reach a British address, because on 9 September 2026 it could not: the checkout offered no shipping option for a GB address at all. That sits on the front page, in the questions and here, and it is the first thing we would check ourselves.

How this site is paid

Aromatherapyoildiffuser.co.uk is published by Scentech V.O.F., which makes and sells the diffusers recommended here. Each link to getscentech.com carries a referral code, so an order that starts here is credited here. You pay nothing extra and the offer page is the same for everyone. The other brands named here pay us nothing and have no say in the text. That is why the label at the top of the article and the line in the footer both call this an advertisement feature.

Update log

9 September 2026 — first version. Scentech figures from the maker's English product pages and help centre; competitor figures from John Lewis, tisserand.com and homedics.co.uk; oil prices from Holland & Barrett and John Lewis; candle figures from yankeecandle.co.uk and John Lewis; Trustpilot score and count read the same day. Corrections are welcome at support@scentech.store.