How we compare

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

The five criteria and what each is worth

We judge an electric oil diffuser on five things, weighted in this order, because this is the order in which they went wrong for us at home.

Water or no water — roughly a third of the judgement. A machine with a tank brings daily filling, limescale and a cleaning routine, and it dilutes the oil on the way out. A waterless, cold-air machine has neither problem and a different one instead: you are tied to the maker's refills. This is the largest single split in the category, so it carries the most weight.

Stated coverage in m² — roughly a quarter. 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 most of the time.

Stated noise in dB — roughly a fifth. These things run in the evening and overnight. A figure near 40 dB and a figure near 60 dB describe two different objects to live with.

Refill cost per month — roughly a seventh. Price per ml multiplied by the stated life of a refill, at a fixed assumption of 12 hours a day, so that the numbers can be lined up next to one another.

Controls — the remainder. Timer, weekly schedule, an app that pairs over Bluetooth rather than depending on wi-fi, and a control on the machine itself for when the phone is elsewhere.

We publish no 0-to-10 scores on this site. A guide with a single recommendation does not need a league table, and inventing decimals would imply a precision we do not have.

Where the facts come from

Every number here comes from one of four kinds of source, and each carries the date we read it:

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

Where a maker publishes no figure we write "not stated" rather than estimating one. Where a figure is our own arithmetic, such as pence per burn hour or pounds per millilitre, the sum is printed beside it so that you can check the working.

What we did not do

There is no laboratory behind this site, no sound meter, no test room and no side-by-side trial, and we never describe our work as one. What we did was compare specifications and public customer reviews. That is a real method with real limits: a specification is a claim made by the seller, and a review score is an aggregate of strangers' experiences.

We also did not price everything. Amazon.co.uk would not show us British prices from where we work, and Argos, Dunelm, Boots and The Range would not let us read their pages on the day, so those retailers are named without figures rather than with numbers we could not verify. No consumer body has published a comparison of these machines that we could find: neither Stiftung Warentest nor the Consumentenbond had one as of 8 September 2026, and we found no British equivalent.

The shipping caveat

We have not verified that a Scentech order can be delivered to a British address, because on 9 September 2026 it could not: the checkout offered no shipping option for a GB address at all. That appears on the guide, in the FAQ and here, and it is the first thing we would check ourselves.

How this site is paid

Aromatherapydiffuser.co.uk is published by Scentech V.O.F., which makes and sells the diffusers recommended here. Every link to getscentech.com carries a referral code, so an order placed through this site is attributed to it. You pay nothing extra as a result and the offer page is the same for everyone. The other brands named on this site pay us nothing, have not seen the text and cannot change it. That is why the label at the top of the guide 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, homedics.co.uk and neomwellbeing.com; candle and oil figures from yankeecandle.co.uk, John Lewis and Holland & Barrett; Trustpilot score and count read the same day. Corrections are welcome at support@scentech.store.