How we compare

This page explains exactly how the recommendation on the guide is arrived at, so you can disagree with it on the merits. Version of 9 September 2026.

The five criteria and what they weigh

We judge an aroma diffuser on five things, and we weight them 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. An ultrasonic machine holds a tank of water with a few drops of oil in it, which brings daily filling, limescale and a cleaning routine. A waterless, cold-air machine has none of that. This is the single largest split in the category, so it carries the most weight.

Stated coverage in m², roughly a quarter. A machine that cannot reach across the room is not quiet, cheap or beautiful enough to make up for it. We take the maker's own figure and note when there is none, which is most of the time.

Stated noise in dB, roughly a fifth. A diffuser runs in the evening and at night, so a figure of around 40 dB and a figure of around 60 dB are genuinely different products to live with.

Refill cost per month, roughly a seventh. Price per ml multiplied by stated life per refill, at a fixed assumption of 12 hours a day, so the numbers are comparable.

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

We do not score in these criteria on a 0-to-10 scale on this site. An advertorial with a single recommendation does not need a league table, and inventing decimals would suggest a precision we do not have.

Where the facts come from

Every number on this site comes from one of four places, and each is dated where it appears:

• Manufacturer specification tables, read from the maker's own product page.
• Retailer product pages: John Lewis, homedics.co.uk, tisserand.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 number of reviews, 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" instead of estimating one. Where a figure is our own arithmetic, such as pence per burn hour, the sum is printed next to it so you can check it.

What we did not do

There is no laboratory, no sound meter, no test room and no side-by-side trial behind this site, and we do not describe our own work as one. What we did was compare specifications and public customer reviews, which is a real method with real limits: a specification is a claim 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 and Dunelm would not let us read their pages on the day, so those retailers appear by name with no figures attached rather than with numbers we could not verify.

And we have not verified that a Scentech order can be delivered to a British address. On 9 September 2026 it could not: the checkout offered no shipping option for a GB address. That is stated on the guide, in the FAQ and here.

How this site makes money

Buyaromadiffuser.co.uk is published by Scentech V.O.F., which makes and sells the Scentech 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 identical for every visitor. The other brands named on this site pay us nothing, have not seen the text in advance, and cannot change it. That is also why the footer of every page, and the label at the top of the guide, calls 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; candle and oil figures from yankeecandle.co.uk, John Lewis and Holland & Barrett; Trustpilot score and count read on the same day. Corrections are welcome at support@scentech.store.