How we compare

The six criteria, and what each is worth

Every model is scored out of ten on six criteria. The weights are ours and they are debatable; they are published so that you can disagree with them in an informed way rather than wonder what is behind a number.

1. Stated coverage in square metres — 25%. The single most useful figure and the one most often missing. A manufacturer or retailer either commits to a room size or it does not. Of the five non-Scentech models we ranked, one does.
2. Noise — 20%. A decibel figure, published by the maker. A device that hisses at the head of the bed gets switched off within a week, so an unpublished figure costs points here.
3. The cost of scent per month, and how refilling works — 20%. Price per millilitre of whatever the device actually consumes, the stated life of a fill or a pod, and whether refilling means decanting, dropping or clicking.
4. Control — 15%. App, dial, timer, schedules. A one, three or seven-hour timer scores; a schedule you set once and forget scores higher.
5. Maintenance — 10%. Water in and out, limescale, cleaning, and how often the device asks something of you.
6. The public review base — 10%. Score and, more importantly, count. Eleven reviews at 4.6 tells you less than a hundred and three at 4.2, and we print the count next to every score for that reason.

A figure nobody publishes cannot earn points. That is why several competent water-based diffusers lose ground on coverage and noise without a word being said against them: the information is not there to read.

Where the facts come from

Manufacturer specification tables (getscentech.com, tisserand.com, homedics.co.uk), retailer product pages at John Lewis & Partners, and public review platforms — Trustpilot for Scentech, John Lewis's own reviews and the Yotpo widget on Tisserand's shop. Candle and essential-oil prices come from yankeecandle.co.uk, John Lewis and Holland & Barrett. Every price and rating here was read on 9 September 2026 and carries that date.

Where we do arithmetic on published figures — cost per hour, cost per millilitre, cost per month — the assumption is printed next to the result, usually "twelve hours a day" or "the maker's own stated life". Nothing is presented as measured that was calculated.

What we did not do

We ran no laboratory test and took no measurements of our own, and we had none of the competing devices in the house; those entries are built from what their makers and retailers publish and from dated customer reviews. We do not call this site or this ranking independent, because it is not: see About this site.

Two limits are worth naming. Amazon.co.uk could not be priced reliably in pounds from where we were working, so the Amazon own-label diffusers — a large part of what British buyers actually purchase — are missing here. And we left the Ellia Gather from HoMedics out of the ranking despite having its price and stated ten-hour runtime, because no rating or review count is published on the retailer's page and one of our six criteria would have had nothing to work with. It is a real product at £59.99; we simply cannot score it the way we score the others.

We also found no published test of aroma diffusers from Which?, from Stiftung Warentest or from the Dutch Consumentenbond as of September 2026. That is a statement about what we could find, not a claim that none exists.

How this site makes money

Every link to getscentech.com here carries a tracking code. If you order there after following one, that order is attributed to this site, and Scentech — which both makes the products and publishes this site — earns the sale. Nothing is added to your price, and there is no commission from John Lewis, Tisserand or HoMedics; those links exist so that you can check our figures and carry no tracking of ours.

Because the whole page is built around links of that kind, we treat the entire article as an advertisement, label it as one at the top of every page, and repeat the disclosure in the footer.

Update log

• 9 September 2026 — first version. Seven models ranked. All Scentech figures read from the manufacturer's English product pages; all competitor prices and ratings read from John Lewis, tisserand.com and homedics.co.uk on the same day. Scentech's pound prices were read from a United Kingdom shop session and are conversions of the euro prices. At the time of writing, the Scentech checkout did not return a shipping option for a British delivery address; that is stated in the FAQ on the home page rather than buried here, and we will update this entry when it changes.