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 arguable; they are printed so that you can disagree with them knowingly rather than wonder what sits behind a number.

1. Stated coverage in square metres — 25%. The most useful figure and the one most often absent. 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. What the oil costs and how refilling works — 20%. Price per millilitre of whatever the machine actually consumes, the stated life of a fill or a pod, and whether refilling means decanting, dropping or clicking. A closed pod system and an open essential-oil system both score here, on arithmetic and on effort — not on how free they leave you, which is a preference we cannot weigh for you.
3. Noise — 20%. A decibel figure, published by the maker. A machine that hisses beside a bed is unplugged within the week, so an unpublished figure costs points.
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 machine 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, which is why the count is printed beside every score on this site.

A figure nobody publishes cannot earn points. That is why several competent water-tank 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. Essential oil prices come from Holland & Barrett, candle prices from yankeecandle.co.uk and John Lewis. Every price and rating on this site was read on 9 September 2026 and carries that date.

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

What we did not do

We ran no laboratory test and took no measurements of our own, and we had none of the competing machines in the house; those entries are built from what 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.

Three 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 share of what British buyers actually put in the basket — are missing here. The Ellia Gather from HoMedics is left out of the ranking although we have its price and its 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. And no manufacturer of a water-tank diffuser publishes oil consumption per hour, so the monthly oil cost of five of the seven models on this page cannot be calculated at all — we say so rather than estimating it.

We also found no published test of oil 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 earns

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

Because the whole article is built around links of that kind, we treat the entire page 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, two of them pod machines and five of them essential oil diffusers with water tanks. 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; oil prices from Holland & Barrett. Scentech's pound prices were read in a United Kingdom shop session and are conversions of the euro prices. At the time of writing the Scentech checkout returned no shipping option for a British delivery address; that is stated in the FAQ on the home page rather than buried here, and this entry will be updated when it changes.