Your options, ranked by long-term cost
Where you buy filters matters far more over the life of a machine than the price of any single cartridge. The cheapest sticker price often carries the highest lifetime cost, because proprietary shapes tie you to one supplier. Here are the main routes, ranked by what they cost you over the years rather than on day one.
| Where | Best for | Watch out for |
|---|---|---|
| Standard-size MERV-13 (CleanAirKits) | Luggable / CR-box owners; lowest lifetime cost, no lock-in | uses standard 16×25 / 20×25 sizes, not proprietary shapes |
| Brand direct (Levoit, Philips, Dyson…) | guaranteed-correct proprietary cartridge | highest ongoing price; you're locked in |
| Amazon / marketplaces | convenience, third-party "compatibles" | variable grade; many generics test below their claimed rating |
How to avoid overpaying
- Buy by size, not brand, wherever your unit allows it — standard 16×25 and 20×25 MERV-13 is cheap and widely stocked.
- Rebuy in pairs and change once a year for most homes — that’s around £58–£75 a year, not £80–£160.
- Skip “subscriptions” unless they genuinely beat the rebuy price; a standard-size filter you can order any time needs no lock-in plan.
- Check the grade: a real MERV number or true HEPA (H13), not vague “HEPA-type” wording.
What to watch out for with marketplace “compatibles”
Amazon and eBay are full of third-party filters advertised as matching a given brand. Some are fine; many are not. Common problems are thinner pleats, less carbon than the original, and efficiency claims that don’t hold up under testing. If you go this route, favour sellers who state a specific grade and dimensions, read recent reviews for airflow and fit complaints, and be sceptical of anything priced far below everything else. A filter that doesn’t seal properly lets air bypass it entirely, undoing the point of buying one.
Match the size before you buy
Whatever the source, the filter has to fit. Read the size printed on the edge of your current filter, or measure the slot, before ordering — our filter sizes guide explains why a “16×25” filter actually measures slightly smaller, and how to round an odd measurement to the nearest standard size. If you are choosing between grades, the HEPA vs MERV-13 guide covers which one suits a home.
The cheapest no-lock-in route
If you own, or are willing to switch to, a standard-size purifier, genuine 3M Filtrete MERV-13 in 16×25 and 20×25 is the lowest lifetime cost we know of in the UK: it ships from Oxford, with free delivery over £100 and 10% off with code FILTER10. Because the sizes are standard, you are never tied to one supplier and the filters never get discontinued out from under you. For how long a set lasts and what that works out to per year, see our replacement filter guide and the notes on replacement intervals.
Think in cost per year, not per filter
The number that matters is annual spend, not the price on a single box. A £20 proprietary cartridge needed three times a year costs more than a £58 standard pair that lasts twelve months. Before you buy, work out how often your machine actually needs a change — our notes on replacement intervals help — and multiply by the real price including delivery. Doing that sum once, at the point of buying the machine, tends to change which purifier looks like the bargain.
Buying for a proprietary machine
If your purifier only takes a proprietary cartridge, your realistic choices are the brand direct or a reputable marketplace seller. Buying direct guarantees the correct part but at the highest price; a good third-party compatible can save money if it states a genuine grade and the exact model fit. Whichever you pick, avoid unbranded “bargains” with no stated specification — a filter that under-performs or leaks air is no saving at all.