Find Your Air Purifier

Answer three questions and you'll know which kit fits: how big is the room, what are you fighting, and how quiet does it need to be?

Rule of thumb

Match CADR to room size: you want the purifier to move the room's entire air volume at least 3–4 times an hour. Bigger room, more air changes needed, higher CADR. When in doubt, size up and run it slower — quieter and just as clean.

Question 1: how big is the room?

Room size is the single biggest factor, and it is about volume, not just floor area. A purifier's clean-air delivery rate (CADR) tells you how much filtered air it produces each hour; you want that figure to be several times the room's air volume so the air is turned over three to four times an hour. As a rough guide, take the floor area in square metres, multiply by the ceiling height (2.4m is typical, but many London period rooms are far higher), and match a kit whose CADR clears that volume comfortably. The 5-Fan at 369 m³/h suits bedrooms and medium rooms up to around 450 sq ft; the XL 7-Fan at 681 m³/h covers large and open-plan spaces of 600 to 750 sq ft. When a room falls between sizes, size up and run the kit on a lower speed — you get the same clean air more quietly, which is almost always the better trade. If you want the reasoning behind picking quiet-over-small, the bedroom guide works through it.

Question 2: what are you actually filtering?

A particle filter and an odour filter are not the same tool. High-airflow MERV-13 media captures the fine particulate — PM2.5, dust, pollen, pet dander — that makes up the bulk of indoor air-quality problems, and it does so at around 93% capture per pass while letting the fan move a lot of air. What it cannot do is remove gases and smells: cooking odours, VOCs from paint or cleaning products, or the lingering smell of smoke. Those are gas-phase and need activated carbon, as in the Carbon 4-Fan. So the honest rule is: for particles (allergies, dust, general London traffic haze) a MERV-13 kit is all you need; add carbon only if odours or fumes are a specific concern.

• Pollen and dust, hayfever or allergy relief → 5-Fan or XL, depending on room size.
• Pets and the dander and smells that come with them → the pet guide covers whether you need carbon too.
• Smoke — wood burner, cooking or wildfire — is particles plus odour; see the smoke guide for why you may want both filter types.
• Damp and musty smells point to a moisture problem first; the mould guide explains what a purifier can and cannot fix.

Question 3: how quiet does it need to be, and the budget route

Noise decides whether a purifier stays switched on. A larger kit run on a low setting is both quieter and more effective than a small one at full speed, so if the purifier will live in a bedroom or a home office, size up and run it gently. If budget is the tighter constraint, the same physics that powers these kits — a high-airflow fan pulling air through MERV-13 media — can be assembled at home for the price of the parts: our Corsi-Rosenthal box guide shows how. However you decide, cross-check the pick against how the kits compare on clean-air delivery per pound in the 2026 rankings.