What is CADR and why does it matter?

CADR, or Clean Air Delivery Rate, measures how much filtered air a purifier delivers, usually in cubic metres per hour. It is the most useful figure for comparing purifiers because it reflects real airflow through the filter. A higher CADR clears a given room faster or handles a larger room to the same standard.

How many air changes per hour do I need for hay fever?

For allergy and hay fever relief, aim for 4 to 5 air changes per hour, meaning the purifier filters all the air in the room four to five times each hour. General freshness only needs about 2 air changes, so allergy sufferers should choose a unit rated for roughly double the room size that basic guidance suggests.

How do I work out air changes per hour myself?

Multiply your room's floor area by the ceiling height to get its air volume, then divide the purifier's CADR by that volume. The result is the number of air changes per hour. In practice the sizing table already builds in the 4 to 5 target, so you rarely need to calculate it by hand.

Should I size up for high ceilings or open-plan rooms?

Yes. Sizing tables assume standard 8-foot ceilings, so a taller room holds more air and needs more capacity. Likewise, treat connected open-plan areas, or rooms with doors kept open, as a single larger space and choose a purifier rated for the combined size.

What Size Air Purifier Do I Need? UK Room-Sizing Guide

Buy an air purifier that is too small and it will struggle to clear the air in your room; buy one far larger than you need and you pay for capacity you never use. Getting the size right comes down to two ideas: how quickly a purifier moves clean air, measured by its CADR, and how many times per hour it can refresh the air in your room, known as air changes per hour. This guide explains both in plain terms and gives a simple rule of thumb so you can match a purifier to your space with confidence.

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CADR: how fast a purifier cleans the air

CADR stands for Clean Air Delivery Rate. It measures how much filtered air a purifier delivers, usually in cubic metres per hour (m3/hr) or, on some packaging, cubic feet per minute. A higher CADR means the unit cleans a given volume of air more quickly, or can handle a larger room to the same standard.

Air changes per hour: the target that matters for allergies

Air changes per hour (ACH) tells you how many times the purifier can filter all the air in a room within one hour. An ACH of 4 means the full volume of air passes through the filter roughly every fifteen minutes. For general freshness, 2 air changes is adequate, but for hay fever, dust and allergy relief the widely recommended target is 4 to 5 air changes per hour, because allergens are constantly being stirred up and reintroduced.

ACH links CADR to your room. To work it out, take the purifier's CADR, divide by your room's air volume (floor area multiplied by ceiling height), and that gives changes per hour. In practice you rarely need to do the maths yourself, because the rule of thumb below already builds in a sensible target for allergy sufferers.

How to measure your room

Start with floor area. Multiply the length of the room by its width to get the area in square feet or square metres. Most UK homes have ceilings around 2.4 metres (roughly 8 feet), which the standard sizing guidance assumes.

Measure length and width, then multiply them for floor area.
If your ceilings are noticeably higher than 8 feet, size up, because there is more air to move.
For open-plan spaces, treat the whole connected area as one room rather than each zone separately.
If you keep doors open between rooms, choose capacity for the combined space.

Once you know the floor area, the table below points you to a suitable model without further calculation.

Sizing rule of thumb table

This table pairs typical UK room sizes with a recommended model, based on delivering the 4 to 5 air changes per hour that suits hay fever and allergy relief. If you fall between two sizes, or have high ceilings or an open-plan layout, choose the larger option.

Common sizing mistakes to avoid

The most frequent error is buying for the floor area alone and ignoring ceiling height or open doorways, both of which increase the real volume of air to clean. The second is choosing a unit rated only for general freshness (2 air changes) when you actually need allergy-grade performance (4 to 5 air changes), which effectively halves the room size it can serve well.

Under-sizing: a small unit in a big room runs constantly on high and still cannot keep up.
Ignoring ceiling height: taller rooms hold more air, so size up.
Forgetting noise: an under-sized unit forced to maximum is louder in daily use than a larger unit running gently.
Overlooking placement: keep the intake and outlet clear of walls and furniture so airflow is not blocked.

Matching the purifier to how you use the room

Sizing sets the floor for performance, but how you run the unit matters too. In a bedroom, a purifier sized generously for the room can run on a lower, quieter setting overnight and still hit the air-change target, which is more restful than a smaller unit labouring on high. In a living room used through the day, extra capacity clears the air faster after windows have been open or people have come in from outside carrying pollen.

If you plan to move the purifier between rooms, size it for the largest space it will serve. A unit that comfortably handles your biggest room will simply clean smaller rooms faster and more quietly, giving you flexibility without buying a second machine.

What size air purifier do I need? A UK room-sizing guide