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Sizing an Air Purifier to Your Room
Sizing an Air Purifier to Your Room
An air purifier is only as useful as the match between its rated coverage and the room it stands in. Get that right and the unit is genuinely effective; get it wrong and it runs constantly while achieving very little.
What a coverage rating means
A coverage figure in square feet describes the largest room in which the unit can achieve a specified number of complete air changes per hour, usually at its highest fan speed. It assumes a standard ceiling height, typically eight feet.
Two things follow. First, the rating applies at maximum fan speed, which is also maximum noise — the quiet setting covers considerably less. Second, a room with high ceilings has more air volume than the square footage suggests.
In a loft or a pre-war apartment with eleven-foot ceilings, reduce the effective coverage by roughly a third.
Air changes per hour, and why the number matters
Air changes per hour, or ACH, describes how many times the unit can process the entire volume of the room in an hour. Two ACH is light filtration; four to five is the level generally recommended where air quality is a priority.
A unit rated for 1650 square feet at two ACH is a different device from one rated for 1650 square feet at five, even though the headline number is identical.
Where a manufacturer publishes both, use the ACH figure to compare. Where they publish only coverage, assume the least favourable reading.
One room, not one apartment
Air purifiers work on the air in front of them. They do not meaningfully clean air in an adjacent room behind a doorway, because the exchange rate through a door is far lower than the unit's throughput.
The practical approach is one unit in the room where you spend the most time — usually a bedroom — rather than one large unit in a hallway serving nothing.
Closing the door of the room being filtered makes the unit substantially more effective.
What HEPA does and does not remove
A true HEPA filter captures at least 99.97 percent of particles at 0.3 micrometres, which is the hardest size to catch. Larger and smaller particles are captured at higher rates.
That covers dust, pollen, mould spores, pet dander and a large share of smoke particulates. It does not remove gases or odours, which require an activated carbon stage.
Units that claim odour removal without carbon are relying on the particulate filter catching the particles that carry some smells, which is partial at best.
Pre-filters and the maintenance that people skip
A washable pre-filter catches hair, lint and larger dust before it reaches the HEPA media. Rinsing it monthly is the single highest-value maintenance task, because a clogged pre-filter starves the whole unit of airflow.
A purifier that has become noticeably quieter or less effective is usually one with a blocked pre-filter, not a failing motor.
Let it dry completely before refitting. A damp filter in a moving airstream is a mould risk.
HEPA replacement intervals
HEPA media cannot be washed; washing destroys the fibre structure that does the filtering. Replacement intervals are typically six to twelve months, depending on use and air quality.
Running a unit continuously on high in a dusty apartment will reach the shorter end. Occasional use in clean air will reach the longer.
Buy on the availability and cost of replacement filters as much as on the unit itself. A cheap purifier with expensive proprietary filters is not cheap.
Noise, and where the unit will actually live
Published noise figures are usually given at the lowest and highest speeds. The number that matters is the noise at the speed you will actually run it at overnight, which for a bedroom is the lowest.
A unit sized generously for the room can run on a lower, quieter speed and still achieve good air changes. That is the practical argument for buying one size up.
Place it with at least a foot of clearance around the intake, and not tucked behind furniture.
Realistic expectations
A purifier reduces airborne particulate in the room it is in, while it is running, with the door closed. It does not eliminate allergens from soft furnishings, it does not replace ventilation, and it does not remove gases without carbon.
Within those limits it is genuinely effective, and the improvement in a bedroom is often noticeable within a night or two.
Our purifier listings state the maker's coverage rating and filter type, and nothing beyond what they publish.
Where to go next
HEPA purifiers with washable pre-filters are in Air Care, listed with their rated coverage.
- Browse Air Care or the full shop.
- Sizing, delivery and payment questions are answered on our FAQ page.
- Delivery times are set out in the Shipping Policy; returns in the Return & Refund Policy.
- Still stuck? Contact us — we reply within 24 hours on business days.
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