How to Choose an Air Purifier | Sizing, CADR, and Filters

Buying an air purifier comes down to one main decision: matching the machine to the room where it will actually run. Most shopping mistakes happen when the advertised room size and the real space don’t line up. The number that matters most is CADR — Clean Air Delivery Rate — which tells you how fast the unit can filter the air. Get that right, and the rest of the decision gets easier.

What Size Air Purifier Do You Need?

Measure your room first, then work backward to the purifier. You need the room’s square footage, and ideally its ceiling height, since volume matters more than floor space. A common rule of thumb from the Wirecutter testing team: pick a CADR that’s about two-thirds of the room’s square footage. For a 300-square-foot room, that means looking for roughly 200 CFM (cubic feet per minute).

The deeper goal is air changes per hour. Consumer-focused guides, including Wirecutter, use at least 4 ACH as a minimum test threshold, and aiming for 4.8–5 gives you real headroom. Oversized units aren’t a problem — they just filter faster. An undersized one is the classic failure, because it simply can’t keep up with the room’s volume.

CADR and True HEPA: The Two Numbers That Matter

CADR is the speed metric. It’s usually expressed in cubic feet per minute and tells you whether a purifier can actually deliver enough clean air for the space. Manufacturers’ room-size claims are marketing; the CADR rating is the measurable spec.

Filter type is the second non-negotiable. Wirecutter requires true HEPA filters in its testing, and the Victorian government’s buying guide recommends a HEPA filter rated at least H13. True HEPA captures particles like dust, pollen, and pet dander. If odors or VOCs matter to you, look for activated carbon as a second stage. Be wary of vague “HEPA-like” language — if it doesn’t say HEPA explicitly, it probably isn’t.

Noise, Filters, and Positioning: The Details That Decide

Noise matters more than most buyers expect, because the purifier should run constantly. Wirecutter recommends using the machine 24/7 on Auto or a quiet setting, and its “quiet” benchmark is 50 dBA or less. Check the dB rating at the speed you’ll actually use, not just the highest setting.

Ongoing cost is part of the purchase. Consumer Reports advises cleaning or replacing filters on schedule, and many manufacturers recommend annual replacement. Those filter cartridges add up, so factor them into your budget before buying, not after.

Setup and placement decide whether the machine works at all. Many units ship with filters installed but wrapped in plastic — unwrap and reinstall them before first use. Give the purifier at least 12 inches of clearance from walls and furniture, and ideally place it near the room’s midpoint with the intake unobstructed. One unit per room is the general rule; a single purifier won’t cover multiple separate spaces.

For a specific allergen problem, our roundup of the best air purifiers for dust mites tests the top models against real bedroom conditions.

Common Buying Mistakes to Avoid

Knowing what to skip is half the battle with air purifiers:

• Don’t trust the box’s room-size claim — verify with CADR and ACH instead.
• Don’t buy undersized; it’s the most common mistake in consumer guidance.
• Don’t ignore replacement filter and electricity costs in your total.
• Don’t block the intake or push the unit flat against a wall.
• Don’t assume “HEPA-like” equals true HEPA.
• Be cautious with ionizers — general guidance suggests avoiding or switching them off, as they may produce ozone.

Price and model specifics change constantly, so check current manufacturer listings before committing. The framework above — measure the room, verify the CADR, confirm true HEPA, check noise and filter costs — holds across every brand and every price point.