Air Purifier Size Calculator

What size air purifier do I need?

An air purifier is sized by how much clean air it can deliver, not by watts or filter size. That number is the clean-air delivery rate, or CADR, measured in cubic feet per minute. The right CADR depends on two things: how big the room is, and how often you want the air in it fully cleaned each hour — a figure called air changes per hour, or ACH.

Work out the room volume — floor area times ceiling height — then multiply by your ACH target and divide by 60 to get the CADR in CFM. A 168 sq ft bedroom with a ceiling about 8 feet is 1,344 cubic feet; at 4.5 air changes an hour for allergies, that needs about 100 CFM of CADR.

Choosing an air-changes target

• 2 air changes/hr — general freshness in a low-pollution home with no specific concerns.
• 4–5 air changes/hr — the range allergy and asthma organisations recommend for sensitive people.
• 5–6+ air changes/hr — smoke, wildfire smoke, cooking odours, or homes with pets.

This lines up with the industry's well-known rule of thumb: an air cleaner's smoke CADR should be at least two-thirds of the room's floor area on a standard ceiling about 8 feet. That rule is simply the ~5-air-change point of the same formula, which is why the allergy setting lands close to it.

Frequently asked questions

What is CADR? Clean-air delivery rate is the volume of filtered air a purifier produces each minute, tested independently by AHAM. Units carry three CADR numbers — smoke, dust, and pollen. Size to the one that matches your concern; smoke is the toughest and the safest to size on.

Is a higher CADR always better? A higher CADR cleans the room faster, or the same room more times per hour, and lets you run the unit quietly on a lower speed. Oversizing does no harm beyond cost and size, unlike an air conditioner. The main trap is buying a unit rated for a room smaller than yours.

How does room-size rating relate to CADR? Manufacturers print a 'recommended room size' that usually assumes about 5 air changes an hour and a ceiling about 8 feet. If your ceiling is taller, or you want more air changes, size by CADR directly rather than trusting the room-size sticker.

Where should I place it, and should it run all the time? Put it where air can reach it on all sides, not tucked against furniture, and run it continuously on a low speed rather than in bursts — steady filtration keeps the particle count down far better than occasional high-speed blasts.

These are planning estimates based on published ACH guidance. For a whole-home approach and HVAC filtration, see our heating and cooling guides.