Air Purifier Size Calculator
Short answer: size an air purifier by its CADR (clean air delivery rate), not a marketing “covers X sq ft” claim. The AHAM 2/3 rule: the smoke CADR should be at least two-thirds of the room’s floor area (8-ft ceiling) — so a 150 sq ft bedroom needs a CADR around 100, a 300 sq ft living room around 200. That’s ~5 air changes per hour. For allergies or wildfire smoke, size up so the unit can run on a quieter low speed and still clean the air. This free calculator runs that math for your room.
Enter your room size, ceiling height, and what you’re fighting (general dust, allergies, pets, or wildfire smoke), and get the minimum CADR you need — or switch modes and enter a purifier’s rated CADR to check whether it’s big enough for the room. The method is the EPA air-change approach and the AHAM CADR standard, both shown transparently so nothing is hidden.
Know the square footage already? Put it in the length box and enter 1 for width.
AHAM-certified purifiers list three CADR numbers (smoke, dust, pollen). Use the smoke number — it’s the lowest and most conservative. It’s on the box or the AHAM Verifide directory.
How the calculator works
It sizes by air changes per hour (ACH) — how many times the purifier can clean the room’s full volume of air every hour — which is the method behind the AHAM 2/3 rule:
| Step | What it does |
|---|---|
| Room volume | Floor area (length × width) × ceiling height = cubic feet of air to clean. |
| Target ACH | General air quality → 4 changes/hr; allergies, dust, or pets → 5; wildfire smoke or asthma → 6 (the EPA suggests more changes during smoke events). |
| Minimum CADR | The clean-air output needed to hit that target: CADR = room volume × target ACH ÷ 60 (CFM). |
| Unit class | The smallest common purifier class whose certified smoke CADR meets or beats that minimum — with a nudge to size up so it can run quiet on low. |
The exact formula: CADRmin = (length × width × ceiling × ACH) ÷ 60. At an 8-ft ceiling and 5 ACH this reduces to the familiar AHAM 2/3 rule — CADR ≈ 0.67 × floor area — which is why a 300 sq ft room lands near a 200 CADR.
The mistake to avoid: trusting the “covers up to X sq ft” box claim
Most boxes advertise a coverage area like “cleans rooms up to 1,500 sq ft.” That number almost always assumes just two air changes per hour — fine for a faint odor, far too weak for allergies or smoke. The same unit that “covers 1,500 sq ft” at 2 ACH only covers about 600 sq ft at the 5 ACH you actually want. Size on the certified smoke CADR, run the ACH math above, and the marketing number stops mattering — see CADR, ACH & room size explained.
Frequently asked questions
What size air purifier do I need for my room?
Match CADR to the room. The AHAM 2/3 rule: smoke CADR ≥ two-thirds of the floor area (8-ft ceiling), so a 150 sq ft bedroom needs ~100 CADR and a 300 sq ft living room ~200. That’s ~5 air changes/hr. Size up for allergies or smoke so it runs quiet on low. Full sizing guide.
What is CADR and why does it matter?
Clean Air Delivery Rate — an AHAM-certified CFM measure of filtered air for smoke, dust, and pollen. Higher CADR cleans a bigger room faster. It’s more honest than a “covers X sq ft” claim, which usually assumes only 2 air changes/hr. CADR explained.
How many air changes per hour do I need?
General air quality ~4/hr; allergies, dust, or pets ~5/hr; wildfire smoke or asthma ~5–6/hr (the EPA suggests more during smoke events). ACH = CADR × 60 ÷ room volume in cubic feet.
Is it better to get one that’s too big?
Yes, slightly. A bigger unit hits your target ACH on a lower fan speed — quieter, more efficient, and the filter lasts longer because the motor isn’t maxed out. Comfortably exceeding the room’s minimum CADR beats running a too-small unit on high all day.
One big purifier or two smaller ones?
For an open great room or a whole floor, two well-placed mid-size units often beat one giant one — better air distribution, redundancy, and you can run both on low (quiet). The calculator flags when your room is into “two-unit” territory.
Related: What size air purifier do I need? (full guide) · CADR, ACH & room size explained
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