Air purifier room size calculator
Air Purifier Room Size & CADR Calculator
Calculate the CADR a room needs, or reverse the same ACH formula to find the maximum area and volume an existing purifier can cover.
Calculate room CADR or purifier coverage
Use smoke CADR—not maximum fan airflow. Size a room or reverse the formula to find how much space an existing purifier can cover.
Room filtration plan
Room volume
1,440 ft³
Required smoke CADR
120 CFM
Planned smoke CADR
120 CFM
Achieved air changes/hour
5 ACH
Purifiers needed
1
Idealized cleanup time
Add a starting and lower target PM2.5 to estimate cleanup time.
At 120 CFM per purifier, use 1 unit(s) to reach at least 5 ACH in this idealized model.
Important assumptions
The model treats the room as well mixed, assumes no continuing particle source or extra air leakage, and assumes rated smoke CADR at the selected fan speed with clean filters. Real performance can be lower.
What CADR do I need for a 45 m² room?
For a 9 m × 5 m room with a 2.5 m ceiling, the volume is 112.5 m³. Choosing 5 air changes per hour in this example gives a required smoke CADR of 562.5 m³/h. Use the calculator’s example button to load these inputs, then calculate your result.
- Size the room
- 2 units rated at 300 m³/h provide 600 m³/h in the model, or 5.3 ACH in this room.
- Check an existing purifier
- One 300 m³/h unit covers at most 24 m² at the same 2.5 m ceiling and 5 ACH target. Select “Find purifier coverage” to reverse the calculation.
This is a well-mixed-room planning example. Use the smoke CADR at the fan setting you will run. Open doors, ongoing pollution and poor mixing can reduce actual performance.
Connect room filtration to your measured air quality
CADR, ACH and cleanup-time formulas
- Required CADR in CFM equals room volume in cubic feet multiplied by target ACH, divided by 60.
- Maximum purifier coverage reverses that formula: volume equals existing smoke CADR multiplied by 60, divided by target ACH; floor area then equals volume divided by ceiling height.
- When start and target PM2.5 are provided, idealized cleanup time equals ln(start ÷ target) divided by achieved ACH, then multiplied by 60 minutes.
- These formulas assume a well-mixed room, no continuing particle source and no extra leakage; placement, doors and filter condition affect real results.
Air quality tools for the decision in front of you
Start with the number or question you already have. Every tool leads to a concrete calculation, comparison, or action plan instead of another generic article.
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Calculate electricity, filter, annual, and three-year costs for one or more air purifiers.
DIY air cleaner & Corsi–Rosenthal box calculator
Compare EPA-tested DIY designs by estimated smoke CADR, room ACH, unit count, and material cost.
Daily air pollution exposure planner
Combine the places in your day into a time-weighted PM2.5 comparison.
Trust Signals
What CADR and ACH can—and cannot—tell you
Review policy
August 8, 2026. We update supporting pages when calculator logic, live data behavior, or source framing changes.
Primary sources
AQI uses EPA / AirNow breakpoint logic. Cigarette-equivalent framing follows Berkeley Earth’s PM2.5 interpretation.
Live data layer
City pages use EPA AirNow where supported, with cached fallbacks and planning baselines if the API fails or coverage is unavailable.
Air purifier sizing questions
Which CADR should I enter?
Use the smoke CADR from an AHAM-verified or manufacturer specification. Dust and pollen CADR are not interchangeable with smoke CADR.
Can I add two smaller purifiers together?
For a planning estimate, yes: add their smoke CADR when both run in the same room. Real airflow still depends on placement and obstructions.
Does a larger purifier remove gases or odors?
Not necessarily. CADR measures particle removal. Gases and odors require suitable activated-carbon or other gas-phase media.