Air Purifier Sizing Calculator
CADR needed from room size and air changes per hour.
About this calculator
This calculator recommends a minimum Clean Air Delivery Rate (CADR) — the industry-standard measure of how much filtered air a purifier produces per minute, tested and certified by the Association of Home Appliance Manufacturers (AHAM) — sized to your room using two independent methods, then takes the larger (more conservative) of the two. The first method targets your chosen air changes per hour: it multiplies your room's volume by your target ACH and converts to cubic feet per minute, so a bigger or taller room, or a higher target ACH, both push the required CADR up. The second method applies the AHAM '2/3 rule,' a widely cited industry rule of thumb stating a purifier's CADR for smoke — the most penetrating and hardest-to-filter common particle type AHAM tests — should be at least two-thirds of the room's floor area in square feet, regardless of ceiling height, since it's calibrated against typical residential ceiling heights rather than true room volume.
Taking the larger of the two results means the calculator won't under-recommend for a room that's large in floor area but has a low ceiling (where the AHAM rule would dominate) or one that's tall with a small footprint (where the ACH-based volume calculation would dominate). The calculator also estimates how many typical mid-size units (around 250 CADR each) would be needed to hit that target, along with rough annual filter replacement and electricity costs assuming continuous operation — the filter estimate assumes $40 per filter with 2 changes per year per unit, and the energy estimate assumes 50 watts per 250-CADR unit running 24/7, both illustrative planning figures rather than a specific product's real specs. What this doesn't capture: CADR ratings differ by particle type (smoke, dust, and pollen each get a separate AHAM-tested number, and this calculator's recommendation is calibrated to the smoke figure, the most demanding of the three), room layout and airflow obstructions that reduce real-world effectiveness below the rated CADR, and the fact that a single large purifier is often more efficient than several small ones even when their combined CADR is similar.
Inputs
Results
Recommended CADR (cfm)
120
How to Use This Calculator
- Enter Room length (ft), Room width (ft), and Ceiling height (ft).
- Set Target air changes/hour.
- Review the Recommended CADR (cfm) result.
- Use ACH-based CADR (cfm) and Room volume (ft³) to inform your decision.
How the result changes with Room length (ft)
| Room length (ft) | Recommended CADR (cfm) |
|---|---|
| 7.5 | 60 |
| 11 | 88 |
| 23 | 184 |
| 38 | 304 |
What each input means
- Room length (ft)
- Length of the room in feet.
- Room width (ft)
- Width of the room in feet.
- Ceiling height (ft)
- Floor-to-ceiling height. Standard residential is 8 ft.
- Target air changes/hour
- Desired air changes per hour. 2–4 general, 4–6 allergy, 6–12 medical.
What each result means
- Recommended CADR (cfm)
- Minimum Clean Air Delivery Rate needed, accounting for AHAM 2/3 rule.
- ACH-based CADR (cfm)
- CADR calculated from room volume and target air changes per hour.
- Room volume (ft³)
- Total room volume in cubic feet.
- Floor area (ft²)
- Room floor area in square feet.
- Units needed (250 CADR each)
- Number of typical 250-CADR purifiers required.
- Annual filter cost ($)
- Estimated yearly cost for replacement filters.
- Annual energy (kWh)
- Estimated annual electricity usage running 24/7.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRoom length (ft) = 15, Room width (ft) = 12, Ceiling height (ft) = 8, Target air changes/hour = 4 = 4 input(s) provided
- Calculate Recommended CADRRecommended CADR = max(requiredCADR, ahamMinCADR)120 = 120
- Calculate ACH-based CADRACH-based CADR = (roomVolumeFt3 * targetACH) / 6096 = 96
- Calculate Room volumeRoom volume = roomLength * roomWidth * ceilingHeight1440 = 1440
Figures and sources
- AHAM Clean Air Delivery Rate (CADR) standard and the AHAM 2/3 sizing rule (2020) — ANSI/AHAM AC-1 (most recently revised 2020), the Association of Home Appliance Manufacturers' test standard certifying an air cleaner's CADR — cfm of smoke, dust, and pollen particles removed — plus AHAM's published '2/3 rule' recommending a room's smoke CADR be at least two-thirds of its floor area in square feet, for rooms up to 8 ft ceilings. The EPA's consumer guide to air cleaners independently confirms the same smoke/dust/pollen CADR structure and a comparable cfm-per-square-foot sizing ratio.
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator take the larger of two different CADR estimates instead of picking one method?
The air-changes-per-hour method scales with full room volume (including ceiling height), while the AHAM 2/3 rule scales only with floor area — so a room that's unusually tall for its footprint will get a higher recommendation from the volume-based method, while a room with a large footprint but a standard or low ceiling will get a higher recommendation from the AHAM rule. Taking the larger of the two ensures the recommendation is never under-sized just because one particular room shape happens to make one method look small.
What target ACH should I use for allergies versus general air quality?
Commonly cited guidance suggests roughly 2-4 air changes per hour for general residential air quality, 4-6 for allergy or asthma sensitivity, and 6-12 for medical-grade or high-sensitivity environments — these are broad, widely referenced ranges rather than a single universal number, and your actual needs depend on factors like local outdoor air quality, whether pets are present, and individual sensitivity. Starting at 4-6 ACH is a reasonable middle ground for most households dealing with allergy concerns.
Why does the calculator specifically reference the smoke CADR rating rather than dust or pollen?
AHAM tests and certifies purifiers separately against three particle types — smoke, dust, and pollen — and smoke particles are the smallest and hardest to filter of the three, so a purifier's smoke CADR is typically the lowest of its three published numbers. Sizing against the smoke rating is the more conservative, safer choice, since a unit that adequately handles smoke-sized particles will generally also handle the larger dust and pollen particles at that same airflow rate.
Is running multiple smaller purifiers as good as one large one with the same combined CADR?
Not necessarily — while the AHAM 2/3 rule and ACH-based sizing both work off total CADR regardless of unit count, several smaller units typically add more combined electricity draw and more filters to replace on a recurring basis than a single larger unit rated for the same total CADR. Multiple units can be useful for distributing air circulation across a large or oddly shaped space, but for a single roughly rectangular room, one appropriately sized unit is usually the more efficient choice.
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