Understanding your result
The headline is the smoke CADR the room needs, with the air changes per hour and the size class in the sub-line. The stats give the CADR, the air changes and the class. The table shows the room volume, the CADR the AHAM rule gives and its air changes, the CADR for your target if it differs, the minutes per pass (how long it takes for a room’s worth of air to go through the filter), the room size a box might print for this CADR, and the electricity on a middle setting.
The minutes per pass row is the intuitive version of air changes: at 5 ACH the room’s air goes through the filter every 12 minutes. The room size a box may claim row is there because manufacturers are allowed to print the area the CADR covers under the two-thirds rule, and some print a much larger area under a lower air-change assumption; compare CADRs, not room sizes.
The air changes calculator puts this rate next to ventilation and exhaust targets, the whole-house ventilation calculator sizes the fresh air a purifier cannot provide, and the filter cost calculator budgets the furnace filter, which is the other half of a house’s particle control.
How we calculate this
The two-thirds rule is AHAM’s own guidance for matching a purifier to a room: the smoke CADR should be at least two-thirds of the room’s area in square feet, which at an 8 ft ceiling is 4.8 air changes per hour. The calculator scales it for taller or lower ceilings so it stays a volume rule. The electricity uses 0.25 W per CFM of CADR, typical of efficient units on a middle fan setting; ENERGY STAR purifiers must deliver at least 2.0 CADR per watt. The particle targets and the air-change figures are on the ventilation and exhaust minimums page.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Sizing rule | Smoke CADR ≥ ⅔ × floor area, 8 ft ceiling | AHAM guidance under ANSI/AHAM AC-1 |
| Rating used | Smoke CADR | The smallest particle and lowest of the three CADRs |
| Air-change targets | 4, 5 or 6 per hour | Common guidance for particle reduction, allergy and wildfire smoke (EPA guide to air cleaners) |
| Size classes | Under 100, 100 to 200, 200 to 350, 350+ CFM | Typical portable purifier ranges |
| Power | 0.25 W per CFM of CADR | Efficient units on a mid setting; ENERGY STAR requires 2.0+ CADR per watt |
Assumptions last reviewed October 8, 2026.
The calculator assumes the air in the room mixes well, which it does when the purifier is in the open and not behind furniture, and it does not account for particles brought in through open windows or from cooking during the period. It says nothing about gases, ozone-producing devices (to be avoided) or the filter class beyond recommending true HEPA. The guide to MERV filters and airflow covers whole-house filtration, which complements a room purifier.
Two worked examples
A bedroom by the AHAM rule
168 sq ft (12 × 14 ft), 8 ft ceiling, AHAM rule, running 12 hours a day.
- Volume: 168 × 8 = 1,344 cu ft
- AHAM rule: ⅔ × 168 = 112 CFM smoke CADR
- Air changes: 112 × 60 ÷ 1,344 = 5 per hour; one pass every 12 minutes
- Class: medium; electricity about 0.34 kWh a day
A purifier with a smoke CADR of 120 to 150 covers this room and runs quietly on a middle setting at night, which is the setting that will actually be used. A unit sold for “up to 500 sq ft” may have a CADR of only 110; it is the CADR that matters.
A living room with a 9 ft ceiling during smoke season
350 sq ft, 9 ft ceiling, target 5 air changes per hour, running 24 hours a day.
- Volume: 350 × 9 = 3,150 cu ft
- CADR for 5 ACH: 5 × 3,150 ÷ 60 = 263 CFM
- One pass every 12 minutes; class large
- Electricity about 1.6 kWh a day
The taller ceiling adds an eighth to the requirement, which the plain two-thirds rule would miss. A unit rated 300 or more, or two units around 150, will hold the room clean during a smoke event; closing the windows and running the central system’s fan with a MERV 13 filter adds whole-house filtration.
Where to find your inputs
Room area and ceiling. Length × width and the ceiling height; for open-plan spaces, include the whole connected area.
Target. The AHAM rule for general use, 5 for allergies or smoke, 6 for high protection.
CADR of a unit. On the AHAM Verified label on the box, given as three numbers for smoke, dust and pollen; use the smoke figure.
Common mistakes
- Buying by the box’s room size. It may assume 1 or 2 air changes per hour; compare smoke CADR.
- Using the pollen CADR. It is always the highest; smoke is the one to size by.
- Ignoring the ceiling. A 10 ft ceiling needs 25% more CADR than the plain rule gives.
- Running it on low. The rating is on high; size with a margin so a quiet setting does the job.
- Putting it in a corner behind furniture. It cleans the air near it; give it open space.
- Expecting it to ventilate. It does not add fresh air or remove CO₂ or humidity.
Questions people ask
- What CADR do I need for my room?
- By the AHAM rule, a smoke CADR of at least two-thirds of the room's floor area in square feet, for an 8 ft ceiling. A 12 × 14 ft bedroom of 168 sq ft needs a CADR of about 112 CFM, which cycles the room's air through the filter about 5 times an hour. A 350 sq ft living room with a 9 ft ceiling needs about 263 for 5 air changes per hour. Scale up for taller ceilings, because the rule is about volume.
- What does CADR mean?
- Clean air delivery rate, the volume of particle-free air a purifier delivers per minute, measured by the Association of Home Appliance Manufacturers' ANSI/AHAM AC-1 test. It is given for three particle types, tobacco smoke (the smallest), dust and pollen (the largest). The smoke figure is the lowest and the one to size by, because a purifier that handles smoke handles the larger particles too. CADR combines airflow and filter efficiency; a fast fan with a poor filter scores low.
- How many air changes per hour should an air purifier give?
- Four to five through the filter for a noticeable reduction in fine particles, which is what the AHAM two-thirds rule delivers at an 8 ft ceiling (about 4.8). For allergy and asthma relief, or during wildfire smoke, 5 to 6 is better. Box claims of large room sizes often assume 1 or 2 air changes per hour, which cleans the room far too slowly to matter.
- Do air purifiers use much electricity?
- Not much. An efficient purifier draws about 0.2 to 0.3 W per CFM of CADR on a middle setting, so a 112 CADR unit uses about 30 W and roughly 0.34 kWh in 12 hours, about 6 cents a day at average prices. Running on high doubles or triples the draw and the noise. The filter replacement, often $40 to $100 a year, usually costs more than the electricity.
- Is one big purifier better than two small ones?
- In a large room or an L-shaped space, two medium units at opposite ends clean more evenly and run more quietly than one large unit on high, because air near a single purifier is cleaned much more often than air in the far corner. For a single bedroom, one unit sized by the rule is enough. Above a CADR of about 350, two units are usually the better buy.
- Can an air purifier replace ventilation?
- No. A purifier removes particles, such as dust, pollen, smoke and some microbes on particles, from recirculated air. It does nothing for carbon dioxide, moisture or most gases and odours, and it adds no fresh air. Whole-house ventilation under ASHRAE 62.2 is still needed; the two work together. Purifiers with activated carbon remove some odours and gases, but the CADR rating does not measure that.