The room-size number on an air purifier box can be misleading. Two machines with the same airflow may claim very different room sizes because the manufacturers used different assumptions.
A better way to choose an air purifier is to look at its clean air delivery rate, or CADR. Then match that value to the volume of your room and the number of air changes you want each hour.
Quick answer
For a room with an 8-foot ceiling, choose a smoke CADR that is at least two-thirds of the room's floor area in square feet.
For example, a 180-square-foot bedroom needs a smoke CADR of about 120 cubic feet per minute, or CFM.
This quick rule is consistent with the room-sizing method used by the Association of Home Appliance Manufacturers. It gives about 4.8 equivalent air changes per hour in a room with an 8-foot ceiling.
If your ceiling is higher than 8 feet, use the room-volume formula instead:
Required CADR in CFM = room length × room width × ceiling height × target eACH ÷ 60
For the AHAM room-sizing basis, use 4.8 eACH as the target.
How to calculate the CADR you need
You can calculate the required CADR from the volume of your room.
Using feet and CFM
- Multiply the room's length, width, and ceiling height to find its volume in cubic feet.
- Multiply the room volume by your target equivalent air changes per hour.
- Divide the result by 60.
Formula
Required CADR (CFM) = room volume (ft³) × target eACH ÷ 60
Using metres and m³/h
- Multiply the room's length, width, and ceiling height to find its volume in cubic metres.
- Multiply the room volume by your target equivalent air changes per hour.
Formula
Required clean-air delivery (m³/h) = room volume (m³) × target eACH
To convert between units:
1 CFM ≈ 1.70 m³/h
These formulas follow the relationship between airflow, room volume, and air changes described by CDC's National Institute for Occupational Safety and Health and in AHAM's air-cleaner room-size methodology.
What does CADR mean?
CADR estimates how much air an air cleaner delivers after removing a specified test particle.
It combines two parts of performance:
- how much air the machine moves
- how well it removes particles from that air
A high-efficiency filter alone does not tell you how quickly the whole machine will clean a room. A very efficient filter paired with a weak fan may still have a modest CADR.
Certified labels may show three CADR values:
- Smoke CADR for small particles
- Dust CADR for medium particles
- Pollen CADR for larger particles
For fine particles such as smoke and much of outdoor PM2.5 pollution, use the smoke CADR when sizing the unit. The U.S. Environmental Protection Agency also advises looking for a high smoke CADR when the goal is to remove smaller particles.
CADR applies to particle removal. It does not tell you how well a machine removes gases, odours, or volatile organic compounds, often called VOCs. There is no comparably widespread consumer rating for gas removal.
Why manufacturer room-size claims differ
A room-size claim is incomplete unless it tells you how many air changes per hour were assumed.
Imagine an air purifier with a smoke CADR of 200 CFM. A manufacturer could describe it as suitable for:
- a smaller room cleaned several times per hour
- a much larger room cleaned only once per hour
The machine has not changed. Only the calculation has.
This is why CADR is usually more useful than a large room-size number printed on the box.
AHAM's verified suggested room size is based on about 4.8 equivalent air changes per hour and an 8-foot ceiling. Under that method:
Suggested room area (ft²) = smoke CADR × 1.55
You can reverse the calculation:
Required smoke CADR ≈ room area × 0.65
That is close to AHAM's easier two-thirds rule.
What is eACH?
ACH means air changes per hour. It describes how many room volumes of air are replaced or cleaned in one hour.
For a portable air cleaner, equivalent ACH, or eACH, is the more precise term. The device is not bringing in a full room of outdoor air. It is recirculating indoor air and removing particles. Its effect is expressed as an equivalent amount of clean air.
More eACH generally means particles are removed faster, provided that air mixes reasonably well in the room.
It does not mean every particle disappears after one air change. New particles may continue to enter from cooking, smoking, open windows, people, pets, or outdoor air. Some particles also settle on surfaces or return to the air later.
Which air-change target should you use?
For ordinary particle-focused room sizing, 4.8 eACH is a practical starting point because it is the basis of AHAM's verified suggested room-size calculation.
This is a sizing convention, not a guarantee that a room will be healthy or risk-free.
NIOSH advises workplaces to aim for five or more total air changes per hour when reducing airborne viral particles. That total can include outdoor-air ventilation and filtered or otherwise cleaned air. NIOSH also says five ACH is a rough guide, not an optimum for every space and not a guarantee against infection.
Do not treat five ACH as a universal healthy-home standard. The right amount of clean air depends on the pollutant source, the people in the room, outdoor conditions, building ventilation, and how the space is used.
During wildfire smoke
AHAM recommends a smoke CADR at least equal to the room's floor area in square feet during wildfire smoke events.
For a 200-square-foot room, that means looking for a smoke CADR of at least 200 CFM. This is more demanding than the usual two-thirds rule.
Keep doors and windows closed when outdoor smoke is severe, as long as indoor heat and carbon monoxide risks are controlled. Follow local public-health instructions during an active smoke event.
Worked example: a typical bedroom
Suppose a bedroom is 12 feet long, 15 feet wide, and 8 feet high.
Room area
12 × 15 = 180 ft²
Room volume
12 × 15 × 8 = 1,440 ft³
CADR needed for 4.8 eACH
1,440 × 4.8 ÷ 60 = 115.2 CFM
Choose a unit with a smoke CADR of at least 115 CFM. Rounding up to 120 CFM or more gives a simple shopping target.
The quick two-thirds rule gives the same practical result:
180 × ⅔ = 120 CFM
Worked example: a room with a high ceiling
Now suppose the same 180-square-foot room has a 12-foot ceiling.
Room volume
12 × 15 × 12 = 2,160 ft³
CADR needed for 4.8 eACH
2,160 × 4.8 ÷ 60 = 172.8 CFM
The room needs about 173 CFM, not 120 CFM.
This is why floor area alone can undersize an air purifier in rooms with high or vaulted ceilings.
Worked example: metric units
Suppose a room is 4 metres long, 5 metres wide, and 2.5 metres high.
Room volume
4 × 5 × 2.5 = 50 m³
Clean-air delivery needed for 4.8 eACH
50 × 4.8 = 240 m³/h
This is about 141 CFM.
What about an open-plan room?
Measure the full connected space that the purifier is expected to serve.
For an open kitchen, dining room, and living room, add the floor areas and use the usual ceiling height. If ceiling heights differ, calculate each section's volume and add the volumes together.
One air purifier may not distribute clean air evenly through a large or oddly shaped space. Walls, alcoves, furniture, doorways, and weak air movement can create poorly mixed areas.
Two well-placed units may work better than one unit at the far end of the space. For a first estimate, add their smoke CADR values. Real performance still depends on placement, fan speed, and air mixing.
Should you buy a larger air purifier than the calculation says?
Some extra capacity can be useful.
Published CADR values are commonly measured at the unit's highest fan setting. At a lower and quieter speed, the purifier moves less air and usually delivers less clean air.
A larger unit may meet your target at a more comfortable noise level. But do not assume that half speed provides exactly half the published CADR unless the manufacturer supplies tested values for each setting.
Consider these practical factors:
- Noise: Check sound ratings if the purifier will run in a bedroom or office.
- Filter cost: Compare replacement prices and expected replacement intervals.
- Energy use: The ENERGY STAR label addresses energy efficiency, not how effectively a unit cleans the air.
- Space: The intake and outlet need clearance. Do not hide the purifier behind furniture or curtains.
- Operating time: A purifier only cleans air while it is running.
HEPA or CADR: which matters more?
They answer different questions.
HEPA describes the particle-removal efficiency of a qualifying filter under its test conditions. CADR describes the tested particle-cleaning rate of the complete appliance.
When comparing portable units for a room, look for both:
- an appropriate particle filter, often a genuine HEPA filter
- enough smoke CADR for the room volume
Also check whether the replacement filter is readily available and affordable. A purifier with an overdue or poorly fitted filter may not perform as intended.
What an air purifier cannot fix
An air purifier can reduce airborne particles. It cannot correct every indoor-air problem.
The EPA recommends controlling pollution at its source and using clean outdoor-air ventilation when appropriate. Filtration is a useful supplement.
A portable air purifier does not:
- repair a leak or stop condensation
- remove settled mold from building materials
- make indoor smoking safe
- replace a working kitchen range hood
- guarantee protection from respiratory infection
- reliably remove gases or VOCs unless it contains enough suitable sorbent media
If you have visible mold or a persistent musty smell, find and correct the moisture source. An air purifier may capture some airborne particles, but it will not repair the moisture problem or remove mold growing on materials.
Avoid devices intentionally producing ozone. The EPA describes ozone as a lung irritant and advises against using ozone-generating air cleaners in occupied spaces.
Air purifier buying checklist
Before buying, check the following:
- Measure the room. Use length, width, and ceiling height.
- Calculate the room volume. Do not rely only on floor area when the ceiling is higher than 8 feet.
- Choose a target. Use 4.8 eACH as the AHAM room-sizing basis, or use a target required by your specific situation.
- Check smoke CADR. Use it when fine-particle removal is the main goal.
- Check the fan speed. The advertised CADR may apply only at maximum speed.
- Consider noise. A purifier you can tolerate running is more useful than one you switch off.
- Check ongoing costs. Include filters and electricity, not only the purchase price.
- Avoid ozone generators. Do not use them in occupied rooms.
- Plan the location. Keep air inlets and outlets clear.
- Control the source. Use exhaust, ventilation, cleaning, moisture control, or smoke avoidance where appropriate.
Bottom line
Ignore room-size claims that do not state their air-change assumption.
For a normal 8-foot ceiling, a smoke CADR equal to about two-thirds of the floor area is a practical starting point. For high ceilings, open-plan spaces, or any room where accuracy matters, calculate from room volume.
Use CADR to compare particle cleaning. Do not use it as a rating for VOC removal, ventilation, mold remediation, or overall health protection.
References
- U.S. Environmental Protection Agency. Guide to Air Cleaners in the Home. Accessed September 22, 2026.
- Association of Home Appliance Manufacturers. AHAM's Air Filtration Standards. Accessed September 22, 2026.
- Association of Home Appliance Manufacturers. Air Cleaner Air Changes per Hour and Room Size Methodology. Published February 2026. Accessed September 22, 2026.
- National Institute for Occupational Safety and Health. How Much Ventilation Is Enough?. Centers for Disease Control and Prevention. Accessed September 22, 2026.