Air Purifier CADR Calculator: Find the Right Room Size

The right air purifier is the one that can move enough filtered air through your actual room. Enter the room dimensions below and the calculator will convert its volume into a target clean air delivery rate, or CADR.

For a normal 8-foot ceiling, the useful shorthand is about 65 CFM of smoke CADR per 100 square feet for everyday sizing. Wildfire smoke can justify more headroom, especially if the room leaks outdoor air, connects to a hallway, or you will not run the purifier on its loudest setting.

This is a sizing tool, not a medical recommendation. It uses published EPA and AHAM methods. CADR ratings are normally measured at a purifier's highest speed, so a unit running quietly may deliver less clean air than its headline number.

Room-size calculator

Calculate the CADR your room needs

Measure the whole connected space. The result assumes the purifier runs at the speed used for its published CADR rating.

Units

Feet / CFMMetres / m³/h

Length (ft)Width (ft)Ceiling (ft)

Cleaning target

Everyday sizing4.8 eACH, the AHAM suggested-room-size methodWildfire headroom7.5 eACH, equal to room area at an 8 ft ceiling

Target eACHPurifier smoke CADR (optional)

Your target

192 CFM

About 327 m³/h at 4.8 eACH

Room area

300 ft² / 28 m²

Everyday 4.8 eACH

192 CFM

Wildfire headroom

300 CFM

Purifiers needed

Enter a model CADR

CADR is usually measured on the highest fan setting. If you plan to run a quieter speed, buy more capacity or use two units.

Quick verdict

Use 4.8 equivalent air changes per hour (eACH) as a practical everyday starting point. That is the basis of AHAM's suggested-room-size method and closely matches the EPA table of 65 CFM for each 100 square feet with an 8-foot ceiling.

Use the wildfire headroom result when smoke is the main reason you are buying. It targets smoke CADR equal to the room area for an 8-foot ceiling. That is more conservative than the EPA's two-thirds minimum and gives some margin for higher particle loads, imperfect air mixing, and the fact that most people do not want to listen to maximum fan speed all day.

Buy for the higher result only when the room and use case justify it. A huge purifier is wasted if it is too loud, too expensive to filter, or awkward enough that you stop running it.

Who this calculator is for

Use it if you are trying to answer any of these questions:

• What CADR do I need for a bedroom, office, nursery, or living room?
• Does a manufacturer's “cleans up to” claim assume one slow air change or repeated cleaning?
• Is a purifier still large enough when the ceiling is above 8 feet?
• Would two smaller air purifiers make more sense than one large unit?
• Is the model's smoke CADR high enough for wildfire season?

It is also useful when two purifiers use different room-size language. CADR and room volume give you a common basis for comparison.

Who should skip it

Do not use CADR alone to choose a purifier for gases, strong odours, solvents, or carbon monoxide. CADR is a particle-cleaning measure. It does not tell you how much activated carbon is inside a filter, how quickly that carbon will saturate, or whether ventilation is needed.

Skip the simple single-room calculation if the purifier will sit in a large open-plan area with stairwells, permanently open doors, or several connected rooms. Measure the entire connected volume, or treat each closed room as a separate cleaning job.

If somebody in the home has a medical condition affected by indoor air, use the calculator as a shopping aid and follow advice from a qualified clinician or public-health authority.

How the CADR calculation works

The formula is straightforward:

Required CADR (CFM) = room volume (ft³) × target eACH ÷ 60

A 20 × 15-foot room with an 8-foot ceiling has 2,400 cubic feet of air. At 4.8 eACH:

2,400 × 4.8 ÷ 60 = 192 CFM

At 7.5 eACH, the same room needs 300 CFM. That higher target makes the smoke CADR equal to the 300-square-foot floor area.

The calculator also converts CFM to cubic metres per hour. If you enter metric room dimensions, it converts them internally and returns both units so you can compare US and international product specifications.

Why ceiling height changes the answer

Room-size labels often assume an 8-foot ceiling. A 300-square-foot room with a 12-foot ceiling holds 50% more air than the same floor area with an 8-foot ceiling. The purifier has to process that extra volume.

This is why the floor-area-only shortcut can undersize lofts, older homes, studios, and open living rooms. Measure height rather than assuming it does not matter.

Everyday sizing versus wildfire headroom

There are two useful targets because “right size” is not a single universal number.

Everyday baseline: 4.8 eACH

AHAM's current suggested-room-size method uses 4.8 equivalent air changes per hour. At an 8-foot ceiling, that works out to 0.64 CFM for every square foot of floor area. The EPA's consumer table rounds this to roughly 65 CFM per 100 square feet.

This is a sensible starting point for pollen, dust, pet dander, and routine particle control. It assumes the purifier can actually deliver its rated CADR in the room and at the chosen fan speed.

Wildfire headroom: 7.5 eACH

AHAM's wildfire sizing shortcut matches smoke CADR to room area in square feet. With an 8-foot ceiling, that is equivalent to 7.5 eACH. It is a headroom target, not proof that every room always requires 7.5 changes.

EPA guidance uses smoke CADR of at least two-thirds of room area as a baseline for wildfire smoke and notes that some situations benefit from more. Size toward the higher result if the room is leaky, smoke intrusion is persistent, the ceiling is high, or the purifier will run below maximum speed.

CADR is usually a maximum-speed number

This is the catch that product pages rarely make prominent.

The EPA notes that CADR is typically measured at the purifier's highest speed. If a model publishes 300 CFM but you keep it on sleep mode, you should not assume it is still delivering 300 CFM.

Manufacturers do not always publish CADR at every speed. Without that curve, the practical choices are:

• oversize the purifier and run it below maximum;
• accept more fan noise when fast cleaning matters;
• use two medium units at lower speeds;
• check independent measurements when they identify the exact model and speed.

Do not treat an unverified low-speed estimate as a specification.

Worked Blueair example: 311i Max versus 311i+ Max

Blueair currently lists the Blue Pure 311i Max at 283 CFM smoke, 314 CFM dust, and 365 CFM pollen CADR, with a 439-square-foot suggested room size at 4.8 eACH. The larger 311i+ Max is listed at 339 CFM smoke, 370 CFM dust, and 446 CFM pollen, with a 525-square-foot suggested room size at the same 4.8 eACH method.

Those are manufacturer specifications, not SolderMag lab measurements. They show why the pollutant-specific number matters: pollen CADR is higher than smoke CADR on both models, but smoke is the more conservative figure for fine-particle sizing.

For a 300-square-foot room with an 8-foot ceiling:

The bigger model is not automatically better. The regular 311i Max has enough published airflow for the everyday target in this room. The plus model makes more sense if wildfire headroom is the priority or if you expect to run below maximum.

Read the full Blueair 311i Max vs 311i+ Max comparison before buying; the names and replacement filters are easy to confuse.

High ceilings, open doors, and awkward rooms

The formula handles ceiling height, but it cannot see how air moves through the room.

High ceilings

Enter the real height. If the ceiling slopes, use a reasonable average. Being slightly conservative is better than pretending the entire room is 8 feet high.

Open-plan rooms

Measure the connected space, not the corner where the purifier sits. A living room that is permanently open to a kitchen and hallway behaves like a larger air volume.

Closed bedrooms

A purifier in a closed bedroom mainly cleans that bedroom. It will not reliably clean a separate room through a doorway. Buy for the room where people spend time, particularly the sleeping space during smoke events.

Poor placement

Keep the intake and outlet clear. A purifier wedged behind a sofa may mix air poorly even if its CADR looks adequate on paper. Follow the manufacturer's clearance instructions.

When two purifiers are better than one

Two units can be the practical answer when:

• the room is long, L-shaped, or divided by furniture;
• one large purifier would be too loud near a desk or bed;
• the required CADR exceeds a single model's rating;
• separate closed rooms need protection at the same time.

Add the smoke CADR of two units only when both serve the same connected space and can circulate air effectively. Two 150-CFM purifiers can theoretically provide 300 CFM together. One in a closed bedroom and one in a closed office do not combine into a 300-CFM whole-home system.

Enter a candidate model's smoke CADR in the calculator to estimate the number of identical units required. Round up, because a fraction of a purifier is not useful.

What CADR does not tell you

CADR is useful because it compresses airflow and particle-removal efficiency into one comparable number. It still leaves out several ownership questions.

Noise

A high CADR on a speed you cannot tolerate is poor value. Look for noise data at the speeds you expect to use, not only the quietest sleep-mode figure.

Gases and odours

Particle filters do not remove carbon monoxide, and thin carbon sheets have limited capacity for gases and persistent odours. Ventilation, source control, and substantial sorbent media are separate considerations.

Filter cost

Check the exact replacement-filter model, current local availability, and a year or two of likely ownership cost. A good purifier with unavailable filters becomes useless.

Sensors and auto mode

An onboard PM2.5 sensor can be convenient, but it does not replace a clearly sized purifier. Auto modes may favour quiet operation. A dedicated air-quality monitor is more useful if you want separate readings or better placement flexibility.

Real-room leakage

Outdoor smoke, open windows, cooking, vacuuming, and constant pet activity add particles while the purifier removes them. CADR is capacity, not a guarantee that a room will stay clean under every source load.

What to check before buying

1. Measure the full connected room, including ceiling height.
2. Use smoke CADR for fine-particle and wildfire sizing.
3. Decide whether 4.8 eACH everyday sizing or extra smoke headroom fits the problem.
4. Check whether the headline CADR is measured at maximum speed.
5. Make sure the purifier is tolerable at the speed you will actually use.
6. Verify replacement-filter cost and exact model compatibility.
7. Check that particle filtration, not odour control, is the job you need done.
8. Plan a clear intake and outlet path.

Once the room math is settled, compare our current air purifier picks. We also have focused guides to air purifiers for allergies and air purifiers for pet homes.

Alternatives to a portable purifier

A portable purifier is not the only way to reduce indoor particles.

• A central HVAC system may accept a higher-efficiency filter, but the system only filters while its fan runs and must be able to handle the filter's resistance.
• A well-built DIY air cleaner can be useful during smoke events, although performance varies with the fan, filter, seal, shroud, and construction.
• Ventilation helps when outdoor air is cleaner than indoor air. It can make smoke or pollen exposure worse when outdoor air is the problem.
• Source control still matters. Use local exhaust for cooking, manage moisture and mould at the source, and do not use an air purifier as a substitute for a carbon-monoxide alarm.

FAQ

What CADR do I need for a 300-square-foot room?

For an 8-foot ceiling, about 192 CFM provides 4.8 eACH, which rounds to the EPA consumer-table baseline of roughly 195 CFM. For extra wildfire-smoke headroom, 300 CFM matches the room area and provides 7.5 eACH. Buy with the fan speed you will actually use in mind.

Is CADR the same as ACH?

No. CADR is a filtered-airflow rating, usually in CFM or m³/h. ACH describes how many room volumes that clean airflow represents each hour. The same purifier produces a higher ACH in a small room than in a large room.

Should I use smoke, dust, or pollen CADR?

Use smoke CADR when fine particles or wildfire smoke are the concern. It is usually the lowest and most conservative of the three ratings. Pollen and dust CADR remain useful for comparing performance on those larger particle groups.

Can I add the CADR of two air purifiers?

Yes, as an estimate when both units run in the same connected room and air can mix between them. Do not add ratings across closed rooms and call the result whole-home coverage.

Does a higher CADR remove smells better?

Not necessarily. CADR measures particle cleaning. Odour and gas removal depends heavily on the type and quantity of sorbent media, its condition, and the specific gas. Many consumer purifiers contain only a thin carbon layer.

Sources and methodology

• U.S. EPA, Guide to Air Cleaners in the Home, for CADR, room-size guidance, higher ceilings, multiple units, and the maximum-speed caveat: https://www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home
• U.S. EPA, Wildfire Smoke: A Guide for Public Health Officials (2026), for the two-thirds smoke-CADR baseline and cases where more capacity can help: https://www.epa.gov/system/files/documents/2026-06/wildfire-guide-for-public-health-officials-2026.pdf
• AHAM Verifide, Air Cleaners and Air Changes per Hour, for the 4.8 eACH suggested-room-size method and wildfire headroom method: https://ahamverifide.org/wp-content/uploads/2026/02/AHAM-White-Paper_Air-Cleaner-ACH_2025.pdf
• ENERGY STAR, Air Cleaners, for current room-size and efficiency context: https://www.energystar.gov/products/air_cleaners
• Blueair Blue Pure 311i Max technical specifications: https://www.blueair.com/products/blue-pure-311i-max-smokeblock-bundle
• Blueair Blue Pure 311i+ Max technical specifications: https://www.blueair.com/products/blue-pure-311i-plus-max

This guide is based on published standards and manufacturer specifications. SolderMag has not conducted hands-on CADR testing of the Blueair examples.