Quick answer
For a 15 x 12 ft room with an 8 ft ceiling, a practical everyday target is about 115 CFM smoke CADR, or 196 m3/h. A purifier rated at 200 CFM would provide about 8.3 ACH in that room.
How to use this air purifier CADR calculator
Measure the room length, width, and ceiling height. Choose a clean-air target based on how aggressively you want the room filtered, then compare the required smoke CADR with air purifier product specifications.
If you already have a product in mind, enter its listed smoke CADR to estimate the air changes per hour it would provide in the room. If a product only lists square-foot coverage, try to find the CADR rating before comparing. Before buying more than one unit, use the Multiple Air Purifiers ACH Calculator to add CADR in one open room, or use the Air Purifier Room Size Calculator to see what room size a listed CADR can cover. Check long-term filter cost with the Air Purifier Filter Replacement Cost Calculator. To estimate the power cost of running one or more units every day, use the Air Purifier Electricity Cost Calculator.
CADR sizing is a room-volume calculation. For comfort and energy planning, you may also want the AC Room Size Calculator or the AC Electricity Cost Calculator.
Air purifier CADR formula
room volume = length x width x ceiling heightrequired CADR (CFM) = room volume (ft3) x target ACH / 60required CADR (m3/h) = room volume (m3) x target ACHactual ACH = purifier CADR (CFM) x 60 / room volume (ft3)CADR is a flow rate. In imperial units, CFM means cubic feet per minute, so the formula multiplies by 60 minutes per hour. In metric units, m3/h is already an hourly flow rate.
Assumptions and methodology
This calculator focuses on portable air purifier particle filtration. It estimates a smoke CADR target from room volume and target air changes per hour. The default everyday target is 4.8 ACH, which is close to common room-size guidance for an 8 ft ceiling.
- The AHAM-style quick check is approximately two-thirds of room area in square feet for a typical 8 ft ceiling.
- A smoke-heavy planning target uses a higher ACH target, so the required CADR is closer to the room area in square feet.
- CADR values add only when multiple purifiers run in the same open space with clear airflow.
- Results are planning estimates. Real performance depends on fan speed, filter condition, room layout, source control, and placement.
Example calculations
Air purifier CADR examples
A 15 x 12 ft bedroom with an 8 ft ceiling has about 1,440 ft3 of air volume. At 4.8 ACH, it needs about 115 CFM smoke CADR.
A 20 x 20 ft living room at a smoke-heavy target needs about 400 CFM smoke CADR. That does not mean one purifier is always required; two smaller units can be practical when their combined CADR is high enough and airflow is well distributed.
Air purifier CADR chart by room size
This chart uses an 8 ft ceiling and a 4.8 ACH target. Use the calculator above for rooms with taller ceilings, metric dimensions, or a stronger smoke target.
| Room type | Example size | Area | Target | Required CADR |
|---|---|---|---|---|
| Small bedroom | 10 x 10 ft | 100 sq ft | 4.8 ACH | 64 CFM / 109 m3/h |
| Bedroom or office | 10 x 12 ft | 120 sq ft | 4.8 ACH | 77 CFM / 130 m3/h |
| Large bedroom | 12 x 12 ft | 144 sq ft | 4.8 ACH | 92 CFM / 157 m3/h |
| Primary bedroom | 15 x 12 ft | 180 sq ft | 4.8 ACH | 115 CFM / 196 m3/h |
| Living room | 20 x 20 ft | 400 sq ft | 4.8 ACH | 256 CFM / 435 m3/h |
| Open living area | 25 x 20 ft | 500 sq ft | 4.8 ACH | 320 CFM / 544 m3/h |
CADR, room coverage, and ACH
Room coverage claims can be hard to compare because they may use different air-change targets. CADR is easier to calculate because it describes how much filtered air the purifier delivers per minute or per hour.
For example, 200 CFM is strong for a typical bedroom but may be modest for a large open living room. Always connect the CADR number back to the room volume, ceiling height, and target ACH instead of relying only on a square-foot headline.
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FAQ
What CADR do I need for my room?
A practical starting point is to calculate room volume and target air changes per hour. For example, a 15 x 12 ft room with an 8 ft ceiling needs about 115 CFM smoke CADR for a 4.8 ACH target. Larger rooms, taller ceilings, and stronger smoke targets need more CADR.
Should I use smoke CADR, dust CADR, or pollen CADR?
Use smoke CADR when sizing an air purifier from a single number. Smoke particles are small, so smoke CADR is usually the conservative number to compare across air purifier models.
Is CADR the same as room coverage?
No. CADR is a clean-air delivery rate, while room coverage is a marketing or certification claim tied to assumptions about ceiling height and air changes per hour. The same CADR covers less area if the ceiling is taller or if the target ACH is higher.
How does CADR relate to ACH?
ACH means air changes per hour. For a room measured in cubic feet, actual ACH is purifier CADR in CFM multiplied by 60, then divided by room volume. Required CADR reverses the same formula.
Does ceiling height matter for air purifier sizing?
Yes. A 300 sq ft room with an 8 ft ceiling has less air volume than the same floor area with a 10 ft or vaulted ceiling. More air volume requires more CADR for the same ACH target.
Can I use two smaller air purifiers instead of one large unit?
Often yes. Add their CADR values together only when both units are running in the same open space and airflow is not blocked. Placement, fan speed, room partitions, and noise limits still matter.
Does CADR tell me whether an air purifier removes odors or gases?
Not by itself. CADR is mainly a particle-cleaning metric. Odors, gases, and VOCs depend on filter media such as activated carbon, source control, ventilation, and product design.