What can we help you find?
Short answer (AEO): Measure your room’s area and ceiling height, pick a target ACH (4 ACH is a common baseline; 5–6 ACH for allergies or smoke), then use this formula to get the CADR you need in CFM: Required CFM = (ACH × area × ceiling height) ÷ 60. Compare that CFM to the purifier’s CADR in the same units; if the listing gives m³/h, convert first (CFM ≈ m³/h ÷ 1.699). Choose a purifier whose CADR meets or slightly exceeds the required CFM and account for placement, open doors, and filter aging by adding a safety margin.
Why this calculation matters
CADR (Clean Air Delivery Rate) is the effective airflow of filtered air a device delivers, typically reported in CFM (cubic feet per minute) in North America. ACH (air changes per hour) expresses how many times the room’s entire air volume is replaced in an hour at that CADR. The formula links the performance goal (ACH) with the room volume so you buy a purifier that will achieve the clearance speed you expect. If you pick a purifier with CADR below the calculated CFM, it simply can’t reach your ACH target — it will take longer to lower particle levels.
Step-by-step: calculate the CADR (CFM) you need
- Measure the room area in square feet: length × width. Note ceiling height in feet. If ceiling is not standard (e.g., vaulted), use an average height or break the room into zones.
- Choose a target ACH:
- 3–4 ACH — reasonable for general background improvement and typical bedroom use.
- 5–6 ACH — recommended when you want faster clearing for allergies, wildfire smoke, post-cooking, or after visitors.
- Apply the formula:
Required CFM = (ACH × area × ceiling height) ÷ 60
Photo by Bailey Alexander on Unsplash The ÷60 converts hourly air changes into minutes (CFM = cubic feet per minute).
- Unit check and conversion:
If a product lists CADR in cubic meters per hour (m³/h), convert to CFM:
CFM ≈ (m³/h) ÷ 1.699
Example: 280 m³/h ÷ 1.699 ≈ 165 CFM.
Photo by Vitaly Gariev on Unsplash - Choose a purifier with CADR ≥ required CFM. Add a practical safety margin (commonly 10–25%) to allow for reduced performance from filter loading, placement that’s not ideal, or open doors/windows.
- Consider whether you need a single high‑CADR unit or multiple smaller units. For very large or irregular open-plan spaces, two units placed separately often give more even mixing and faster overall clearance than one central unit with the same total CADR.
Worked examples
- Small bedroom — 12 × 12 ft (144 ft²), 8 ft ceiling, target 4 ACH:
CFM = (4 × 144 × 8) ÷ 60 = 76.8 → round to 77 CFM. Choose a purifier with CADR ≥ ~85 CFM if you want a margin.
- Living room — 16 × 20 ft (320 ft²), 8 ft ceiling, target 4 ACH:
CFM = (4 × 320 × 8) ÷ 60 = 170.7 → ~171 CFM. A 200 CFM unit provides a comfortable safety margin.
- Open plan or heavy smoke — 20 × 25 ft (500 ft²), 9 ft ceiling, target 5 ACH:
CFM = (5 × 500 × 9) ÷ 60 = 375 CFM. This is a high requirement — you may reach it with one high-CADR unit (if available) or two units that together deliver ≥375 CFM.
How to interpret product CADR values (examples from the site)
Listings on the site report CADR in different ways (CFM, m³/h, or simply a number without units). Always confirm units before you assume coverage.
- MERONTE MR5866 — listing gives CADR: 200 CFM. That would cover the 16×20 living-room example at ~4 ACH (required ≈171 CFM) with room to spare.
- Afloia Europa — product text indicates a CADR of 280 m³/h. Converting: 280 ÷ 1.699 ≈ 165 CFM, close to the living-room requirement at 4 ACH but below the 375 CFM needed in the heavy-smoke example.
- Coway model — the features list CADR numbers as “Dust 445, Smoke 439, Pollen 450.” Listings sometimes omit units; AHAM CADR figures are usually reported in CFM. If those Coway numbers are CFM (as AHAM reports), they would exceed the heavy-smoke example requirement of 375 CFM. Verify the seller’s spec sheet or AHAM verification before relying on the listing.
If the listing omits units or test method, check the detailed spec sheet, look for an AHAM verification label, or contact the manufacturer to confirm whether the CADR values are in CFM or m³/h and whether they come from AHAM testing.
Practical trade-offs and things to watch for
- CADR is particle-focused. It indicates particle removal for dust, smoke, and pollen; it does not measure gas/VOC removal. For odors or VOCs look for performance data on gas-phase adsorption (activated carbon) or manufacturer testing for VOCs.
- Rated CADR is often measured at maximum fan speed. Lower fan speeds lower CADR (and noise). If you plan to run a purifier quietly at night, check CADR at lower speeds or expect longer clearance times.
- Filter loading and maintenance: as filters collect particles, effective CADR falls. Replace filters on the schedule recommended by the manufacturer; if you expect heavy use (smoke, pets), budget for more frequent replacements and buy a unit with a higher starting CADR.
- Placement and room layout matter: place the purifier away from obstructions, not tucked in a corner. In open-plan spaces or rooms with multiple doors, either increase required CADR or use multiple units placed in different zones for better mixing.
- Noise and power: higher CADR usually means higher fan speed and noise. If noise matters, choose a model with enough CADR at an acceptable noise level or plan to run higher speed only when occupancy allows (e.g., run on high while you’re out, then low at night).
- Verify test method: AHAM-verified CADR is a standardized, comparable number. A manufacturer’s in-house CADR or a number given without test context may not be directly comparable.
Decision checklist — pick and use a purifier
- Measure area and ceiling height; calculate required CFM with the formula above and pick target ACH (4 baseline; 5–6 for allergies/smoke).
- Convert device CADR to CFM if needed (m³/h ÷ 1.699).
- Choose a unit with CADR ≥ required CFM and add 10–25% margin for real-world factors.
- Confirm whether CADR values are AHAM-tested (preferred) and whether the numeric CADR quoted is for smoke, dust, or pollen — choose the relevant pollutant CADR for your concern.
- Plan for placement, maintenance (filter replacements), and acceptable noise. For large or irregular spaces, consider multiple units.
- See our curated picks at /best/air-purifiers/ for shopping and compare manufacturer spec sheets before buying.
Quick reference conversions
- CFM ≈ m³/h ÷ 1.699
- m³/h ≈ CFM × 1.699
- Round up slightly when sizing — it’s better to oversize than to underperform.
Related FAQs
What ACH should I choose for allergies or smoke?
Use 5–6 ACH for faster particle clearance (allergies, wildfire smoke). For everyday use in a bedroom or living room, 3–4 ACH is often sufficient.
Does CADR cover VOCs/odors?
No. CADR measures particle removal. For VOCs and odors, look for substantial activated-carbon capacity and manufacturer testing for gas-phase removal.
Is AHAM verification important?
AHAM verification applies a standardized test and makes CADR numbers directly comparable. If a listing lacks AHAM data, request detailed test documentation from the seller or manufacturer before relying on the number.
Sources and further reading
- U.S. Environmental Protection Agency — guidance on using air cleaners and HVAC filters for indoor air quality.
- Association of Home Appliance Manufacturers (AHAM) — CADR testing standards and AHAM-verified program.
- Manufacturer spec sheets and AHAM labels for any model you consider — always use the primary source when confirming CADR units and test method.
