Air Purifier CADR Calculator
Work out the CADR an air purifier needs for your room — by dimensions, floor area or volume, target air changes per hour or clean air per occupant — and check a purifier’s ACH, clearing time and running cost.
How it works
Room volume = length × width × ceiling height · CADR (CFM) = volume (ft³) × ACH ÷ 60 · CADR (m³/h) = volume (m³) × ACH · Occupant target = occupants × clean air per person · CADR needed = max(room target, occupant target) × (1 + extra capacity) · ACH delivered = CADR × 60 ÷ volume · Time to clear 90% = ln(10) ÷ ACH hours · AHAM rule: CADR ≥ ⅔ × floor area (ft²) ≈ 4.8 ACH · 1 CFM = 1.699 m³/h = 0.472 L/s
Clean Air Delivery Rate measures how much genuinely filtered air a purifier delivers, so matching it to a room means multiplying the room’s volume by how many times per hour you want the air cleaned. In cubic feet the result is divided by 60 to get CFM; in cubic meters it is already m³/h. AHAM’s two-thirds rule — CADR in CFM at least two-thirds of the floor area — targets about 4.8 air changes an hour at an 8-foot ceiling. The CDC suggests 5 or more air changes of clean air per hour, and people with allergies or facing wildfire smoke should aim for 6 to 8 or more. For shared rooms such as offices, classrooms or waiting rooms, a per-person target (ASHRAE Standard 241 or the WHO minimum) can call for more clean air than the room size does, so the calculator takes whichever is larger. You can subtract clean air already supplied by ventilation or a good HVAC filter, add headroom so the purifier can run on a quieter speed, split the total across several units, and check a specific purifier: its air changes per hour, how long it takes to remove 90% and 99% of airborne particles (assuming well-mixed air, the fraction left falls as e^(−ACH × hours)) and what it costs a year to run.
Worked example
A 15 × 12 ft room with an 8 ft ceiling holds 1,440 ft³. For the AHAM standard of 4.8 air changes an hour, CADR = 1,440 × 4.8 ÷ 60 = 115.2, so look for at least 116 CFM (about 196 m³/h). The two-thirds rule agrees: 180 ft² × ⅔ = 120 CFM. At 4.8 ACH, 90% of airborne particles are gone in about 29 minutes (ln 10 ÷ 4.8 = 0.48 h). A purifier rated 150 CFM would give 150 × 60 ÷ 1,440 = 6.25 ACH; running it 24 hours a day at 40 W uses 350 kWh a year, about $60 at $0.17 per kWh.
Sources: Clean air delivery rate (Wikipedia) · AHAM Verifide — Air changes per hour FAQ · CDC/NIOSH — Aim for 5 air changes per hour
Frequently asked questions
What is CADR?
Clean Air Delivery Rate is a standardised measure — in cubic feet per minute (CFM) or cubic metres per hour — of how much filtered air a purifier delivers, tested separately for smoke, dust and pollen. A higher CADR clears a given room faster. Compare the smoke figure, the lowest of the three, with the CADR this calculator gives.
How many air changes per hour do I need?
Around 4–5 ACH suits general use and underpins AHAM’s room-size ratings; the CDC recommends aiming for 5 or more. If you have allergies or asthma, or face wildfire smoke, aim for 6–8 ACH, and 10–12 for a sickroom. Each step up needs proportionally more CADR.
What is the AHAM two-thirds rule?
A quick sizing shortcut: pick a purifier whose CADR in CFM is at least two-thirds of the room’s floor area in square feet. For a 200 ft² room that means about 134 CFM, roughly 4.8 air changes an hour at an 8-foot ceiling. It undersizes rooms with higher ceilings, so the calculator also works from the actual volume.
How long does an air purifier take to clean a room?
In a closed, well-mixed room the airborne particle level falls by about 63% for each air change, so 90% removal takes ln(10) ÷ ACH hours — about 29 minutes at 4.8 ACH and 17 minutes at 8 ACH — and 99% takes twice as long. Leaks, open doors and new pollution slow this down.
Is one big purifier or two smaller ones better?
For the same total CADR, two or more smaller units placed around a large or L-shaped room usually mix and clean the air more evenly, and each can run on a quieter speed. Set the number of purifiers under More options to see the CADR each one needs.
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This calculator is for educational purposes. Double-check important results before acting on them.