Start with volume
Length × width × ceiling height gives the amount of room air the purifier must process.
Air purifier CADR calculator
Enter your room dimensions. Get the minimum smoke CADR in CFM and m³/h as you type.
Uses full room volume and one consistent air-change target—not a vague “room coverage” claim.
✓ Ceiling height
✓ CFM + m³/h
✓ Multiple units
✓ Actual ACH check
Why room “coverage” can mislead
A purifier does not clean a square foot. It delivers a volume of filtered air each minute. That is why a 12-foot ceiling needs more CADR than an 8-foot ceiling over the same floor.
Choose one ACH target
ACH means equivalent room volumes of clean air per hour. Comparing every model at the same target makes coverage claims comparable.
Buy by smoke CADR
EPA guidance uses smoke CADR for smaller particles. Do not substitute a raw fan-airflow figure.
Imperial formula
required CADR (CFM) = length × width × height × target ACH ÷ 60Metric formula
required CADR (m³/h) = room volume (m³) × target ACHCADR calculator FAQ
What CADR do I need for a 300 sq ft room?
At an 8-foot ceiling and 4.8 ACH, the calculation is 300 × 8 × 4.8 ÷ 60 = 192 CFM. We default to 4.8 ACH because that is the basis of AHAM Verifide’s suggested room size.
Why do other sizing guides say 200 CFM? AHAM’s two-thirds rule calls for smoke CADR of at least two-thirds of the floor area in square feet: 300 × 2 ÷ 3 = 200 CFM. At an 8-foot ceiling, that is 5 ACH in our formula. The shortcut and our 4.8 ACH calculation use slightly different targets.
To use the shortcut’s 5 ACH equivalent, choose “Five air changes · 5 ACH” above. Still enter your actual ceiling height: the same 300 sq ft room with a 10-foot ceiling needs 250 CFMat 5 ACH. Compare verified smoke CADR at the fan speed you will use; this room-volume estimate does not certify a product’s coverage.
AHAM guidance checked October 7, 2026.
How do I measure an open-plan or L-shaped room?
Include the whole connected area the purifier will serve. For an L-shaped space, split the floor into rectangles that do not overlap and add their areas. With the same ceiling height throughout, a 12 × 15 ft living area plus a 10 × 12 ft dining area totals300 sq ft.
To enter that area here, use an equivalent rectangle of 20 × 15 ft and your actual ceiling height. At 8 ft and 4.8 ACH, the target is192 CFM. Entering only the 180 sq ft living area would give 116 CFM and leave the connected dining area out of the estimate. This shortcut assumes one ceiling height and reasonably mixed air; size rooms separated by closed doors individually.
EPA’s room-sizing guidance says to consider the entire open floor plan and keep airflow unobstructed. Source checked October 17, 2026.
Can I add the CADR of two purifiers?
For planning, add the smoke CADR ratings of units serving the same room. Two 100 CFM units give about 200 CFM total. Real results still depend on placement, obstacles, open doors, and air mixing.
Is CADR the same as CFM?
CADR is often reported in CFM in the United States, but the terms are not interchangeable. CFM can describe unfiltered fan airflow; CADR represents delivered clean air after accounting for particle removal.
Why does this calculator default to 4.8 ACH?
AHAM states that its Verifide suggested room size is based on 4.8 air changes per hour. The dropdown also lets you compare at 5 ACH, use a higher planning target, or enter your own.
Does an air purifier remove gases and odors?
Particle CADR does not rate gas removal. EPA says gas removal needs an activated-carbon or other gas-specific filter, and there is no widely used equivalent performance rating for it.
Primary sources · checked September 11, 2026
Numbers you can audit
US EPA · Guide to Air Cleaners in the Home ↗US EPA · Indoor Air Filtration for wildfire smoke ↗AHAM Verifide · Air Changes per Hour FAQ ↗CDC/NIOSH · Aim for 5 or more ACH ↗
This is a sizing estimate, not medical or engineering advice. Building ventilation, leakage, room shape, filter loading, placement, and fan speed change real performance.