Air Purifier Size Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Smoke CADR: Clean air delivery rate for smoke-sized particles, measured in cubic feet per minute (CFM). It is the product rating used for the candidate comparison. [1]

Clean-air cycles per hour (ACH): The number of times per hour that clean air equal to the room's air volume is delivered. This calculator uses a 5 ACH planning target. [2]

Room air volume: The amount of air in the room, in cubic feet. It equals Room size times Ceiling height.

CADR headroom or shortfall: Combined tested Smoke CADR minus the CADR needed for 5 clean-air cycles per hour. A positive number is extra capacity; a negative number is the amount still needed.

EPA minimum Smoke CADR benchmark for an 8 ft room: A separate area-based benchmark of 0.65 CFM per square foot that assumes an 8 ft ceiling. [1]

Mistake: Entering a room coverage claim in Tested Smoke CADR for each purifier .
Fix: Enter the tested Smoke CADR in CFM from the product specifications, not a claimed room size.

Mistake: Using floor area as Ceiling height or leaving the height at 8 ft for a vaulted room.
Fix: Enter the actual floor-to-ceiling height in feet so Room air volume is calculated correctly.

Mistake: Adding different purifier models under Number of identical purifiers .
Fix: Use this field only for units with the same tested Smoke CADR; compare mixed models separately.

Mistake: Treating a negative CADR headroom or shortfall as zero.
Fix: A negative value is the additional combined tested Smoke CADR needed to reach the 5 ACH target.

Mistake: Comparing the EPA minimum Smoke CADR benchmark for an 8 ft room with a tall-room result as though both use the same height.
Fix: Use Smoke CADR needed for 5 clean-air cycles per hour for your entered Ceiling height; the EPA benchmark always assumes 8 ft.

• The 5 ACH result is a planning estimate for particle cleaning. It assumes the listed purifier CADR supplies all clean air counted toward the target.
• Natural ventilation, HVAC filtration, open doors, leaks, and other air cleaners can change actual clean-air cycles in the room.
• Use the tested Smoke CADR, not fan airflow or a marketing coverage claim. Product performance can vary by fan setting and filter condition.
• The EPA minimum Smoke CADR benchmark for an 8 ft room is intentionally kept separate from the entered Ceiling height. It is not adjusted for tall or vaulted ceilings.
• A passing particle-CADR comparison does not prove removal of gases, odors, or all pollutants. It also does not predict noise, energy use, filter life, or infection risk.
• The optional multi-unit check assumes every listed purifier has the same tested Smoke CADR and runs in the same room.

Calculation method

First, the calculator finds Room air volume from the room floor area and ceiling height.

Room air volume (cu ft) = Room size (sq ft) x Ceiling height (ft)

It then calculates the Smoke CADR needed to provide five clean-air cycles per hour. CADR is in cubic feet per minute, so the hourly air volume is divided by 60 minutes.

Smoke CADR needed for 5 clean-air cycles per hour (CFM) = Room air volume x 5 / 60

The separate EPA 8 ft benchmark uses room area only and a factor of 0.65 CFM per square foot. [1]

EPA minimum Smoke CADR benchmark for an 8 ft room (CFM) = Room size x 0.65

When a candidate rating is entered, the calculator multiplies the tested Smoke CADR for each purifier by the Number of identical purifiers. It compares that combined rating with the 5 ACH requirement. The 5 ACH planning target follows CDC NIOSH guidance for reducing airborne viral particles. [2]

Combined tested Smoke CADR (CFM) = Tested Smoke CADR for each purifier x Number of identical purifiers

CADR headroom or shortfall (CFM) = Combined tested Smoke CADR - Smoke CADR needed for 5 clean-air cycles per hour

Clean-air cycles provided per hour (ACH) = Combined tested Smoke CADR x 60 / Room air volume

Mini-example

A 300 sq ft room with an 8 ft ceiling has 2,400 cu ft of air. Its 5 ACH requirement is 2,400 x 5 / 60 = 200 CFM. One purifier rated at 250 CFM has 50 CFM of headroom and provides 6.25 ACH. The EPA 8 ft benchmark is 300 x 0.65 = 195 CFM.