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Calculation Machine

Air Purifier CADR Calculator

CADR needed for a room and resulting ACH.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
CADR required
200 CFM
Two-thirds rule check
198 CFM
Your unit's ACH
6.25
Units needed
0.8
Room volume
2,400 cu ft

Quick answer

A purifier's CADR should be at least two-thirds of the room's floor area in square feet for 4-5 air changes per hour.

The formula

CADR ≈ volume x ACH / 60

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Room area300 sq ft
  • Ceiling height8 ft
  • Target air changes5 ACH
  • Purifier CADR (optional)250 CFM

Result

  • CADR required200 CFM
  • Two-thirds rule check198 CFM
  • Your unit's ACH6.25
  • Units needed0.8
  • Room volume2,400 cu ft

How to use the Air Purifier CADR Calculator

  1. 1Enter the room dimensions and how many air changes you want.
  2. 2Optionally enter a specific model's smoke CADR to check it.
  3. 3Run the unit on a speed you can live with — rated CADR is usually the loudest setting.

Frequently asked questions

Which CADR number matters?

Smoke CADR is the toughest test and best proxy for fine particles.

Two small units or one big one?

Two smaller units usually mix the room better and run quieter.

About this calculator