Air Purifier CADR Calculator
CADR needed for a room and resulting ACH.
Enter your values
Enter the room area in sq ft.
Enter the ceiling height in ft.
Enter the target air changes in ACH.
Enter the purifier cadr (optional) in CFM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
CADR ≈ volume x ACH / 60
At a glance
| What it does | CADR needed for a room and resulting ACH. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room area, Ceiling height, Target air changes, Purifier CADR (optional) |
| Main output | CADR required |
| Formula | CADR ≈ volume x ACH / 60 |
| Cost | Free — no sign-up, no download |
How to use the Air Purifier CADR Calculator
- 1Enter the room dimensions and how many air changes you want.
- 2Optionally enter a specific model's smoke CADR to check it.
- 3Run the unit on a speed you can live with — rated CADR is usually the loudest setting.
Inputs explained
- Room area(sq ft)
- Enter the room area in sq ft.
- Ceiling height(ft)
- Enter the ceiling height in ft.
- Target air changes(ACH)
- Enter the target air changes in ACH.
- Purifier CADR (optional)(CFM)
- Enter the purifier cadr (optional) in CFM.
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
Results
- CADR required200 CFM
- Two-thirds rule check198 CFM
- Your unit's ACH6.25
- Units needed0.8
- Room volume2,400 cu ft
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.
What do I need to enter into the Air Purifier CADR Calculator?
Just 4 values: room area, ceiling height, target air changes, purifier cadr (optional). Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Air Purifier CADR Calculator work out the 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. It applies the formula CADR ≈ volume x ACH / 60 and shows the working so you can check each step by hand.
Is the Air Purifier CADR Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.