How to Size an Air Purifier for Your Room

Buying an air purifier is easy, but buying the right size is where most people go wrong. If the unit is too small, it will run constantly on max speed (noisy!) and still leave the air stale. If it is too large, you overpay for capacity you will not use. This guide makes sizing simple and practical using the CADR metric.

1. Why Room Size is Not Enough

Manufacturer "recommended room size" ratings are often misleading. They might assume the unit is running on maximum turbo speed, or they might calculate based on only 1 or 2 air changes per hour (ACH). For real health benefits—especially for allergies or smoke—you need much more circulation.

2. Calculate Your Room Volume

First, measure your room. Don't just guess.

• Metric: Length (m) × Width (m) × Height (m) = Volume (m³).
  Example: 4m × 5m × 2.5m = 50 m³.
• Imperial: Length (ft) × Width (ft) × Height (ft) = Volume (ft³).

3. Determine Your Target ACH

Air Changes Per Hour (ACH) means how many times the purifier filters the entire volume of air in the room in 60 minutes.

• 4-5 ACH: Recommended for general health, better sleep, and light dust.
• 6-8 ACH: Essential for allergy sufferers, asthmatics, or homes with pets/smoke.

4. The CADR Formula

CADR (Clean Air Delivery Rate) is the most important number on the box. It measures the volume of filtered air the unit pumps out per minute (or hour).

Example: For a 50 m³ room needing 5 ACH:
50 × 5 = 250 m³/h.

If the unit is rated in cubic feet per minute (CFM), the rule of thumb is: CFM should be at least 2/3 of the room's square footage (assuming 8ft ceilings).

5. Real-World Factors

Math is perfect; life is not. Consider these adjustments:

• Noise Tolerance: Ratings are for max speed. If you want a quiet bedroom unit, buy a model rated for a room double your size. This lets you run it on "Medium" or "Low" and still get sufficient airflow.
• Ceiling Height: If you have high ceilings (over 2.5m / 8ft), standard square footage ratings will undersize your unit. Stick to volume calculations.
• Open Doors: If you leave the door open, the "room" is effectively your whole floor. Upsize significantly or add a second unit.

Summary Checklist

• Ignore the "Recommended Room Size" on the box; look for the CADR number.
• Calculate: Room Volume × 5.
• Check noise levels (dB) at the speed you will actually use.
• When in doubt, go bigger. You can always turn a large unit down, but you cannot make a small unit work better than its limit.