How to Choose an Air Purifier for Your Room Size

Why Room Size Matters

An undersized air purifier cannot clean the room fast enough to keep up with incoming pollution. An oversized one wastes money and electricity. Getting the size right is the single most important factor in air purifier effectiveness.

Understanding CADR

CADR (Clean Air Delivery Rate) measures how many cubic feet of air a purifier can clean per minute. It is tested for three pollutants:

• Smoke CADR: Most relevant for Indian conditions (PM2.5)
• Dust CADR: Relevant for larger particles
• Pollen CADR: Relevant for seasonal allergies

For Indian pollution, always use the smoke CADR rating.

The CADR Formula

The rule of thumb: CADR should be at least 2/3 of the room area in square feet.

• Room area (sq ft) × 0.67 = Minimum smoke CADR (CFM)

Examples:

Room Size	Room Area	Minimum CADR	Typical Purifier
Small bedroom	120 sq ft	80 CFM	Entry-level
Medium bedroom	180 sq ft	120 CFM	Mid-range
Large bedroom	250 sq ft	167 CFM	Mid-to-high range
Living room	400 sq ft	267 CFM	High-end
Open living/dining	600 sq ft	400 CFM	Premium or two units

Calculating for Ceiling Height

Standard formula assumes 8-foot ceilings. If your ceiling is higher, adjust:

Adjusted area = Actual area × (Ceiling height / 8)

For example, a 200 sq ft room with 12-foot ceilings: 200 × (12/8) = 300 sq ft equivalent. You need CADR of 200 CFM.

How Many Air Changes Per Hour (ACH)

ACH measures how many times the purifier cleans the entire room volume per hour. For polluted Indian cities, target 4–5 ACH.

ACH formula: ACH = (CADR × 60) / Room volume (cubic feet)

For a 150 sq ft room (1200 cubic feet) with a 120 CFM purifier: ACH = (120 × 60) / 1200 = 6 ACH — excellent.

Single Room vs Whole Home

Most air purifiers are designed for single rooms. Claims of “covers up to 500 sq ft” usually assume low pollution levels and low fan speed.

Practical advice for Indian homes:

• Buy separate units for bedrooms and living areas
• Bedrooms are most important (you spend 8 hours there)
• A single large unit in the living room cannot clean adjacent bedrooms
• For open-plan apartments, buy one unit sized for the entire open area

Placement Tips

• Place the purifier in the centre of the room or against a wall (not in a corner)
• Keep at least 1–2 feet clearance on all sides
• Place near the door or window (highest infiltration area) if you cannot seal the room
• Do not place behind furniture or curtains
• For bedrooms, position near the bed — within 6–8 feet for best results
• Run the purifier at medium or high speed during sleep, not just low

Common Mistakes

• Buying based on maximum coverage (which assumes very low pollution levels)
• Using the purifier only when pollution is visibly bad (should run 24/7)
• Placing the purifier in a corner where airflow is restricted
• Not cleaning or replacing filters on schedule
• Expecting one purifier to clean an entire apartment

Key Takeaways

• Minimum smoke CADR = Room area (sq ft) × 0.67
• Target 4–5 air changes per hour for polluted Indian cities
• Buy separate units for bedrooms and living areas
• Place with 1–2 feet clearance, near the highest infiltration point
• Run the purifier 24/7 during high pollution seasons, not just when you notice smog