Air Purifier CADR Rating Explained: How to Choose the Right Size for Your Room
Updated March 2026 · 12 min read · India-specific guide
Key Takeaway
CADR (Clean Air Delivery Rate) is the single most important specification when buying an air purifier. Look for smoke CADR specifically — it measures fine particle (PM2.5) removal. For a standard 20 m² Indian bedroom, aim for smoke CADR ≥ 250 m³/h with H13 HEPA filtration.
What is CADR?
CADR stands for Clean Air Delivery Rate. It measures how many cubic meters (or cubic feet) of clean air an air purifier delivers per hour, tested at a specific particle size. The AHAM (Association of Home Appliance Manufacturers) standard tests three particle sizes:
- Smoke CADR — fine particles 0.09–1 μm (most relevant for PM2.5)
- Dust CADR — medium particles 0.5–3 μm
- Pollen CADR — large particles 5–11 μm (easiest to filter)
For Indian urban air quality, where PM2.5 from vehicle exhaust, industrial emissions, crop burning, and construction dust is the primary health concern, smoke CADR is the number you must compare. A purifier might have smoke CADR 220 and pollen CADR 380 — do not be misled by the higher pollen number.
CADR Recommendations by Room Size
| Room Size | Min CADR (m³/h) | Target ACH | Example Models |
|---|---|---|---|
| Small bedroom (10–15 m²) | 150–250 m³/h | 4–5 ACH | Mi Air Purifier 4 Lite, Coway AP-1512HH |
| Standard bedroom (15–25 m²) | 250–380 m³/h | 4–5 ACH | Philips AC2887, Dyson HP07 |
| Living room (25–40 m²) | 380–550 m³/h | 4–5 ACH | Blueair 480i, IQAir HealthPro 250 |
| Open plan / Large room (40–60 m²) | 550–800 m³/h | 4–5 ACH | IQAir HealthPro Plus, Blueair 605 |
| Office / commercial (60–100 m²) | 800–1,200 m³/h | 4–5 ACH | IQAir GC MultiGas, Blueair Pro M |
Note: For Delhi, Kanpur, Patna, or other cities with regular AQI above 200, use 5–6 ACH as your target, not 4.
HEPA Filter Grades — H11 vs H13 vs H14
| Grade | Efficiency | Best For | Note |
|---|---|---|---|
| HEPA H11 | 95% at 0.3 μm | Low-pollution areas (AQI < 100) | Not adequate for high-pollution Indian cities |
| HEPA H13 | 99.95% at 0.3 μm | Most Indian cities (AQI 100–300) | Good standard for most residential use |
| HEPA H14 | 99.995% at 0.3 μm | Delhi/Kanpur in winter (AQI 300+) | Higher filter resistance; some purifiers throttle airflow |
| Medical-grade HEPA (ULPA) | 99.9995% at 0.12 μm | Hospitals, immunocompromised patients | Very expensive; overkill for residential use in most cases |
The ACH Formula for Indian Conditions
Required CADR (m³/h) = Room Area (m²) × Ceiling Height (m) × ACH
Standard conditions: ACH = 4
Delhi/Kanpur winter, Diwali, AQI > 200: ACH = 5–6
Asthma/COPD patients, children, elderly: ACH = 5–6
Example: 18 m² bedroom, 2.7m ceiling, Delhi winter → 18 × 2.7 × 5 = 243 m³/h minimum
Features That Actually Matter (vs. Marketing Noise)
✅ Important: True CADR rating (AHAM certified)
Look for AHAM certification, not just 'HEPA filter.' Some brands claim CADR numbers without certification. Trusted brands with verified CADR: IQAir, Blueair, Coway, Philips, Winix, Levoit (certain models).
✅ Important: H13 or better HEPA grade
Many budget purifiers use H11 HEPA — adequate for Western cities, inadequate for Delhi or winter north India. Demand H13 or better for Indian urban conditions.
✅ Important: Auto mode with PM2.5 sensor
A built-in laser particle sensor that automatically increases fan speed when PM2.5 rises is extremely useful — you don't have to manually remember to turn it up during Diwali or pollution events.
⚠️ Useful but not critical: Activated carbon filter
Carbon filters remove VOCs and odors (cooking smells, paint fumes, etc.) but do NOT remove PM2.5. They're a nice addition but don't let their absence (or presence) drive your purchase decision.
❌ Marketing noise: Negative ions, UV-C, photocatalysis
Ionizers and UV-C additions have minimal evidence of PM2.5 removal effectiveness and some ionizers produce ozone as a byproduct. Focus on CADR and HEPA grade first.
Frequently Asked Questions
What does CADR mean and why does it matter?
CADR stands for Clean Air Delivery Rate — the volume of clean air (in cubic meters per hour or cubic feet per minute) that a purifier delivers at a specific particle size. It was developed by AHAM (Association of Home Appliance Manufacturers) and is tested in a standardized room under controlled conditions. CADR is the single most important specification when buying an air purifier because it directly tells you how fast the machine cleans the air. A machine can have a true HEPA filter but a weak fan and low CADR — it would clean air slowly, leaving your room polluted for longer. Higher CADR = faster cleaning. Always look for CADR, not just 'HEPA certified'.
How do I calculate the right CADR for my room?
Use the 'ACH rule': you need at least 4 Air Changes per Hour (ACH) for healthy air in a polluted environment, 5+ ACH during high AQI events. Formula: Required CADR (m³/h) = Room area (m²) × Ceiling height (m) × 4 (for ACH). Example: a 20 m² bedroom with 2.7m ceiling: 20 × 2.7 × 4 = 216 m³/h minimum CADR. For Delhi winter smog or if you have asthma, use ACH = 5 or 6: 20 × 2.7 × 5 = 270 m³/h. Always round up — it's better to have too much capacity than too little. Running a large purifier on medium speed is quieter and uses less energy than running a small purifier at full blast.
Is PM2.5 CADR different from dust or pollen CADR?
Yes, and this distinction is crucial for Indian consumers. AHAM standard CADR tests measure three particle sizes: smoke (fine particles around 0.09–1 μm, most relevant for PM2.5), dust (larger particles 0.5–3 μm), and pollen (5–11 μm). Smoke CADR is the most relevant for Indian air quality concerns — it correlates best with PM2.5 removal efficiency. A purifier might have a smoke CADR of 250 and a pollen CADR of 380 — the pollen number is higher because large particles are easier to capture. Always compare smoke CADR between models when evaluating for PM2.5 pollution.
Does air purifier placement affect performance?
Significantly. Air purifiers work by drawing in polluted air and expelling clean air — they need clear paths for both. Optimal placement: (1) Place in the room where you spend the most time — bedroom for most people. (2) Keep at least 30–50 cm clearance on all sides, especially the intake and outlet. (3) Avoid placing behind furniture, in corners, or under tables — this reduces effective airflow. (4) In open-plan spaces, a central location maximizes coverage. (5) During high AQI events (Delhi winter, Diwali), run the purifier in the bedroom continuously with the door closed — you'll achieve higher clean air concentration than trying to clean a large open space.
How often should I replace HEPA filters in Indian conditions?
Filter replacement intervals vary significantly by city pollution level. Manufacturer recommendations (typically 12 months) are calibrated for Western cities with AQI 20–50. In Indian conditions: Delhi residents typically find HEPA filters visibly black after 6–9 months. Mumbai and Chennai: 10–14 months. Hyderabad and Pune: 10–12 months. A practical rule: visually inspect the filter every 3 months. If the pre-filter (the first-layer foam/mesh) is visibly grey or black, it's time to consider replacement. Running a heavily loaded HEPA filter past saturation can cause filter bypass — where air flows around rather than through the saturated filter media. Most modern purifiers have filter-life indicator lights; these are generally reliable but may not account for extremely high pollution events.