> ## Documentation Index > Fetch the complete documentation index at: https://docs.sonohealth.com/llms.txt > Use this file to discover all available pages before exploring further. # CADR, ACH, and Room Size: How to Size an Air Purifier > Understand CADR and air changes per hour (ACH), and learn how to match an air purifier to your room size for effective, real-world air cleaning. Two acronyms decide whether an air purifier will actually clean your room: **CADR** and **ACH**. Understanding them helps you size a unit correctly instead of buying one that's too small to keep up. ## What is CADR? CADR (Clean Air Delivery Rate) measures how much clean air a purifier delivers, typically for smoke, dust, and pollen. A higher CADR means the unit can clean a larger space faster. CADR tells you throughput; room size tells you how much air there is to clean — you need to match the two. ## What is ACH (air changes per hour)? ACH is how many times a purifier can filter the entire volume of air in a room each hour. For general air quality, around **4–5 ACH** is a good target; for allergy or asthma sensitivity, higher is better. More air changes means cleaner, more consistently filtered air. ## How does the AirPro's coverage translate? The [AirPro](/air-purifier/overview) is rated to clean about **430 sq ft in 10 minutes** and **1,200 sq ft in 30 minutes** with its [360-degree airflow](/air-purifier/room-coverage). A complete cleaning every 10 minutes for a 430 sq ft room works out to roughly 6 air changes per hour for that space — comfortably above the general target. In a larger open area it still delivers strong, if slower, turnover. ## How do I size a unit to my room? * Measure the room's square footage. * Match it to the purifier's rated coverage with a little margin to spare. * For sensitive users, choose a unit rated for a **larger** room than yours so it delivers more air changes. * For rooms over the rated coverage or multiple rooms, use more than one unit — doors and walls block airflow between rooms. See [multi-unit recommendations](/air-purifier/room-coverage). ## Why does placement affect real-world performance? Even a well-sized unit underperforms if airflow is blocked. Keep 6–12 inches of clearance around the intake, avoid tucking it in tight corners, and elevate it toward your breathing zone where possible — full guidance is on the [room coverage page](/air-purifier/room-coverage). Manufacturer coverage figures assume an empty room and ideal placement. In a furnished room with normal activity, give yourself extra capacity for the best real-world results, and run the unit continuously rather than in short bursts. **Related:** [Room Coverage Guide](/air-purifier/room-coverage) · [HEPA 14 Filtration](/air-purifier/hepa-14-filtration) · [Smart Features](/air-purifier/smart-features)