Every air purifier claims to cover a certain room size. One says "up to 200 sq ft," another says "up to 500 sq ft." But what do these numbers actually mean? Often, not what you'd hope.
The Problem with Room Size Claims
Room size claims aren't standardized. Different manufacturers calculate them differently:
- Some calculate for 2 air changes per hour (slow, minimal cleaning)
- Others calculate for 4-5 air changes (effective cleaning)
- Some measure at max speed (loud, impractical for constant use)
- Others don't specify their assumptions at all
A purifier rated for "500 sq ft" by one company might be rated for "300 sq ft" by another with more conservative methodology—same purifier, different marketing.
A Better Approach: Use CADR
CADR (Clean Air Delivery Rate) is standardized and meaningful. Use this formula:
Room Size Calculation
Minimum CADR needed = Room square footage × 0.67
This targets approximately 4.8 air changes per hour in a room with 8-foot ceilings—effective cleaning without oversizing.
Room Size Reference Table
| Room Type | Typical Size | Min CADR | Recommended CADR |
|---|---|---|---|
| Small bedroom | 100-150 sq ft | 100 | 150+ |
| Medium bedroom | 150-200 sq ft | 135 | 200+ |
| Living room | 200-300 sq ft | 200 | 300+ |
| Large living room | 300-400 sq ft | 270 | 400+ |
| Open plan space | 400-600 sq ft | 335 | 500+ |
Factors That Affect Sizing
Ceiling Height
Standard calculations assume 8-foot ceilings. For higher ceilings, you need more CADR:
- 9-foot ceilings: Add 12% to CADR requirement
- 10-foot ceilings: Add 25% to CADR requirement
- 12-foot ceilings: Add 50% to CADR requirement
Open vs Closed Rooms
A closed bedroom with a door effectively traps air for the purifier to clean. An open-plan living/kitchen area has more air volume and potential pollution sources.
Pollution Level
If you have pets, smoke nearby, or live in a polluted area, sizing up provides faster cleaning and better maintenance of air quality during pollution events.
Running Speed
CADR is usually measured at maximum speed. If you plan to run at medium or low speed for noise reasons, the effective CADR is lower. Size up if you won't run at max.
Practical Apartment Sizing
The One-Room Strategy
You don't need to purify your entire apartment. Focus on the room where you spend the most time with the door closed—usually the bedroom. A properly-sized purifier here provides 7-8 hours of clean air while you sleep.
Multiple Purifiers vs One Large Unit
For apartments with separate rooms, multiple smaller purifiers often work better than one large unit. Air doesn't flow well through doorways, so a single unit can't effectively clean closed-off rooms.
Open Floor Plans
If your apartment is open (living room flows into kitchen, etc.), treat it as one large space. A higher-CADR unit positioned centrally works better here.
When to Size Up
Consider getting a purifier larger than minimum if:
- You have pets
- You or household members smoke (or neighbors do)
- You live in an area with air quality issues
- You want to run at lower (quieter) speeds
- You cook frequently without good exhaust ventilation
When Minimum Is Fine
Minimum sizing works well if:
- You just want general dust and allergen reduction
- You have good ventilation already
- You don't mind running at higher speeds
- Budget is a primary concern