To size an air purifier correctly, divide your room's cubic footage by 60, then multiply by your target air changes per hour (ACH)—the result is the minimum CADR (Clean Air Delivery Rate) you need in CFM. For a standard 300 sq ft bedroom with 8-foot ceilings, you need a purifier with at least 200 CFM CADR to achieve 5 ACH.
Getting the size wrong is the most common air purifier mistake. An undersized unit just can't cycle enough air to make a measurable difference, no matter how good its filters are. An oversized unit wastes money upfront but gives you the flexibility to run on lower (quieter) speeds while still achieving excellent filtration.
The Air Purifier Sizing Formula
Here's the math behind proper sizing:
Required CADR (CFM) = (Room Volume in cubic feet × Target ACH) ÷ 60
Breaking that down:
- Room Volume = Length (ft) × Width (ft) × Ceiling Height (ft)
- Target ACH = How many times per hour you want all the air filtered
- ÷ 60 = Converts cubic feet per hour to cubic feet per minute (CFM)
For a 15 ft × 20 ft room with 8 ft ceilings: Volume = 2,400 cubic ft. At 5 ACH: Required CADR = (2,400 × 5) ÷ 60 = 200 CFM.
Target ACH Rates by Use Case
Not everyone needs the same air change rate. Your target ACH depends on why you're running the purifier and what contaminants you're addressing.
The quiet operation rule: Buy a purifier with a max CADR that gives you your target ACH, then run it on medium. You'll hit approximately 50–65% of max CADR at a significantly lower noise level. A 400 CFM purifier on medium in a 300 sq ft room delivers roughly 200–260 CFM—achieving 5–6.5 ACH while staying much quieter than running at full speed.
Room Size Quick-Reference Chart
If you don't want to do the math, use this chart. It assumes 8-foot ceilings and 5 ACH (appropriate for most allergy and general use scenarios).
The "Recommended CADR" column adds a 30% buffer so you can run the unit below maximum speed while still hitting your target.
Ceiling Height Matters More Than You Think
Standard sizing charts assume 8-foot ceilings. But if your home has 9-foot, 10-foot, or vaulted ceilings, the air volume increases dramatically—and your CADR requirement increases proportionally.
A 300 sq ft room with 12-foot ceilings needs the same CADR as a 450 sq ft room with 8-foot ceilings. If you follow a chart that only accounts for square footage, you'll undersize by 50%.
Vaulted and cathedral ceilings: If your ceiling slopes, estimate the average height. A room with a vaulted ceiling that peaks at 14 ft and slopes to 8 ft at the walls has an average height of roughly 11 ft. Use that average in your calculation.
Open Floor Plans: How to Size Connected Rooms
Open floor plans are the trickiest to size. Here's the rule:
If two rooms are connected by a doorway wider than 6 feet or have no wall between them, treat them as a single space. Add the square footage together and size accordingly.
If rooms are connected by a standard doorway (under 4 feet wide) or a hallway, treat them as separate spaces. Air exchange through narrow openings is too limited for a purifier in one room to effectively clean the other.
Example: Open Kitchen/Living/Dining
A common modern layout: 180 sq ft kitchen opens directly into a 300 sq ft living room, which opens into a 120 sq ft dining area. Total: 600 sq ft open space with 9-foot ceilings.
Volume: 600 × 9 = 5,400 cu ft Required CADR at 5 ACH: (5,400 × 5) ÷ 60 = 450 CFM
You'd need a single high-CADR unit (like the Coway Airmega 400 at 400 CFM or the Blueair Blue Pure 211i Max at 350 CFM) or two mid-range units placed at opposite ends of the space.
Example: L-Shaped Living Room
L-shaped rooms create airflow challenges because the bend blocks direct air circulation. Treat the two legs of the L as partially separate zones. If the total L-shaped room is 500 sq ft, you'll get better results from two 250 CFM units (one in each leg) than one 500 CFM unit in a corner.
Multiple Purifiers vs. One Large Unit
Sometimes two smaller units outperform one large unit, even with the same combined CADR.
Best practice for large rooms (500+ sq ft): Use two units placed at opposite ends of the room. This creates better air circulation and more uniform particle removal. Place one unit near the primary pollution source (kitchen, entry door, pet area) and one in the breathing zone (where you sit or sleep).
Real-World Sizing Examples
Example 1: Studio Apartment (450 Sq Ft, 8 ft Ceilings)
Lisa lives in a 450 sq ft studio with standard 8 ft ceilings. She has mild allergies and one cat.
Volume: 3,600 cu ft. Target ACH: 5 (allergies + one pet). Required CADR: (3,600 × 5) ÷ 60 = 300 CFM. Recommended CADR (with buffer): 390+ CFM.
Best pick: Coway Airmega 400 (CADR 400 smoke / 400 dust) — covers the entire studio on high, achieves target ACH on medium-high. She can place it centrally for uniform coverage.
Example 2: Two-Story Home (Main Living Areas)
Dave wants to purify his 2,200 sq ft two-story home. Downstairs: 700 sq ft open plan + 150 sq ft home office (with door). Upstairs: 300 sq ft master bedroom + 200 sq ft kid's room + 250 sq ft guest room.
He needs separate units per zone (walls and doors block airflow between rooms):
| Zone | Sq Ft | Volume (8 ft) | ACH Target | CADR Needed | Recommended Unit |
|---|---|---|---|---|---|
| Open plan (downstairs) | 700 | 5,600 | 4 | 373 CFM | 2× Blueair 211i Max |
| Home office | 150 | 1,200 | 5 | 100 CFM | Levoit Core 300S |
| Master bedroom | 300 | 2,400 | 6 | 240 CFM | Coway AP-1512HH |
| Kid's room | 200 | 1,600 | 6 | 160 CFM | Winix 5500-2 |
| Guest room | 250 | 2,000 | 4 | 133 CFM | Levoit Core 400S |
Total investment: 6 units covering 5 zones. This is realistic for whole-home portable purification.
Example 3: Basement Home Theater (500 Sq Ft, 7 ft Ceilings)
Mark has a 500 sq ft finished basement with 7 ft ceilings. The low ceilings actually help—less volume to filter.
Volume: 500 × 7 = 3,500 cu ft. Target ACH: 5 (general use, no specific concerns). Required CADR: (3,500 × 5) ÷ 60 = 292 CFM.
Basements often have higher humidity and more mold spore concerns. Mark should pair his purifier with a dehumidifier and aim for the higher end of CADR for mold prevention.
Example 4: Commercial Office (1,200 Sq Ft, 10 ft Ceilings)
A small business office with 1,200 sq ft and 10 ft commercial ceilings.
Volume: 12,000 cu ft. Target ACH: 5 (multiple occupants, COVID-era guidelines). Required CADR: (12,000 × 5) ÷ 60 = 1,000 CFM.
No single consumer unit delivers 1,000 CFM. Options: three Coway Airmega 400 units (400 CFM each = 1,200 combined), or a commercial unit like the Austin Air HealthMate Commercial (up to 400 CFM) supplemented with additional units.
Common Sizing Mistakes to Avoid
Mistake 1: Using the manufacturer's room size claim. Manufacturers often claim room coverage at just 2 ACH—barely useful. A unit "rated for 500 sq ft" by the manufacturer might only achieve 2 ACH in that space. Check the actual CADR number and do your own calculation.
Mistake 2: Ignoring doors. A purifier in the living room doesn't clean the bedroom. Closed doors create separate air zones. Even open doors only allow limited air exchange through the doorway.
Mistake 3: Not accounting for pollution level. A room that's generally clean needs 3–4 ACH. A room with smokers, heavy cooking, or multiple pets needs 6–8 ACH. Wildfire season? 8–12 ACH. The same room can need radically different CADR depending on conditions.
Mistake 4: Sizing for max speed. If you size your purifier so it only hits your target ACH on the highest speed, you'll run it on high 24/7. That means maximum noise and maximum energy use. Size for your target ACH at medium speed by buying a unit with 40–60% more CADR than the minimum.
Mistake 5: Forgetting ceiling height. This bears repeating: a 400 sq ft room with 10 ft ceilings has 25% more air volume than the same room with 8 ft ceilings. Standard charts that only reference square footage undersize for anything above 8 ft.
CADR vs. Manufacturer Claims: A Reality Check
The Coway Airmega 400 claims 1,560 sq ft coverage. At its 400 CFM CADR, it achieves just 2 ACH in that space—barely enough to notice. For meaningful filtration at 5 ACH, it covers 480 sq ft. That's a 225% gap between the marketing claim and practical performance. Always verify with the CADR number.
When to Size Up vs. Add a Second Unit
Size up (single larger unit) when:
- Your room is compact and roughly square
- You want simpler maintenance (one filter to replace)
- Noise from a single point is acceptable
- Budget is a primary concern
Add a second unit when:
- Your room is longer than 25 feet or L-shaped
- You want quieter operation (two units on low > one on high)
- You need redundancy (one fails, other still works)
- You're covering a large open plan (500+ sq ft)
- You want to address both particle and gas/VOC concerns with specialized units
Key Takeaways
- Use the formula: CADR = (Length × Width × Ceiling Height × Target ACH) ÷ 60
- Don't trust manufacturer room size claims—they often assume 2 ACH, which is insufficient
- Aim for 5 ACH for general allergy use, 6–8 ACH for smoke or asthma, 8–12 ACH for wildfire smoke
- Add 30–50% CADR buffer so you can run on medium speed instead of maximum
- Account for ceiling height—every foot above 8 ft adds 12.5% to your CADR requirement
- Open floor plans need combined square footage; closed rooms need individual units
- Two smaller units often outperform one large unit in rooms over 500 sq ft