Air Purifier Room Size Guide
The room size number on an air purifier box is almost always misleading. Here is how to calculate the CADR you actually need — based on your room's dimensions, how you'll use the unit, and what you need it to achieve.
Why the Box Claims Mislead
Air purifier packaging typically states a room coverage figure — "Covers rooms up to 500 sq ft" or similar. This claim is based on achieving just 2 air changes per hour (ACH) — the minimum threshold for any measurable air quality improvement.
The problem: 2 ACH is inadequate for allergy relief, asthma management, smoke reduction, or meaningful PM2.5 reduction. The evidence base for these benefits is built on studies achieving 4–6 ACH.
AHAM (the air purifier standards body) publishes its own guideline: CADR should be at least ⅔ of the room's square footage, which delivers approximately 5 ACH. This is consistently higher than the box coverage claims — often by 2.5×.
Step-by-Step: Calculate Your Required CADR
Step 1: Measure your room
Length (ft) × Width (ft) = Square footage. For non-rectangular rooms, estimate the total floor area.
Step 2: Determine your target ACH
- General air quality improvement: 3 ACH
- Pollen and dust allergy: 4–5 ACH
- Pet dander or asthma: 5 ACH
- Smoke, wildfire, or serious respiratory concern: 5–6 ACH
Step 3: Calculate required CADR at full speed
CADR (CFM) = Room sq ft × Ceiling height (ft) × Target ACH ÷ 60
Example: 250 sq ft bedroom, 8 ft ceilings, target 5 ACH:
250 × 8 × 5 ÷ 60 = 167 CFM minimum CADR
Step 4: Account for actual fan speed
If you plan to run the purifier at medium speed (bedroom, night use), effective CADR is typically 50–60% of the rated maximum. Multiply your required CADR by 1.8–2× to get the headline CADR you should look for:
167 × 1.8 = ~300 CFM headline CADR needed for bedroom use on medium speed
Room-by-Room Recommendations
| Room | Typical size | Use case | Required CADR (full speed, 5 ACH) | Headline CADR needed (medium speed) |
|---|---|---|---|---|
| Small bedroom | 100–150 sq ft | Sleep, allergy | 67–100 CFM | 120–180 CFM |
| Standard bedroom | 150–220 sq ft | Sleep, allergy | 100–147 CFM | 180–265 CFM |
| Master bedroom | 220–320 sq ft | Sleep, allergy | 147–213 CFM | 265–385 CFM |
| Home office | 100–180 sq ft | General, VOC | 67–120 CFM | 120–215 CFM |
| Living room | 250–400 sq ft | General, smoke | 167–267 CFM | 300–480 CFM |
| Open-plan kitchen/living | 400–700 sq ft | Smoke, general | 267–467 CFM | 480–840 CFM |
| Basement | 300–600 sq ft | Mould, damp, smoke | 200–400 CFM | 360–720 CFM |
High Ceilings and Non-Standard Rooms
The calculations above assume standard 8 ft (2.4 m) ceilings. Higher ceilings increase room volume and therefore require higher CADR for the same ACH.
| Ceiling height | Volume multiplier vs 8 ft | CADR adjustment |
|---|---|---|
| 8 ft (standard) | 1.0× | Baseline |
| 9 ft | 1.12× | Increase CADR by 12% |
| 10 ft | 1.25× | Increase CADR by 25% |
| 12 ft (vaulted) | 1.5× | Increase CADR by 50% |
| 15 ft (loft/barn conversion) | 1.87× | Nearly double the CADR |
Multiple Rooms: One Unit or Two?
A common question: is it better to buy one large unit and move it between rooms, or two smaller units for the rooms you use most?
One large unit, moved between rooms: only cleans the room it's currently in. When moved to the bedroom at night, the living room is unfiltered. Practical for single-person households who occupy one room at a time.
Two correctly-sized units: better total coverage. A correctly sized bedroom unit running 24/7 on low, plus a living room unit that runs when the room is in use, provides better overall air quality than one large unit moved between spaces.
The most cost-effective approach for most households: buy the best unit your budget allows for the bedroom (where you spend 7–8 hours every night), and add a second unit for the living room when budget allows.
Matching Our Models to Common Rooms
| Model | Smoke CADR | Best room match (5 ACH, medium speed) |
|---|---|---|
| Levoit Core 300 | 145 CFM | Bedrooms up to ~130 sq ft at 5 ACH on medium |
| Coway AP-1512HH | 246 CFM | Bedrooms 130–200 sq ft; small living rooms |
| Winix 5500-2 | 232 CFM | Bedrooms 130–200 sq ft; small living rooms |
| Dyson TP07 | ~192 CFM | Bedrooms 100–175 sq ft at 5 ACH on medium |
| Blueair Blue Pure 211i Max | 350 CFM | Living rooms 200–300 sq ft; large bedrooms |
| Levoit Core 600S | 410 CFM | Living rooms 250–350 sq ft; open-plan spaces up to ~500 sq ft at 3 ACH |
See the full comparison table for noise levels and filter costs alongside CADR. For guidance on where to position a unit once you've chosen it, see our air purifier placement guide.
Sizing Mistakes and How to Avoid Them
- Buying the size for the room at max speed, then running on low — the most common mistake. Always calculate for your intended operating speed, not maximum output.
- Using the box coverage claim at face value — divide by 2.5 for meaningful allergy-level filtration, or calculate from CADR directly.
- Buying one unit for an open-plan space larger than 600 sq ft — at this scale, a single portable unit struggles to achieve 4 ACH regardless of CADR rating. Two units placed at opposite ends work significantly better than one central unit.
- Measuring room size but forgetting ceiling height — in rooms over 9 ft, the extra volume requires a proportionally higher CADR. A unit that's perfectly sized for a standard room will underperform in a high-ceiling space.
- Choosing a unit based on "Covers up to X sq ft" without checking CADR — coverage claims are not standardised and are based on minimal ACH. Only CADR figures from AHAM certification are reliable for comparison.
FAQ
Should I oversize my air purifier?
Modest oversizing — choosing a unit with 20–30% more CADR than your minimum requirement — is sensible. It gives you headroom to run at lower (quieter) speeds while still achieving target ACH, and the unit runs less intensively which reduces noise and extends filter life. Significant oversizing (2× the required CADR) wastes money on purchase price and provides diminishing returns in air quality benefit.
Can I use one large purifier for a whole house?
Not effectively. Air purifiers clean the air in the room they're in. A single unit in a hallway or central space cannot clean the air in closed rooms with any efficiency. For whole-home benefit, you need units in the rooms where people spend most time — typically the bedroom and the main living space.
Does room furniture affect required CADR?
Somewhat. Heavily furnished rooms with large upholstered items (sofas, carpets, curtains) have more surfaces that trap and re-release particles, requiring somewhat more air cycling to achieve the same particle concentration reduction. The effect is modest — typically a 10–15% additional CADR buffer is sufficient for well-furnished rooms.
What if my room has an unusual shape or is divided by a partition?
Calculate the total air volume of the space the purifier needs to serve. For rooms with large open doorways or partial partitions, treat the connected spaces as a single volume. For rooms with narrow doorways or full-height dividers, calculate each section separately. Generally, if air can circulate freely between two spaces, treat them as one room for sizing purposes.
Quick Reference: CADR Needed by Room Size
This table gives the minimum CADR at maximum speed to achieve 5 ACH (allergy-level) with standard 8 ft ceilings. Add 80% for bedroom use at medium speed (the typical night setting):
| Room size | CADR for 5 ACH at max speed | CADR for 5 ACH at medium speed | Recommended model |
|---|---|---|---|
| 100 sq ft | 67 CFM | 120 CFM | Levoit Core 300 |
| 150 sq ft | 100 CFM | 180 CFM | Levoit Core 300 |
| 200 sq ft | 133 CFM | 240 CFM | Coway AP-1512HH |
| 300 sq ft | 200 CFM | 360 CFM | Blueair Blue Pure 211i Max |
| 400 sq ft | 267 CFM | 480 CFM | Levoit Core 600S |
| 600 sq ft | 400 CFM | 720 CFM | Two units recommended |