How to choose the right air purifier size
The single most useful number on an air purifier box is the CADR – Clean Air Delivery Rate. It tells you how many cubic feet of filtered air the unit delivers per minute (cfm) on its highest setting. CADR is measured separately for three particle types: tobacco smoke (the smallest particles), dust and pollen (the largest). Because smoke is the hardest test, its CADR is the most conservative number to compare, and it is the one this calculator uses.
A purifier that is too small will run loud on high speed all day and still leave the air dirty. One that is far too large wastes money and power. The goal is to match the CADR to your room’s air volume and to how clean you need the air to be.
The formula
The calculator multiplies the volume of your room by the number of air changes per hour you need, then divides by 60 to convert hours into minutes:
Required CADR (cfm) = Length × Width × Ceiling height (ft) × ACH ÷ 60
For example, a 15 × 12 ft bedroom with an 8 ft ceiling has 1,440 ft³ of air. For allergies (4 ACH) you need 1,440 × 4 ÷ 60 = 96 cfm, rounded up. Pets, open doorways and smoke add a safety margin because they constantly bring new particles into the room. In metric units the same rule gives the airflow in m³/h: volume (m³) × ACH.
What the “room size” on the box means
Manufacturers often print a recommended room size, usually based on the AHAM “two-thirds rule”: the smoke CADR should be at least two-thirds of the room’s floor area in square feet. That rule assumes an 8 ft ceiling and works out to about 5 air changes per hour, which is a good target for allergy relief. If your ceiling is higher or your goal is stricter, the box rating is too optimistic. That is why the calculator also shows an adjusted rating to look for.
Some brands advertise a much larger area based on just one or two air changes per hour. Always check how the number was calculated, or simply compare smoke CADR values.
Typical CADR by room size (8 ft ceiling, AHAM two-thirds rule ≈ 5 ACH)
| Room | Floor area | Minimum smoke CADR |
|---|---|---|
| Small bedroom / office | 100 ft² (9 m²) | 67 cfm |
| Bedroom | 150 ft² (14 m²) | 100 cfm |
| Master bedroom | 250 ft² (23 m²) | 167 cfm |
| Living room | 350 ft² (33 m²) | 233 cfm |
| Large living area | 500 ft² (46 m²) | 333 cfm |
| Open-plan floor | 800 ft² (74 m²) | 533 cfm (often two units) |
Choosing the filter type
- True HEPA (H13 or better) captures at least 99.97% of particles at 0.3 microns, the hardest size to catch. It is the right choice for allergies, pet dander, dust, mold spores and smoke particles. Avoid “HEPA-type” or “HEPA-like” filters, which have no standard behind them.
- Activated carbon absorbs gases and odors: cooking smells, tobacco smoke odor, paint fumes and other VOCs. HEPA alone does not remove smells. For smoke or odors, look for a thick carbon bed (hundreds of grams, not a thin carbon-dusted sheet).
- Pre-filter catches hair and lint so the expensive HEPA filter lasts longer. A washable pre-filter is a big plus in homes with pets.
- Avoid ozone generators. Ozone irritates the lungs. Ionizers and “plasma” features may emit some ozone; if you buy one, look for certification that ozone emissions are within safety limits (for example, CARB certification in California) and keep the ionizer switched off if you can.
Noise, placement and running cost
CADR is measured on the highest fan speed, which is usually loud (50–60 dB). Most people run purifiers on a lower, quieter speed, especially at night. If the purifier is for a bedroom, buy one with about 30–50% more CADR than you need so it can clean the room on a quiet setting (around 30–40 dB). Place it where air can flow freely: at least a foot from walls and furniture, not behind a sofa or in a corner, and ideally near where you sleep or sit.
Running costs are small but not zero. A typical 200 cfm purifier uses 30–50 W on high and much less on low speeds. Filters are the bigger expense: HEPA filters usually last 6–12 months and carbon filters 3–6 months, depending on hours of use and how dirty the air is. Check filter prices before you buy, because a cheap purifier with expensive filters can cost more over three years.
One big purifier or two smaller ones?
For rooms that need more than about 400 cfm, or for L-shaped and open-plan spaces, two mid-size units placed at opposite ends usually clean the air more evenly than one large unit. They also run quieter, because each fan can stay on a lower speed.