What size air purifier do I need? (CADR explained)
Short answer: Match the air purifier's smoke CADR to your room. AHAM's rule is a smoke CADR of at least two-thirds of the floor area, so a 200 sq ft room needs about 133 cfm (AHAM's own example says at least 132). EPA's chart is nearly the same (130 cfm for 200 sq ft) and assumes an 8-foot ceiling. Go higher for taller ceilings.
What does CADR mean on an air purifier?
CADR stands for clean air delivery rate. It is AHAM’s rating for how fast a portable air cleaner filters particles. It is given in cubic feet per minute (cfm).
Many boxes list three CADR numbers: tobacco smoke, dust, and pollen. EPA explains that these stand in for small, medium, and large particles. For small particles, EPA says to choose a unit with a high smoke CADR.
AHAM’s sizing rule is based on the smoke number, so use that one.
How do I calculate the right CADR for my room?
Measure the floor area: length times width, in square feet. Then apply AHAM’s rule:
Minimum smoke CADR (cfm) = room area (sq ft) × 2/3
AHAM’s own example: a 200 sq ft room needs a smoke CADR of at least 132. (Exact two-thirds of 200 is 133.3.)
EPA publishes a sizing chart that works out to 0.65 cfm per square foot. It assumes an 8-foot ceiling. The two methods land within about 3% of each other.
| Room area (sq ft) | EPA chart (0.65 × area) | AHAM 2/3 rule |
|---|---|---|
| 100 | 65 cfm | 67 cfm |
| 150 | 98 cfm | 100 cfm |
| 200 | 130 cfm | 133 cfm |
| 250 | 163 cfm | 167 cfm |
| 300 | 195 cfm | 200 cfm |
| 350 | 228 cfm | 233 cfm |
| 400 | 260 cfm | 267 cfm |
| 450 | 293 cfm | 300 cfm |
| 500 | 325 cfm | 333 cfm |
EPA’s chart lists only the 100-sq-ft steps. The 50-sq-ft rows are the same formula applied in between.
What the 2/3 rule means in air changes
At an 8-foot ceiling, a CADR of 2/3 × area gives about 5 clean-air changes per hour. The math: (2/3 × area × 60 minutes) ÷ (area × 8 ft) = 5. That is simple arithmetic, not a separate standard.
What if my ceiling is higher than 8 feet?
EPA says to consider a unit sized for a larger area when ceilings are above 8 feet. A simple way is to scale by height.
Adjusted CADR = (area × 2/3) × (ceiling height ÷ 8)
Example: a 300 sq ft room with a 10-foot ceiling. 200 cfm × (10 ÷ 8) = 250 cfm.
A higher CADR is fine. It cleans faster and can run on a quieter low speed.
How loud and how costly is an air purifier to run?
EPA notes that noise depends partly on fan speed. For reference, it says about 50 dB is roughly the sound of a modern refrigerator. Check the decibel rating at the speed you will actually use, especially for bedrooms.
The ENERGY STAR label means the unit meets EPA efficiency rules. EPA stresses it says nothing about how well the unit cleans air. So compare CADR first, then efficiency.
Running cost depends on the wattage on the spec sheet. As an example, assume a unit drawing 40 watts, running 24 hours a day. That is 0.96 kWh per day, or 28.8 kWh per 30 days. EIA’s Short-Term Energy Outlook projects an average U.S. residential price of about 18.2 cents per kWh for 2026.
| Electricity rate | Cost per 30 days (40 W, 24/7) | Cost per year (365 days) |
|---|---|---|
| 12¢/kWh | $3.46 | $42.05 |
| 15¢/kWh | $4.32 | $52.56 |
| 18¢/kWh | $5.18 | $63.07 |
| 25¢/kWh | $7.20 | $87.60 |
| 30¢/kWh | $8.64 | $105.12 |
Your unit may draw more or less. Formula: watts × hours ÷ 1,000 × price per kWh. Replacement filters add to the cost, so check their price before you buy.
Are ozone generators or ionizers safe?
EPA says ozone is a lung irritant. It advises avoiding air cleaners and HVAC filters that intentionally produce ozone. Ozone generators sold as air cleaners do exactly that. EPA notes no federal agency has approved them for use in occupied spaces.
EPA also says some electrostatic precipitators, ionizers, plasma units, and UV lights without proper lamp coatings may emit ozone. A mechanical HEPA filter avoids this issue. EPA notes portable units often reach a high CADR by using a HEPA filter.
Is a portable purifier enough on its own?
A portable unit cleans one room. For the whole house, your HVAC filter does the bulk of the work while the system runs. See how often to change your HVAC filter.
Keep expectations modest. EPA says studies with portable HEPA cleaners found improvements in one or more allergy or asthma symptoms, but not always major ones or in every symptom. Cardiovascular and respiratory gains were typically small.
Quick answers
What CADR do I need for a 300 sq ft room? About 195-200 cfm smoke CADR with an 8-foot ceiling. Go to roughly 250 cfm if the ceiling is 10 feet.
Which CADR number should I use: smoke, dust, or pollen? Use the smoke CADR. EPA recommends a high smoke CADR for removing small particles.
Does ENERGY STAR mean an air purifier works better? No. EPA says the label covers energy efficiency only, not air-cleaning performance.
Can I buy a purifier rated for a bigger room? Yes. A higher CADR cleans faster and can run on a quieter low setting.
Are ozone air purifiers safe? EPA advises avoiding devices that intentionally make ozone, because ozone irritates the lungs.