Enter the room and what the air is for. You get the smoke CADR AHAM's rule asks for, the room size to look for on the box, the air changes an hour it gives, and how many ENERGY STAR certified purifiers reach it.
Your air purifier
A smoke CADR of at least 120 for your 180 sq ft roomAHAM's rule: two-thirds of the floor area · 5 air changes an hour · 175 of 247 ENERGY STAR purifiers reach it
Quick answer: AHAM, which certifies air purifier ratings, says the smoke CADR should be at least two-thirds of the room’s floor area. A 10 × 12 ft room (120 sq ft) wants a smoke CADR of at least 80; a 12 × 15 ft room (180 sq ft), 120. For wildfire smoke, AHAM asks a CADR equal to the floor area. Those figures assume an 8-foot ceiling: a taller room needs more.
How to use the air purifier size calculator
- Measure the room: its length, its width and the ceiling height.
- Say what it is for: everyday air (dust, pollen, pets, cooking) or wildfire smoke.
- Read the answer: the smoke CADR to look for, the room size its AHAM label will show, the air changes an hour it gives, and how many ENERGY STAR certified purifiers reach it. Have one already? Type its smoke CADR to check it.
Damp as well as dusty? The dehumidifier size calculator sizes one from ENERGY STAR’s chart.
The rules, and where they come from
CADR. AHAM’s Clean Air Delivery Rate “indicates the volume of filtered air an air cleaner delivers, with separate scores for tobacco smoke, pollen and dust”. It is in cubic feet a minute, and the label on the box shows all three.
The two-thirds rule. AHAM: “As a rule of thumb, the CADR of your air cleaner should be equal to at least two-thirds of the room’s area. For example, a room with the dimensions of 10 feet by 12 feet has an area of 120 square feet. It would be best to have an air cleaner with a smoke CADR of at least 80.”
Wildfire smoke. “For wild fire smoke, AHAM recommends a Smoke CADR equal to the size of the room in square feet.”
The label’s room size. AHAM’s testing FAQ: “Room Size (square feet – ft2) = cigarette smoke CADR x 1.55”, the room in which the purifier cuts smoke particles by 80%. AHAM says its certified purifiers give “4.8 ACH for their suggested room size”, above “the minimum ventilation recommendation”, three air changes an hour.
Tall ceilings. “If your ceiling height is higher than 8 feet, the square foot coverage of the air cleaner will be less than the room size shown.” The calculator raises the CADR in step with the height, which keeps the same air changes an hour as at 8 feet; that step is our arithmetic, not AHAM’s.
Smoke CADR by room size
| Room, 8 ft ceiling | Everyday: smoke CADR | Wildfire smoke: smoke CADR | ENERGY STAR purifiers that reach the everyday CADR |
|---|---|---|---|
| 10 × 10 ft (100 sq ft) | 67 | 100 | 219 |
| 10 × 12 ft (120 sq ft) | 80 | 120 | 208 |
| 12 × 12 ft (144 sq ft) | 96 | 144 | 200 |
| 12 × 15 ft (180 sq ft) | 120 | 180 | 175 |
| 14 × 16 ft (224 sq ft) | 149 | 224 | 142 |
| 15 × 20 ft (300 sq ft) | 200 | 300 | 115 |
| 20 × 20 ft (400 sq ft) | 267 | 400 | 82 |
| 20 × 25 ft (500 sq ft) | 333 | 500 | 48 |
| 25 × 30 ft (750 sq ft) | 500 | 750 | 6 |
Of 247 ENERGY STAR certified room air cleaners listed on 6 October 2026, with smoke CADRs from 31 to 577.
Worked examples
A smoke CADR of at least 120 for your 180 sq ft room: a 12 × 15 ft bedroom with an 8 ft ceiling. On the box, look for an AHAM room size of 186 sq ft or more.
A smoke CADR of at least 180 for your 180 sq ft room: the same room in wildfire season, when AHAM asks a CADR as big as the floor.
A smoke CADR of at least 150 for your 180 sq ft room: a 10 ft ceiling holds a quarter more air than an 8 ft one, so the CADR goes up by a quarter.
A smoke CADR of 100 is too small for your 180 sq ft room: a purifier rated 100 gives about 4.2 air changes an hour here, short of the 120 the two-thirds rule asks.
Common mistakes
- Buying by the box’s biggest number. AHAM: a purifier “too large for the room” means “unnecessary energy use and noise”; size it to the room.
- Using the pollen or dust CADR to size it. AHAM’s rule and the label’s room size use the smoke CADR, the smallest particles.
- Forgetting the ceiling. A vaulted room holds more air than its floor suggests.
- Hiding it in a corner. AHAM: place it “closer to the center of the room, away from anything that may block or obstruct the air flow”.
More on fresh rooms at Dreamy Home Style
- Living room plants for a fresh atmosphere
- Bathroom plants for a healthier home
- Chill room ideas
- Low-light houseplants
Planning the room itself? Draw it to scale in the free 2D floor planner.
Air purifier size questions
What size air purifier do I need?
One whose smoke CADR is at least two-thirds of the room’s floor area, AHAM’s rule: 80 for a 120 sq ft room, 120 for 180 sq ft, 200 for 300 sq ft, with an 8 ft ceiling.
How do I size an air purifier by room size?
Multiply the room’s length by its width, then take two-thirds: that is the smoke CADR to look for. On the box, the AHAM label’s room size should be at least your room’s.
What is a CADR calculator?
It turns a room into the Clean Air Delivery Rate a purifier needs, or checks a purifier’s CADR against a room. Here it uses AHAM’s own rules and ENERGY STAR’s certified list.
What air purifier do I need for a large room?
For 400 sq ft, a smoke CADR of 267, which 82 of the 247 ENERGY STAR certified purifiers reach; for 750 sq ft, 500, which 6 reach. Past the largest, 577, use two.
How many air changes an hour does an air purifier give?
CADR × 60 ÷ the room’s volume. The two-thirds rule gives 5 an hour with an 8 ft ceiling; AHAM’s labels are tested at 4.8.
What CADR do I need for wildfire smoke?
AHAM recommends a smoke CADR equal to the room’s floor area: 180 for a 12 × 15 ft room.