Multiply your room’s floor area by 0.667 and you have a usable minimum smoke CADR. That shortcut is accurate for a rectangular room with an eight-foot ceiling, and wrong for everything else — which is most rooms people actually want to treat.
Sizing from volume instead takes one extra minute and survives tall ceilings, sloped ceilings, open-plan spaces and L-shaped rooms. Here is the method, and then four worked examples.
The volume method in three steps
Step one: get the volume in cubic feet. Length times width times ceiling height. For a sloped or vaulted ceiling, use the average of the lowest and highest points — that is close enough for this purpose. For an L-shaped room, split it into two rectangles and add them.
Step two: pick a target air change rate. This is the number of times per hour you want the room’s full volume passed through the filter. More on choosing it below.
Step three: convert to CADR.
Required CADR (cfm) = volume (cu ft) × target air changes per hour ÷ 60
The division by 60 is just converting per-hour to per-minute, because CADR is quoted in cubic feet per minute.
Choosing a target air change rate
This is the judgment call, and it is where the honest answer is a range rather than a single figure.
The two-thirds floor-area rule works out to roughly five air changes per hour in an eight-foot room, so five is a reasonable general-purpose target and the one that keeps you consistent with the published guidance. Below about three, a purifier struggles to stay ahead of a room that is being re-polluted by ordinary household activity. Above about eight, you are usually buying fan noise you will switch off.
| Situation | Target air changes per hour | Why |
|---|---|---|
| General dust and background particles | 4–5 | Matches the standard sizing guidance |
| Allergy to pollen or pet dander | 5–6 | Dander is replenished continuously while the pet is in the room |
| Wildfire smoke or outdoor smoke infiltration | 6–8 | Infiltration continues all day; AHAM raises its own guidance for smoke |
| Bedroom, running overnight only | 4–5, oversized unit | Door closed, low occupancy, but you need it quiet |
For wildfire smoke specifically, AHAM’s own recommendation is a smoke CADR equal to the room’s square footage, which is a 50% increase on the general rule. In volume terms that is about seven and a half air changes an hour at an eight-foot ceiling.
Four worked examples
1. Standard bedroom, eight-foot ceiling
11 by 13 feet, 8-foot ceiling. Volume is 11 × 13 × 8 = 1,144 cubic feet.
At five air changes: 1,144 × 5 ÷ 60 = 95 cfm minimum smoke CADR.
The floor-area shortcut gives 143 × 0.667 = 95. Identical, because this is exactly the room the shortcut was built for.
2. Same floor area, ten-foot ceiling
Volume is 11 × 13 × 10 = 1,430 cubic feet.
At five air changes: 1,430 × 5 ÷ 60 = 119 cfm.
The shortcut still says 95, so it has under-sized the room by a quarter. This is the case AHAM is pointing at when it advises choosing a unit rated for a larger room if ceilings exceed eight feet.
3. Open-plan living and dining area
A 15 by 20 foot living area opening onto a 10 by 12 foot dining area, both at 9 feet. Volume is (300 + 120) × 9 = 3,780 cubic feet.
At five air changes: 3,780 × 5 ÷ 60 = 315 cfm.
Few consumer units deliver a smoke CADR that high, which is the real finding here. Two units of about 160 cfm each, placed in the two halves, is usually easier to buy, quieter in use, and more effective than one machine trying to pull air across a 35-foot span.
4. Bedroom during wildfire season
The 11 by 13 foot bedroom again, 8-foot ceiling, but smoke is the reason you are buying.
AHAM’s smoke guidance: CADR equal to floor area, so 143 cfm. In volume terms, 143 × 60 ÷ 1,144 = 7.5 air changes an hour.
Why you should still oversize after doing the arithmetic
CADR is typically measured at the unit’s highest fan speed. If your calculation says 95 cfm and you buy a unit rated at exactly 95, you get 95 only when the fan is flat out — which, in a bedroom, you will not tolerate for more than one night.
So treat the calculated figure as the delivery you need at the speed you will actually run, and buy accordingly. In practice that means picking a unit rated 1.5 to 2 times your calculated minimum, then running it on low or medium. You get the airflow you sized for, at a noise level you can sleep through, and the filter lasts longer because the fan is working less hard.
Where a manufacturer publishes airflow or CADR per speed setting, use that table and buy for the medium figure. It is the single most useful specification most brands bury.
Three things that break the arithmetic
An open door. Every calculation here assumes a closed room. An open door to a hallway effectively adds the hallway’s volume and whatever flows in from the rest of the house, and no portable unit is sized for that. If the door has to stay open, size for the combined space or accept that you are treating a zone rather than a room.
A continuous source. Air change rate tells you how fast the unit clears a given quantity of particles. It says nothing about a source that keeps adding them — cooking on a gas burner with no extraction, a candle, a cat on the bed. With an active source the room reaches an equilibrium, not zero, and the equilibrium concentration depends on the source rate as much as on the CADR.
Placement. A unit pushed into a corner, under a shelf, or behind furniture is drawing from and discharging into the same small pocket of air. Leave clearance on the intake and outlet faces, and put it where room air can circulate past it rather than where it is least visible.
Quick reference
| Room volume | CADR at 4 ACH | CADR at 5 ACH | CADR at 7 ACH |
|---|---|---|---|
| 800 cu ft | 53 | 67 | 93 |
| 1,200 cu ft | 80 | 100 | 140 |
| 1,600 cu ft | 107 | 133 | 187 |
| 2,400 cu ft | 160 | 200 | 280 |
| 3,200 cu ft | 213 | 267 | 373 |
| 4,000 cu ft | 267 | 333 | 467 |
Read the figure, then shop for a unit rated well above it so you can run the fan down.
Related guides
- How to choose an air purifier: the full guide
- CADR explained: what the three numbers mean
- True HEPA vs HEPA-type
Sources: AHAM Verifide air filtration standards for the two-thirds rule, the ceiling-height adjustment and the wildfire smoke recommendation; EPA, Guide to Air Cleaners in the Home (2nd edition, 2018) for the sizing chart and the note that CADR is measured at highest speed. All volume and air-change calculations and the tables above are our own, derived from those figures.