CADR for a 900 sq ft room
Quick answer
Start from the volume: 900 square feet at an eight foot ceiling is 7,200 cubic feet of air. It needs about 540 CFM of clean air delivery rate to reach four and a half air changes an hour, which is the range usually suggested where somebody in the house has allergies, 240 CFM for two changes an hour, and 720 CFM for smoke season at six. AHAM's two thirds convention gives 600 CFM for this room, which is the same arithmetic with an eight foot ceiling and five air changes an hour already filled in.
CADR figures come from the ANSI/AHAM AC-1 test method. The air change targets are guidance, not a standard.
| Target | Air changes an hour | Required CADR | Time to remove 90 percent |
|---|---|---|---|
| General air quality | 2 | 240 CFM | 69 min |
| Allergy households | 4.5 | 540 CFM | 31 min |
| Five changes, the two thirds convention | 5 | 600 CFM | 28 min |
| Smoke and wildfire season | 6 | 720 CFM | 23 min |
MOOKA Air Purifiers for Home Room up to 1076ft2, True HEPA Filter, B-D02L
This machine can be credited with about 861 CFM, derived from the listing's own figures: 1076 sq ft at 6 air changes an hour, assuming an eight foot ceiling. That clears the 540 CFM this room needs for four and a half air changes an hour.
Coverage claims vary in what air change rate they assume, so compare a published airflow figure rather than the room size on the box.
| Ceiling | Volume | CADR at 2 ACH | CADR at 4.5 ACH | CADR at 6 ACH |
|---|---|---|---|---|
| 7 ft | 6,300 cu ft | 210 CFM | 473 CFM | 630 CFM |
| 8 ft | 7,200 cu ft | 240 CFM | 540 CFM | 720 CFM |
| 9 ft | 8,100 cu ft | 270 CFM | 608 CFM | 810 CFM |
| 10 ft | 9,000 cu ft | 300 CFM | 675 CFM | 900 CFM |
| 12 ft | 10,800 cu ft | 360 CFM | 810 CFM | 1080 CFM |
| If a listing says it covers | At this rate | That implies an airflow of |
|---|---|---|
| 900 sq ft | 1 air change an hour | 120 CFM |
| 900 sq ft | 2 air changes an hour | 240 CFM |
| 900 sq ft | 4 air changes an hour | 480 CFM |
| 900 sq ft | 5 air changes an hour | 600 CFM |
How much airflow does a 900 sq ft room need?
The arithmetic is one line. Required CADR in cubic feet per minute equals floor area times ceiling height times the air changes you want, divided by sixty. For 900 square feet at eight feet, four and a half changes an hour works out at 900 x 8 x 4.5 / 60, or 540 CFM.
Nothing in that line is a rule of thumb. It is unit conversion: cubic feet of room, divided into minutes. The only judgement is the air change target, and that is where guidance rather than measurement comes in.
Why the two thirds rule gives the same number
AHAM publishes a shopper's convention: look for a smoke CADR of at least two thirds of the room's floor area. For this room that is 600 CFM. It is not a separate recommendation, it is this same calculation with two values already chosen for you, an eight foot ceiling and five air changes an hour, because 8 x 5 / 60 works out at 0.667.
That matters in two directions. If your ceiling is higher than eight feet, the convention quietly undersizes: at a ten foot ceiling this room needs 750 CFM for the same five changes, not 600. And if two changes an hour is genuinely enough for your situation, the convention oversizes by more than half.
Reading a coverage claim for a room this size
A listing that says it covers 900 square feet has told you almost nothing on its own, because the number depends entirely on the air change rate the manufacturer chose. The same machine is honestly described as covering a large room at one change an hour and a much smaller one at five.
Where a listing states both the area and the rate, the airflow follows by arithmetic and you can compare machines properly. Where it states only an area, the useful move is to treat it as a ceiling rather than a specification, and to look for a published CADR instead.
How long it takes to clear the air
Particle removal in a well mixed room is exponential decay, not a countdown. At 540 CFM in this room the machine delivers four and a half air changes an hour, which removes ninety percent of what is airborne in about 31 minutes and ninety nine percent in about 61 minutes.
That assumes the machine is the only removal path and that nothing new is arriving. During cooking or on a smoke day both assumptions fail, which is the real argument for sizing above the comfort target rather than at it.
When one machine is not the answer
If the required airflow is higher than anything a single unit publishes, two smaller machines in different parts of the room beat one that cannot do the job. Two units also mix the room better, because a single unit in a corner recirculates its own clean air and leaves the far side of the room largely alone.
The other honest answer is to make the room smaller. A door closed during the hours you care about, usually while sleeping, cuts the volume the machine has to serve and does it for free.
Machines that publish enough airflow for 900 sq ft
Every machine here has either a published CADR or a coverage claim that states the air change rate behind it, so the figure can be compared with the requirement above.
- MOOKA
MOOKA H13 HEPA Air Purifier for Home Pet Large Room Up to 1300ft2, Black
- HEPA claimed
- carbon stage
- has a UV lamp
$46.98 Check pricePrices change often - MOOKA
MOOKA Air Purifiers for Home Room up to 1076ft2, True HEPA Filter, B-D02L
- HEPA claimed
- carbon stage
$49.99 Check pricePrices change often - Afloia
Afloia Air Purifier for Large Rooms Up to 2,000 Sq. Ft. with PM2.5 Display
- carbon stage
- washable pre-filter
- particle sensor
- auto mode
$79.99 Check pricePrices change often
Common questions
What size air purifier do I need for a 900 square foot room?
About 540 CFM of clean air delivery rate for four and a half air changes an hour, or 240 CFM if two changes an hour is enough for your situation. The figure comes from floor area times ceiling height times air changes divided by sixty. With an eight foot ceiling this room holds 7,200 cubic feet, so the machine has to move that volume through a filter four and a half times each hour.
Is CADR the same as CFM?
They share a unit but they are not the same claim. CFM is how much air a fan moves. CADR is how much of that air comes out genuinely cleaned, measured under the ANSI/AHAM AC-1 test method in a sealed chamber. A machine can move plenty of air through a poor filter and post a high fan rating with a much lower CADR, which is why the tested figure is the one worth comparing.
Why does a purifier have three CADR numbers?
Because AHAM AC-1 measures three particle sizes separately: smoke, dust and pollen. Smoke is the smallest and usually produces the lowest number, so it is the one to compare when smoke is the reason you are buying. A single headline figure with no particle named is usually the highest of the three.
Does the ceiling height really change the answer?
Yes, proportionally. The calculation is about volume, not floor area, and the floor area only stands in for volume when the ceiling is the assumed eight feet. This room needs 540 CFM at eight feet and 675 CFM at ten for the same four and a half changes an hour, which is a difference big enough to change which machine you buy.
What if the room opens into a hallway?
Then the machine is serving a larger volume than the floor plan says, and it will fall short of the rate you calculated. Either size for the connected volume, close the door during the hours that matter, or accept a lower effective air change rate and know that is what you have chosen.