CADR for a 400 sq ft room
Quick answer
The room is 400 square feet, which at a standard eight foot ceiling is 3,200 cubic feet of air to move. It needs about 240 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, 107 CFM for two changes an hour, and 320 CFM for smoke season at six. AHAM's two thirds convention gives 267 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 | 107 CFM | 69 min |
| Allergy households | 4.5 | 240 CFM | 31 min |
| Five changes, the two thirds convention | 5 | 267 CFM | 28 min |
| Smoke and wildfire season | 6 | 320 CFM | 23 min |
WINIX 5520 Air Purifier for Home Large Room Up to 1882 Ft2 in 1 Hr With Air Quality Monitor, True HEPA, High Deodorization Carbon Filter and Auto Mode, Captures Pet Allergies, Smoke, Dust.
This machine can be credited with about 502 CFM, derived from the listing's own figures: 1882 sq ft at 2 air changes an hour, assuming an eight foot ceiling. That clears the 240 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 | 2,800 cu ft | 93 CFM | 210 CFM | 280 CFM |
| 8 ft | 3,200 cu ft | 107 CFM | 240 CFM | 320 CFM |
| 9 ft | 3,600 cu ft | 120 CFM | 270 CFM | 360 CFM |
| 10 ft | 4,000 cu ft | 133 CFM | 300 CFM | 400 CFM |
| 12 ft | 4,800 cu ft | 160 CFM | 360 CFM | 480 CFM |
| If a listing says it covers | At this rate | That implies an airflow of |
|---|---|---|
| 400 sq ft | 1 air change an hour | 53 CFM |
| 400 sq ft | 2 air changes an hour | 107 CFM |
| 400 sq ft | 4 air changes an hour | 213 CFM |
| 400 sq ft | 5 air changes an hour | 267 CFM |
How much airflow does a 400 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 400 square feet at eight feet, four and a half changes an hour works out at 400 x 8 x 4.5 / 60, or 240 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 267 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 333 CFM for the same five changes, not 267. 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 400 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 240 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 400 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 - 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 400 square foot room?
About 240 CFM of clean air delivery rate for four and a half air changes an hour, or 107 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 3,200 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 240 CFM at eight feet and 300 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.