CADR for a 175 sq ft room
Quick answer
A 175 square foot room with an eight foot ceiling holds 1,400 cubic feet of air. It needs about 105 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, 47 CFM for two changes an hour, and 140 CFM for smoke season at six. AHAM's two thirds convention gives 117 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 | 47 CFM | 69 min |
| Allergy households | 4.5 | 105 CFM | 31 min |
| Five changes, the two thirds convention | 5 | 117 CFM | 28 min |
| Smoke and wildfire season | 6 | 140 CFM | 23 min |
Pet Air Purifier for Home Large Room 2500 Sq Ft, H13 HEPA Double-Sided Filter with Washable Pre-filter, Adsorbs Pet Hair Dander Odor Allergies Smoke Dust,PM2.5 Air Quality Monitor,Auto Mode,P358 Grey
This machine can be credited with about 176 CFM, the listing states this airflow directly. That clears the 105 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 | 1,225 cu ft | 41 CFM | 92 CFM | 123 CFM |
| 8 ft | 1,400 cu ft | 47 CFM | 105 CFM | 140 CFM |
| 9 ft | 1,575 cu ft | 53 CFM | 118 CFM | 158 CFM |
| 10 ft | 1,750 cu ft | 58 CFM | 131 CFM | 175 CFM |
| 12 ft | 2,100 cu ft | 70 CFM | 158 CFM | 210 CFM |
| If a listing says it covers | At this rate | That implies an airflow of |
|---|---|---|
| 175 sq ft | 1 air change an hour | 23 CFM |
| 175 sq ft | 2 air changes an hour | 47 CFM |
| 175 sq ft | 4 air changes an hour | 93 CFM |
| 175 sq ft | 5 air changes an hour | 117 CFM |
How much airflow does a 175 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 175 square feet at eight feet, four and a half changes an hour works out at 175 x 8 x 4.5 / 60, or 105 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 117 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 146 CFM for the same five changes, not 117. 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 175 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 105 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 175 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
Common questions
What size air purifier do I need for a 175 square foot room?
About 105 CFM of clean air delivery rate for four and a half air changes an hour, or 47 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 1,400 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 105 CFM at eight feet and 131 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.