Air purifier sizing for a classroom
Quick answer
A classroom is sized at 6 air changes an hour against a 9 foot ceiling on this site, so a 800 square foot one needs about 720 CFM of clean air delivery. The table below runs from 400 to 1200 square feet.
| Floor area | Volume | At 2 ACH | At 6 ACH | At 6 ACH |
|---|---|---|---|---|
| 400 sq ft | 3,600 cu ft | 120 CFM | 360 CFM | 360 CFM |
| 500 sq ft | 4,500 cu ft | 150 CFM | 450 CFM | 450 CFM |
| 600 sq ft | 5,400 cu ft | 180 CFM | 540 CFM | 540 CFM |
| 700 sq ft | 6,300 cu ft | 210 CFM | 630 CFM | 630 CFM |
| 800 sq ft | 7,200 cu ft | 240 CFM | 720 CFM | 720 CFM |
| 900 sq ft | 8,100 cu ft | 270 CFM | 810 CFM | 810 CFM |
| 1,000 sq ft | 9,000 cu ft | 300 CFM | 900 CFM | 900 CFM |
| 1,200 sq ft | 10,800 cu ft | 360 CFM | 1080 CFM | 1080 CFM |
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 720 CFM this room calls for.
Check the noise at the speed that delivers the airflow, not the figure quoted for the lowest setting.
Every size in this room type
- 400 sq ft 360 CFM needed
- 500 sq ft 450 CFM needed
- 600 sq ft 540 CFM needed
- 700 sq ft 630 CFM needed
- 800 sq ft 720 CFM needed
- 900 sq ft 810 CFM needed
- 1000 sq ft 900 CFM needed
- 1200 sq ft 1080 CFM needed
What makes a classroom different
The usual sources in this room are many people in one room, dust from paper, chalk and carpet, outdoor air quality on smoke days. That mix is what decides whether the carbon stage matters as much as the particle stage, and it is why the same square footage in two different rooms does not always call for the same machine.
A room full of people is the case where the arrival rate is highest and the target should be too, which is why classroom guidance sits at the top of the range rather than the middle. Portable units also need somewhere to sit where nobody trips on the cord, and that is often what decides how many will fit.
Which air change target this room is sized at
This section sizes a classroom at 6 air changes an hour, the smoke and wildfire season case, which suits wildfire smoke, a woodstove that backpuffs, or smoke drifting in from outside. Two changes an hour is the comfort baseline and six is where smoke guidance sits.
The target is guidance rather than a measured threshold, so it is worth knowing what moving it does. Going from two changes an hour to 6 multiplies the airflow you need by 3, and the noise and the filter cost move with it.
Machines sized for this room
Each of these publishes either a CADR or a coverage claim with the air change rate behind it, so it can be compared against the requirement above.
- 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 purifier does a classroom need?
Multiply the floor area by the ceiling height, multiply by the air changes an hour you want, and divide by sixty. At 9 feet and 6 changes an hour, a 500 square foot classroom needs about 450 CFM and a 1200 square foot one needs about 1080 CFM.
Should I buy one large unit or two smaller ones?
Two smaller machines usually mix a room better than one large one, because a single unit recirculates its own clean air in the corner it sits in. One larger machine is quieter for the same total airflow and costs less to keep in filters. If the room is long or L shaped, two is the better answer more often than the arithmetic alone suggests.
Does the door being open change the sizing?
It does, and by more than people expect. An open door connects the room to a hallway and often to a stairwell, and the machine is then serving that whole volume rather than the room on the floor plan. Either size for the connected space or close the door during the hours that matter.
Is a higher air change rate always better?
No. Airflow costs noise, power and filter life, and past a point the room is already as clean as the filter can make it. The honest ceiling is the arrival rate of whatever you are removing: on a smoke day more is genuinely better, and on an ordinary day in an ordinary house it is not.
Do these figures assume anything about the filter?
They assume the machine delivers its rated clean air delivery rate, which a clogged filter does not. A filter at the end of its life moves less air at the same fan speed, so the effective air change rate drops quietly over months without the machine sounding any different.