Air purifier sizing for a basement

Researched from the AHAM AC-1 test method, EPA and CARB guidance, and published manufacturer specifications

Quick answer

A basement is sized at 4.5 air changes an hour against a 7 foot ceiling on this site, so a 700 square foot one needs about 368 CFM of clean air delivery. The table below runs from 300 to 1000 square feet.

Required CADR by floor area, 7 foot ceiling
Floor area	Volume	At 2 ACH	At 4.5 ACH	At 6 ACH
300 sq ft	2,100 cu ft	70 CFM	158 CFM	210 CFM
400 sq ft	2,800 cu ft	93 CFM	210 CFM	280 CFM
500 sq ft	3,500 cu ft	117 CFM	263 CFM	350 CFM
600 sq ft	4,200 cu ft	140 CFM	315 CFM	420 CFM
700 sq ft	4,900 cu ft	163 CFM	368 CFM	490 CFM
800 sq ft	5,600 cu ft	187 CFM	420 CFM	560 CFM
900 sq ft	6,300 cu ft	210 CFM	473 CFM	630 CFM
1,000 sq ft	7,000 cu ft	233 CFM	525 CFM	700 CFM

Meets 368 CFM, the extra large room class

WINIX 5510 Air Purifier (New Generation of 5500-2 with App Support) for Home Large Room Up to 1881 Ft2 in 1 Hr, True HEPA, High Deodorization Carbon Filter and Auto Mode, Captures Pet Allergies, Smoke

This machine can be credited with about 502 CFM, derived from the listing's own figures: 1881 sq ft at 2 air changes an hour, assuming an eight foot ceiling. That clears the 368 CFM this room calls for.

$179.99Check pricePrices change often

Check the noise at the speed that delivers the airflow, not the figure quoted for the lowest setting.

Every size in this room type

300 sq ft 158 CFM needed
400 sq ft 210 CFM needed
500 sq ft 263 CFM needed
600 sq ft 315 CFM needed
700 sq ft 368 CFM needed
800 sq ft 420 CFM needed
900 sq ft 473 CFM needed
1000 sq ft 525 CFM needed

What makes a basement different

The usual sources in this room are mould spores where humidity has been allowed to sit high, musty odour compounds, radon, which no filter removes. 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 basement with a musty smell has a moisture problem, and a purifier treats the symptom. Getting relative humidity under sixty percent with a dehumidifier is what stops mould growing in the first place, and it is the order of operations that matters: dry the space, then filter it.

Which air change target this room is sized at

This section sizes a basement at 4.5 air changes an hour, the allergy and asthma households case, which suits pollen, pet dander or dust mite debris, where somebody in the house notices the difference. 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 4.5 multiplies the airflow you need by 2.3, 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 H13 HEPA Air Purifier for Home Pet Large Room Up to 1300ft2, Black
- HEPA claimed
- carbon stage
- has a UV lamp
$46.98 Check price
Prices change often
MOOKA
MOOKA Air Purifiers for Home Room up to 1076ft2, True HEPA Filter, B-D02L
- HEPA claimed
- carbon stage
$49.99 Check price
Prices change often
MOOKA
MOOKA Air Purifiers for Home Room up to 1076ft2, True HEPA Filter, B-D02L
- HEPA claimed
- carbon stage
$49.99 Check price
Prices 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 price
Prices change often
Winix
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.
- HEPA claimed
- particle sensor
- auto mode
- app or voice
$179.99 Check price
Prices change often
Instant
Instant HEPA Quiet Air Purifier From the Makers of Instant Pot with Plasma Ion Technology for Rooms up to 1140ft2, removes 99% of Dust, Smoke, Odors, Pollen & Pet Hair, for Bedrooms, Offices, Pearl
- carbon stage
- auto mode
- has an ionizer
$199.99 Check price
Prices change often

Common questions

What size purifier does a basement need?

Multiply the floor area by the ceiling height, multiply by the air changes an hour you want, and divide by sixty. At 7 feet and 4.5 changes an hour, a 400 square foot basement needs about 210 CFM and a 1000 square foot one needs about 525 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.