CFM Calculator: How Many CFM Do You Need for Your Room?

How many CFM do you need? It depends on what the airflow is for. An air purifier is sized by how many times an hour it cleans the room’s air, a bathroom fan by floor area or fixtures, and a range hood by a fixed minimum. This calculator covers all three: enter the room, pick the job and it shows the cubic feet per minute (CFM) to look for, plus the source behind the number.

You can also run it the other way round: enter a CFM and see how many air changes per hour it gives the room.

How the calculator works

The core formula is the one CDC/NIOSH uses for air changes:

CFM = room volume (cu ft) × air changes per hour ÷ 60

Air changes per hour = CFM × 60 ÷ room volume (cu ft)

Room volume is length × width × ceiling height. Metric sizes are converted to feet first, because airflow ratings in the US are published in cubic feet per minute; the result is also shown in cubic meters per hour (1 cfm = 1.699 m³/h, the conversion AHAM uses). For “how many CFM do I need”, the result is rounded up to the next whole cfm.

The presets

Job	Target	Source
Air purifier, everyday	4.8 air changes per hour	AHAM: the room size on the box is based on 4.8
Air purifier, CDC	5 air changes per hour	CDC/NIOSH “aim for 5” for occupied rooms
Air purifier, wildfire smoke	7.5 air changes per hour	AHAM: smoke CADR at least equal to the floor area (8 ft ceiling)
Bathroom fan, up to 100 sq ft	1 cfm per sq ft, at least 50 cfm	HVI (1 cfm/sq ft); ASHRAE 62.2 (50 cfm minimum)
Bathroom fan, over 100 sq ft	50 cfm per toilet, shower or tub; 100 cfm per jetted tub	HVI
Kitchen, vented range hood	100 cfm	ASHRAE 62.2
Kitchen, other exhaust fan	300 cfm, or 5 air changes per hour in an enclosed kitchen	ASHRAE 62.2
Kitchen, continuous exhaust	5 air changes per hour of the kitchen’s volume	ASHRAE 62.2 (enclosed kitchens only)

For the bathroom, the calculator takes the larger of HVI’s figure and ASHRAE’s 50 cfm. A bathroom fan that runs continuously instead of being switched on needs only 20 cfm under ASHRAE 62.2.

Worked examples

All rooms have 8 ft ceilings unless noted.

Room	Job	Calculation	Result
Bedroom, 12 x 12 ft (1,152 cu ft)	Air purifier, everyday	1,152 × 4.8 ÷ 60 = 92.2	93 cfm smoke CADR
Same bedroom	Air purifier, CDC	1,152 × 5 ÷ 60	96 cfm
Same bedroom	Air purifier, wildfire smoke	1,152 × 7.5 ÷ 60	144 cfm (equal to the 144 sq ft floor, as AHAM’s rule says)
Living room, 20 x 15 ft, 9 ft ceiling (2,700 cu ft)	Air purifier, everyday	2,700 × 4.8 ÷ 60	216 cfm
Same living room	Air purifier, wildfire smoke	2,700 × 7.5 ÷ 60 = 337.5	338 cfm
Bedroom, 5 x 4 m, 2.5 m ceiling (50 m³ = 1,766 cu ft)	Air purifier, CDC	1,766 × 5 ÷ 60 = 147.1	148 cfm (251 m³/h)
Bathroom, 8 x 5 ft (40 sq ft)	Bathroom fan	HVI 40 cfm; ASHRAE minimum 50	50 cfm
Bathroom, 9 x 10 ft (90 sq ft)	Bathroom fan	HVI 1 cfm × 90 sq ft	90 cfm
Bathroom, 20 x 12 ft, tub, shower and toilet	Bathroom fan	50 + 50 + 50	150 cfm
Kitchen, 12 x 10 ft (960 cu ft), enclosed	Kitchen	Range hood minimum; 960 × 5 ÷ 60 = 80	100 cfm hood, or 80 cfm continuous

The 40 sq ft and 20 x 12 ft bathrooms are HVI’s own examples, so you can check them against the source. The metric example shows the conversion: 5 × 4 × 2.5 m is 50 m³, or about 1,766 cubic feet.

Going the other way: a purifier with a smoke CADR of 200 cfm in a 15 x 12 ft room (1,440 cu ft) gives 200 × 60 ÷ 1,440 = 8.3 air changes per hour. One rated at 233 cfm in a 20 x 15 ft room (2,400 cu ft) gives 5.8, enough for CDC’s 5 but not for AHAM’s 7.5 during wildfire smoke.

For air purifiers, CFM means CADR

A purifier’s fan airflow is not the same as the clean air it delivers. CADR (clean air delivery rate) is measured under ANSI/AHAM AC-1: the particle decay with the purifier running, minus natural decay, times the volume of the test chamber. So it already accounts for the filter, and it’s the CFM that counts toward air changes. Use the smoke CADR, which covers the smallest particles. Our explainer on what CADR means covers the three ratings.

Two cautions:

• CADR is measured on the highest fan speed. EPA notes that CADR at lower settings is usually not published and can be much lower. If the purifier will run on a quiet setting, choose one rated above the result.
• The presets assume one well-mixed room. Doors, furniture and corners slow mixing. Our air changes per hour calculator shows how long particles take to clear at a given rate, and the air purifier size calculator turns your room into the room-size rating printed on the box.

During smoke events, AHAM’s stricter wildfire rule (CADR at least equal to the floor area) is the preset to use; EPA’s own wildfire advice is a smoke CADR of at least two-thirds of the floor area. Our guide to wildfire smoke indoors covers sealing up a clean room.

Bathroom and kitchen exhaust

Exhaust fans remove moisture and odors at the source by sending air outdoors, which is why they come first in our guide on how to reduce humidity in your house. An air purifier can’t do that job: it filters particles and leaves the moisture behind, as our look at the best bathroom air purifier explains.

• Bathrooms. HVI’s 1 cfm per square foot works out to about eight air changes per hour with an 8 ft ceiling. In bigger bathrooms, HVI suggests either one fan sized for the total or a 50 cfm fan over each fixture, and leaving the fan on for 20 minutes after using the bathroom.
• Kitchens. ASHRAE 62.2 sets 100 cfm for a vented range hood regardless of kitchen size. Other kitchen exhaust fans, including downdraft vents, have to move more: 300 cfm, or 5 air changes per hour in an enclosed kitchen. Continuous kitchen exhaust is only allowed in an enclosed kitchen; an open-plan kitchen needs a fan you switch on.

These are minimum rates from ASHRAE 62.2 and HVI’s recommendations. Your local building code may set its own, and for a new installation it’s worth checking with the installer.

If a basement or closed room stays damp even with fans, a dehumidifier is the next step: the dehumidifier size calculator turns room size and dampness into pints per day.