Bathroom fan CFM calculator: what size bathroom exhaust fan do I need?
63 cfm fan
63 sq ft at 1 cfm per sq ft (HVI) · rated at 0.25 in w.c. · for a 9 × 7 ft bathroom
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3/8 or 10' 6".
63 cfm fan
63 sq ft at 1 cfm per sq ft (HVI) · rated at 0.25 in w.c.
Source: International Residential Code 2021, Table M1505.4.4, and HVI bathroom ventilation rates, read October 27, 2026. Read the source
A 9 × 7 ft bathroom needs a 63 cfm exhaust fan: HVI rates 1 cfm per sq ft up to 100 sq ft, and that is over the 50 cfm intermittent minimum of IRC 2021 Table M1505.4.4.
A worked example
A 13 1/2 × 9 ft primary bath with a toilet, a shower and a jetted tub, on a switch needs a 200 cfm fan. 13.5 × 9 is 121.5 sq ft, over 100, so HVI adds the fixtures: toilet 50, shower 50 and jetted tub 100, 200 cfm. HVI also allows splitting it, one fan over each fixture.
How many CFM per square foot for a bathroom fan?
One, up to 100 sq ft, by HVI: a 5 × 8 ft bathroom is 40 sq ft and 40 cfm, an 8 × 10 ft one 80 cfm. The IRC still asks 50 cfm intermittent, so under 50 sq ft the fan is 50 cfm.
What size fan for a bathroom over 100 square feet?
HVI switches to fixtures: 50 cfm each for a toilet, a shower and a bathtub, 100 for a jetted tub. Its own example, a 20 × 12 ft bathroom with a tub, a shower and an enclosed toilet, comes to 150 cfm, as one fan or three of 50.
Can a smaller fan run all the time instead?
Yes: IRC Table M1505.4.4 accepts 20 cfm continuous in place of 50 cfm intermittent, and HVI says the same. HVI adds that a switched fan should keep running 20 minutes after each use, on a timer.
What else it is called
Also searched as a bathroom exhaust fan CFM calculator, bath fan size calculator or bathroom vent fan calculator. CFM is cubic feet per minute, the airflow printed on the fan label and in the HVI-Certified Products Directory.
Fan size by bathroom
| Bathroom | Floor area | HVI rate | Fan, intermittent |
|---|---|---|---|
| 5 × 8 ft | 40 sq ft | 40 cfm | 50 cfm |
| 6 × 8 ft | 48 sq ft | 48 cfm | 50 cfm |
| 7 × 9 ft | 63 sq ft | 63 cfm | 63 cfm |
| 8 × 10 ft | 80 sq ft | 80 cfm | 80 cfm |
| 10 × 10 ft | 100 sq ft | 100 cfm | 100 cfm |
| Over 100 sq ft: toilet and shower | over 100 sq ft | 100 cfm | 100 cfm |
| Over 100 sq ft: toilet, shower and tub | over 100 sq ft | 150 cfm | 150 cfm |
| Over 100 sq ft: toilet, shower and jetted tub | over 100 sq ft | 200 cfm | 200 cfm |
HVI rate: 1 cfm per sq ft of floor up to 100 sq ft; over 100 sq ft, 50 cfm per toilet, shower or tub and 100 per jetted tub. Fan: never under the 50 cfm intermittent of IRC 2021 Table M1505.4.4, rated at 0.25 in w.c.; a fan that runs all the time needs 20 cfm.
Where the numbers come from
- Minimum: International Residential Code 2021, Table M1505.4.4, as adopted by Colorado without amendments: bathrooms and toilet rooms 50 cfm intermittent or 20 cfm continuous, the listed rate at a static pressure of at least 0.25 in w.c. (M1505.3); M1505.2: bathroom exhaust goes outdoors, never into an attic or crawl space. Read October 27, 2026. IRC 2021 chapter 15 on UpCodes
- Rate: Home Ventilating Institute, Bathroom Ventilation. Up to 100 sq ft, 1 cfm per sq ft of floor, about eight air changes an hour; over 100 sq ft, by fixtures: toilet 50 cfm, shower 50, bathtub 50, jetted tub 100. Read October 27, 2026. HVI bathroom ventilation
- HVI, How Much Ventilation Do I Need?: under 100 sq ft, 1 cfm per sq ft with a minimum of 50 cfm, and 20 cfm continuous in place of a 50 cfm intermittent fan. Read October 27, 2026. HVI ventilation guidelines
- The rest is arithmetic: floor area = length × width; over 100 sq ft, the fixture rates added.
Fan ratings are airflow at 0.25 in w.c. of static pressure, the point the IRC uses; a long or bent duct pushes the static pressure up and the airflow down. Steam rooms take a separate fan in the steam room (HVI).
By Javier Miralles · Last updated . How this site is made.
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