Enter the bathroom's size, its ceiling, what is in it and how long the duct runs. You get the fan's airflow (CFM), the duct that carries it by code, the noise to look for and how long to run it.
Bathroom exhaust fan
At least 50 CFMon a 4″ duct (flex to 56 ft, smooth to 114 ft) · 2.0 sones or quieter · runs 20 minutes after each use
Quick answer: a bathroom fan needs 1 CFM for every square foot of floor, and at least 50 CFM (the Home Ventilating Institute). A 5 × 8 ft bathroom takes a 50 CFM fan. With a ceiling over 8 ft, Broan’s rule (length × width × height × .13) asks more. Over 100 sq ft, add 50 CFM for the toilet, the shower and the tub, 100 for a jetted tub.
How to use the bathroom fan calculator
- Enter the bathroom’s length, width and ceiling height, in feet or metres.
- Pick the bath or shower and where the toilet is. They count only in a bathroom over 100 sq ft, or when the toilet has its own room.
- Enter the duct’s length to the outside and its elbows. The result names the duct size the code allows for that run.
- Read the fan’s CFM, the noise to look for and the timer. Copy link keeps the result.
The rules behind the numbers
Up to 100 sq ft: 1 CFM a square foot
The Home Ventilating Institute (HVI), which certifies fan ratings, asks 1 CFM per square foot of floor, with a minimum of 50 CFM. Broan-NuTone counts the ceiling too: length × width × height × .13, rounded up to the next ten (its own example: 10 × 8 × 9 ft × .13 = 93.6, so 100 CFM). That is about 8 air changes an hour. The calculator takes the larger, so a tall ceiling gets more air.
Over 100 sq ft: by fixture
In a bigger bathroom, both size the fan by what is in it: a toilet 50 CFM, a shower 50, a bathtub 50, a jetted tub 100. A bathroom with a toilet, a tub and a separate shower takes 150 CFM.
The code’s least
The International Residential Code asks every bathroom and toilet room for 50 CFM on demand, or 20 CFM running all the time (Table M1505.5), exhausted outdoors. A toilet in its own room needs its own.
The duct
A fan only moves its rated air through a duct that is big and short enough. The code’s Table M1504.2 gives the longest duct of each size for a fan’s airflow, and takes 15 ft off for every elbow. A 4″ flexible duct carries 50 CFM up to 56 ft, but 80 CFM only 4 ft.
Noise and the timer
ENERGY STAR bathroom fans up to 200 CFM make 2.0 sones or less (3.0 up to 500 CFM). The HVI asks for a timer or control that runs the fan at least 20 minutes after each use.
Bathroom fan CFM by room size
| Bathroom | 8 ft ceiling | 9 ft ceiling |
|---|---|---|
| 5 × 8 ft (40 sq ft) | 50 CFM | 50 CFM |
| 6 × 9 ft (54 sq ft) | 60 CFM | 70 CFM |
| 8 × 10 ft (80 sq ft) | 90 CFM | 100 CFM |
| 9 × 10 ft (90 sq ft) | 100 CFM | 110 CFM |
| 10 × 10 ft (100 sq ft) | 110 CFM | 120 CFM |
The larger of the HVI’s 1 CFM a square foot and Broan’s length × width × height × .13, rounded up to the next ten.
Duct size by fan airflow (IRC Table M1504.2)
| Duct | 50 CFM | 80 CFM | 100 CFM | 125 CFM | 150 CFM |
|---|---|---|---|---|---|
| 4″ flex | 56 ft | 4 ft | no | no | no |
| 5″ flex | no limit | 81 ft | 42 ft | 16 ft | 2 ft |
| 6″ flex | no limit | no limit | 158 ft | 91 ft | 55 ft |
| 4″ smooth | 114 ft | 31 ft | 10 ft | no | no |
| 5″ smooth | no limit | 152 ft | 91 ft | 51 ft | 28 ft |
| 6″ smooth | no limit | no limit | no limit | 168 ft | 112 ft |
The longest duct allowed, with no elbows; take 15 ft off for each one. The code reads a fan’s airflow at 0.25 inch of static pressure, as HVI-certified ratings list it.
Three worked examples
A 5 × 8 ft bathroom, 8 ft ceiling. 40 sq ft asks 40 CFM, so the 50 CFM least: a 50 CFM fan. Its 10 ft duct with one elbow counts as 25 ft: a 4″ duct does it, flex or smooth.
An 8 × 10 ft bathroom with a 9 ft ceiling (Broan’s own example). 10 × 8 × 9 × .13 = 93.6, so 100 CFM. Its 20 ft duct and one elbow count as 35 ft: a 5″ duct, since a 4″ smooth one carries 100 CFM only 10 ft.
A 10 × 12 ft main bath with a jetted tub and a shower. Over 100 sq ft, by fixture: toilet 50 + jetted tub 100 + shower 50 = 200 CFM. Over 12 ft of duct and two elbows (42 ft), only a 6″ smooth duct is allowed.
Common bathroom fan mistakes
- Forgetting the ceiling. A 9 or 10 ft ceiling holds more air: size by the volume.
- A bigger fan on the old 4″ flex duct. An 80 CFM fan may run only 4 ft of it; the fan is rated, the duct decides.
- Venting into the attic. The code wants bathroom air exhausted directly outdoors, not into an attic or crawl space.
- Switching it off with the light. Let it run 20 minutes after a shower: a timer does it for you.
Planning the bathroom? Fit the toilet with the free toilet size and rough-in finder, lay the room out in the bathroom layout planner, and draw it to scale in the free 2D room planner.
Plan the whole bathroomRead next
- Spa-inspired bathroom ideas
- Basement bathroom designs
- Black and white bathroom tiles
- Small bathroom makeovers
Bathroom exhaust fan questions
How many CFM do I need for my bathroom fan?
1 CFM for each square foot of floor, and at least 50 (the HVI). With a ceiling over 8 ft, multiply length × width × height by .13 and round up to the next ten (Broan). Over 100 sq ft, add 50 for the toilet, shower and tub each, 100 for a jetted tub.
What size exhaust fan do I need for a small bathroom?
50 CFM, the least the HVI and the code ask, for any bathroom up to about 48 sq ft with an 8 ft ceiling. A 5 × 8 ft bathroom takes 50 CFM.
Is 80 CFM enough for a bathroom?
For one up to 76 sq ft with an 8 ft ceiling, yes. Past that, or with a higher ceiling, go to 90 or 100 CFM, and check the duct: 80 CFM runs only 4 ft of 4″ flex duct.
What size duct does a bathroom exhaust fan need?
By the code’s Table M1504.2: a 50 CFM fan runs 4″ flex duct up to 56 ft; 80 CFM wants 4″ smooth (31 ft) or 5″ flex (81 ft); 100 CFM a 5″ duct. Take 15 ft off for each elbow.
How long should a bathroom fan run after a shower?
At least 20 minutes after each use, the HVI says, which is why it asks for a timer or a control that keeps it running.
Does a toilet room need its own exhaust fan?
Yes: the code lists bathrooms and toilet rooms each at 50 CFM on demand or 20 continuous (IRC Table M1505.5), so a toilet with its own door gets its own fan.
How quiet should a bathroom fan be?
ENERGY STAR bathroom fans up to 200 CFM run at 2.0 sones or less. Compare the sone rating on the box: the lower, the quieter.