How this calculator works
A bathroom fan has to remove steam fast enough that the mirror clears and the moisture does not settle into the walls and ceiling. Two standard rules give the capacity, in cubic feet of air per minute (CFM). For rooms up to 100 ft², the Home Ventilating Institute (HVI) rule is 1 CFM per square foot of floor, with a minimum of 50. For larger rooms, add up the fixtures: 50 CFM for each toilet, shower and ordinary tub, and 100 for a jetted tub. The second method is air changes: a bathroom should replace its air about 8 times an hour.
Worked example: an 8 × 6 ft bathroom with an 8 ft ceiling has 48 ft² of floor, so the HVI rule gives the 50 CFM minimum. Its volume is 384 ft³, and 8 air changes an hour is 384 × 8 ÷ 60 = 51 CFM. The higher figure is 51, so a 60 CFM fan is the next size.
Fan size by bathroom size (8 ft ceiling, one toilet and one shower)
The calculator's answer, rounded up to the next 10 CFM.
| Room | Floor area | Fan size |
|---|---|---|
| 5 × 8 ft | 40 ft² | 50 CFM |
| 6 × 8 ft | 48 ft² | 60 CFM |
| 8 × 10 ft | 80 ft² | 90 CFM |
| 10 × 10 ft | 100 ft² | 110 CFM |
| 10 × 14 ft | 140 ft² | 150 CFM |
Tips before you buy
- Choose a quiet fan. Fans rated at 1.0 sone or less are barely audible, and people use quiet fans, while they turn noisy ones off.
- Vent outdoors through a short, straight duct, never into the attic or the space above the ceiling. Moist air in the attic grows mold and rots the roof.
- Match the duct to the fan: 4 in duct is typical for fans up to about 80 CFM, and 6 in for larger fans. Use rigid or semi-rigid duct, insulated in cold attics so that it does not drip condensation.
- Run the fan during the shower and for 15 to 20 minutes after. A timer switch or a humidity-sensing fan does this for you.
- A fan or light/fan unit installed directly over a shower or tub must be listed for that location (look for "suitable for installation over a tub or shower") and on a GFCI-protected circuit.
- Look for the Energy Star label and the HVI certification, which show that the CFM rating is tested.
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Frequently asked questions
What size bathroom fan do I need?
For a bathroom up to 100 ft², one CFM per square foot of floor with a minimum of 50 CFM. An 8 × 6 ft bathroom needs about 50 to 60 CFM, and a 10 × 10 ft one about 110 CFM.
How many CFM do I need for a 100 square foot bathroom?
At least 100 CFM by the room-size rule, which is also about 107 CFM from 8 air changes an hour with an 8 ft ceiling. A 110 CFM fan is a good choice.
Does a bigger bathroom fan work better?
Up to a point. A fan that is too small leaves steam in the room, but one that is too large is loud and wastes heated or cooled air. Choose by the calculation, and keep to a quiet sone rating.
Can I vent a bathroom fan into the attic?
No. The moist air condenses on the cold roof and the insulation, and leads to mold and rot. The duct should go through the roof or a wall to the outdoors.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.