HVAC Calculators · Calculator
Bathroom Fan CFM Calculator: What Size Exhaust Fan Do You Need?
For bathrooms up to 100 sq ft, plan on 1 CFM per square foot of floor area with a 50 CFM minimum, scaled up for ceilings over 8 ft. Larger bathrooms are sized by fixture: 50 CFM each for a toilet, shower and tub, and 100 CFM for a jetted tub. A 9 × 8 ft bathroom with a 9 ft ceiling needs about 81 CFM, so a 90 CFM fan.
Bathroom fan CFM calculator
The two sizing methods
The Home Ventilating Institute (HVI) guidance that most fan makers follow uses two methods depending on room size.
- Up to 100 sq ft: 1 CFM per square foot of floor, never less than 50 CFM. This works out to about 7.5 air changes per hour in a room with an 8-foot ceiling, which is enough to clear shower steam in a reasonable time.
- Over 100 sq ft: add up the fixtures. Each toilet, shower and standard tub gets 50 CFM; a jetted or whirlpool tub gets 100 CFM. A large primary bath with a toilet in its own enclosure often gets a separate fan for that compartment.
Building codes set a lower floor. The IRC and ASHRAE 62.2 require at least 50 CFM for an intermittent bathroom fan or 20 CFM for one that runs continuously. Code minimums keep you legal; the HVI sizing keeps mirrors and walls dry.
Formula and ceiling-height adjustment
Area method: CFM = max(50, Length × Width × Ceiling height ÷ 8)
Fixture method: CFM = 50 × (toilets + showers + tubs) + 100 × jetted tubs
The 1 CFM per square foot rule assumes an 8-foot ceiling. Taller rooms hold more air, so the calculator scales the area result by ceiling height divided by 8. A 9-foot ceiling adds 12.5%, a 10-foot ceiling adds 25%. The adjustment is applied to the area method only, because fixture values already describe the moisture source rather than the room volume.
Worked example
A 9 × 8 ft hall bathroom with a 9-foot ceiling, one toilet and one tub-shower combo:
- Floor area: 9 × 8 = 72 sq ft, which is under 100, so the area method applies.
- Height-adjusted airflow: 72 × 9 ÷ 8 = 81 CFM.
- Next common fan size: 90 CFM.
That matches the calculator's default result. If the same bathroom were 12 × 10 ft (120 sq ft), the fixture method would take over: 50 + 50 = 100 CFM.
Allow for the duct run
A fan's rated CFM is measured against a small amount of resistance, commonly 0.1 in. w.c., and some fans also list output at 0.25 in. w.c. Real duct runs with elbows, a long flex line and a roof cap often present more resistance than that. A cheap fan rated at 80 CFM can move well under 50 CFM once installed.
| Duct run | What to do |
|---|---|
| Short and straight (under 10 ft, one elbow) | Use the rated CFM at 0.1 in. w.c. |
| Moderate (10–25 ft, two or three elbows) | Pick a fan whose 0.25 in. w.c. rating meets your target |
| Long or 3-inch duct | Step up a size, upgrade to 4- or 6-inch duct, or use an inline fan |
The bathroom fan venting guide covers duct diameter, termination caps and why a fan must never dump into the attic.
Noise: check the sone rating
A fan that is too loud gets switched off early, which defeats the point. Look for 1.0 sone or less for a fan that will run often; that is about as loud as a quiet refrigerator. ASHRAE 62.2 limits continuously running fans to 1 sone and intermittent fans to 3 sones. The sones to decibels converter translates ratings into dB.
Run the fan during the shower and for at least 20 minutes afterwards. A humidity-sensing switch or timer does this automatically and is one of the cheapest moisture fixes in a house.
Why bathroom ventilation matters
A single shower can put somewhere around a quarter to half a pound of water vapor into the air. Without exhaust, that moisture condenses on cold surfaces and feeds mold in grout, behind paint and in wall cavities. It also raises whole-house humidity; see the indoor humidity guide for target levels. To check how quickly a given fan replaces the air in any room, use the air changes per hour calculator.
Fan types: ceiling, inline and combination units
A standard ceiling-mounted fan houses the motor right above the grille. It is the simplest install and suits most bathrooms up to about 110 CFM. An inline (remote) fan sits in the attic or a ceiling cavity and pulls through one or more grilles; because the motor is away from the room it is very quiet, and it can serve a large bathroom with separate shower and toilet pickups. Combination units add a light, night-light or heater. A built-in heater does not change the exhaust sizing, but it does need its own circuit capacity, so check the electrical rating before swapping a plain fan for a heater model.
Whichever type you choose, match the fan's outlet to the duct size you plan to run, and compare models by their airflow at 0.25 in. w.c. rather than the headline number on the box.
Frequently asked questions
Is a 50 CFM fan enough for a small bathroom?
For a bathroom of 50 sq ft or less with an 8-foot ceiling and a short duct run, yes. If the duct is long or the ceiling is high, choose 70–80 CFM.
Can a bathroom fan be too powerful?
In a tight house a very large fan can depressurize the home and pull air back down a fireplace or atmospheric water heater flue. Stay near the calculated size, and have combustion appliances checked if you install several high-CFM exhaust fans.
Does a toilet room need its own fan?
An enclosed toilet compartment in a large bathroom usually gets its own 50 CFM fan, because the main fan cannot pull air through a closed door.
Should the fan go over the shower?
Placing it over or next to the shower captures steam at the source. A fan installed inside a shower enclosure must be rated for wet locations and usually needs GFCI protection.
Last reviewed 2026-10-28