Bathroom Fan CFM Calculator

The short answer

For a bathroom up to 100 sq ft, the fan should move 1 CFM per square foot of floor, and at least 50 CFM. That works out to about eight air changes an hour with a 8 ft ceiling. For a larger bathroom, add 50 CFM for each toilet, shower and bathtub and 100 CFM for a jetted tub, then round up to the next fan size sold.

The International Residential Code’s minimum is 50 CFM, rated at 0.25 in of static pressure, and the duct matters too: the code limits how far each duct size can run.

How to measure

1. The floor: measure the bathroom wall to wall, both ways. Leave out a toilet room with a door, since it gets its own fan.
2. The ceiling: measure from the floor to the ceiling.
3. The fixtures: note the toilet, the shower or tub-shower, a separate bathtub and a jetted tub.
4. The duct: if a fan is already there, measure the duct from the fan to the roof or wall cap along its path, and count its bends. For a new fan, plan the most direct route outside.

How to use the calculator

1. Enter the bathroom’s width, length and ceiling height.
2. Check its fixtures. They count when the bathroom is over 100 sq ft.
3. Enter the duct’s length and its elbows.
4. Read the result: the fan size, the rule behind it, every standard fan labeled Fits or Tight, and every duct from 3 to 6 in, smooth and flex, with how far the code lets it run. Copy link keeps your numbers in the address, and the next step takes the room’s size to the room lumens calculator.

How we calculate

The airflow targets below start at the legal minimum and go up with the room. The minimum and the longest duct runs are set by the International Residential Code.

• Up to 100 sq ft: 1 CFM per square foot of floor, and at least 50 CFM.
• A ceiling over 8 ft: a taller room may need more air. The calculator raises the rate in step with the room’s volume, which keeps about eight air changes an hour.
• Over 100 sq ft: 50 CFM for each toilet, shower and bathtub, and 100 CFM for a jetted tub.
• Fits and Tight: a fan that meets the rate above Fits. One that meets only the code’s 50 CFM is Tight.
• The pick: the smallest standard fan that Fits. Above the largest single fan, two equal fans, one over each wet area.
• The duct: the code’s table gives the longest run for each duct size and fan, less 15 ft for each elbow. A duct within that run and at least the smallest size for the fan Fits. One within the run but narrower is Tight.

The fan’s noise, its controls and where it goes aren’t in the math. The result lists them as notes.

Worked examples

Worked example

A 5 × 8 ft bathroom with a toilet and a tub-shower

What we know

• Bathroom: 5 × 8 ft, 8 ft ceiling
• Fixtures: toilet and shower; duct: 8 ft of duct with 1 elbow

How it works out

1. At 40 sq ft, the bathroom needs the minimum, 50 CFM.
2. A 50 CFM fan covers it.
3. Duct: 4 in smooth, up to 99 ft with 1 elbow: Fits.

A 50 CFM fan.

Worked example

An 8 × 10 ft bathroom with a 10 ft ceiling

What we know

• Bathroom: 8 × 10 ft, 10 ft ceiling
• Fixtures: toilet and shower; duct: 8 ft of duct with 1 elbow

How it works out

1. At 80 sq ft, 1 CFM per square foot gives 80 CFM; with a 10 ft ceiling, 100 CFM, raised in step with the room's volume.
2. A 100 CFM fan covers it.
3. Duct: 5 in smooth, up to 76 ft with 1 elbow: Fits.

A 100 CFM fan.

Worked example

A 12 × 12 ft primary bath with a shower and a tub

What we know

• Bathroom: 12 × 12 ft, 8 ft ceiling
• Fixtures: toilet, shower and tub; duct: 12 ft of duct with 1 elbow

How it works out

1. At 144 sq ft, count the fixtures instead: 50 CFM each for the toilet, the shower and the tub, 150 CFM in all.
2. A 150 CFM fan covers it.
3. Duct: 6 in smooth, up to 97 ft with 1 elbow: Fits.

A 150 CFM fan.

Bathroom fan size chart

The fan for common bathroom sizes at three ceiling heights: the rate per square foot, rounded up to the next fan size sold.

Larger bathrooms

Over 100 sq ft, count the fixtures instead of the floor. One fan over the middle of the room works, and so do smaller fans over the shower, the tub and the toilet.

A toilet in its own room with a door needs its own fan, and 50 CFM is usually enough for it. A steam shower needs its own fan too.

Duct size and length

A long, narrow or bending duct adds resistance and cuts the air a fan moves. Many fans are sold by their airflow at 0.1 in of static pressure, the International Residential Code wants the rating at 0.25 in, and real ducts usually push past that. One fan sold as 80 CFM, for example, moves 61 CFM at 0.25 in.

• Elbows: each one takes 15 ft off the run the code allows. Keep 2 to 3 ft of straight duct before the first one.
• Smooth or flex: choose smooth metal or PVC duct, and stretch flex duct tight so it doesn’t sag. The code lets smooth duct run farther.
• Common fans, small ducts: many 80 CFM fans come with a 4 in collar. The code allows 31 ft of smooth 4 in duct for one, but only 4 ft of flex, and for runs up to 25 ft with up to 3 elbows, plan on 5 in duct.
• Real airflow: once it’s installed, a fan rated 80 CFM may move only about 66 CFM. To deliver 50 CFM, buy a fan rated at least 70 CFM.

Where the fan goes

• Over the shower or tub: put the fan over or near the shower or tub, where the steam is. A fan there must be listed for it and wired to a GFCI circuit.
• Outdoors: the International Residential Code requires the duct to end outside, never in the attic, a soffit or a crawl space. The outlet goes 3 ft from windows, doors and property lines and 10 ft from mechanical intakes.
• Makeup air: leave 3/4 in under the bathroom door so air can come in as the fan pulls it out.

Noise and controls

• Quiet: look for 1 sone or less, about as loud as a refrigerator. A fan you switch on and off should stay at 3 sones or less, and one that runs all the time at 1 sone or less.
• Run time: leave it on 20 minutes after a shower. The International Residential Code asks for a timer with a setting of 30 minutes or less, or a humidity control set between 50 to 80%.
• Running all the time: under the code, a fan that runs nonstop can move as little as 20 CFM.

Once the fan is set, light the room with the room lumens calculator, which takes the bathroom’s size from this one. For the rest of the room, find the vanity that fits your wall and the shower curtain and rod.