Bathroom Fan CFM Calculator

Size a bath exhaust fan with this bathroom fan CFM calculator: the area rule, the HVI fixture rule, and the duct loss that makes delivered CFM lower than the label.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size a bathroom fan, run both standard rules and take the larger. The area rule is 1 CFM per square foot of floor with a 50 CFM minimum; the HVI fixture rule allows 50 CFM each for a toilet, shower or tub. A 10 x 8 ft bathroom with a toilet and a tub-shower gives 80 CFM by area and 100 CFM by fixtures, so buy a 110 CFM fan.

Your measurements

A rectangular volume: length times width times depth.

Bathroom lengthftBathroom widthftCeiling heightftThe area rule assumes an 8 ft ceiling; a taller room holds more air, so the result scales by height / 8.Fixtures in the roomThe HVI fixture method allows 50 CFM each for a toilet, a shower or a bathtub and 100 CFM for a jetted tub. A sink counts as nothing. Use it when the bathroom is over 100 sq ft.Straight duct runftMeasured duct from the fan housing to the cap. Elbows and the cap are added separately as equivalent length.90 degree elbows in the runCount the turn off the housing and the turn at the roof or wall. Each costs about as much pressure as 15 ft of straight duct. A lazy sweep in flex still counts as one.Duct diameterSized as smooth metal. Insulated flex at the same diameter carries about 1.5 times the friction, so it delivers roughly 20% less than the figures here. A 3 in duct is a leftover from 1970s builder fans.

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Results

Fan size to buy: 110 CFM; The larger of the two methods, rounded up to a stock size. Fans are sold at 50 CFM, 70 CFM, 80 CFM, 110 CFM, 150 CFM, 190 CFM, 250 CFM. The fixture method governed here
Area method: 80 CFM; 80 sq ft of floor at 1 CFM per sq ft, scaled for the 8 ft ceiling, with a 50 CFM minimum. HVI applies this rule up to 100 sq ft
HVI fixture method: 100 CFM; A toilet and a tub-shower, at 50 CFM for each toilet, shower or tub and 100 CFM for a jetted tub. This is the rule HVI uses above 100 sq ft
Delivered airflow after the duct run: 76 CFM; 4 in smooth metal over 65 ft of equivalent length passes 76 CFM; insulated flex the same size passes about 61 CFM. Warning: the duct, not the fan, is setting the airflow here, so go up to 6 in or shorten the run
Duct size to run: 6 in round; 10 ft of duct plus 2 elbows at 15 ft each and 25 ft for the cap. 4 in is the practical minimum, and the run terminates outdoors through a roof or wall cap, never in the attic (IRC M1501.1)

Bathroom Fan CFM formula

Area method (CFM) = Length x Width x (Ceiling height / 8), minimum 50 CFM
Fixture method (CFM) = 50 per toilet + 50 per shower + 50 per tub + 100 per jetted tub
Fan size = larger of the two, rounded up to 50, 70, 80, 110, 150, 190 or 250 CFM
Equivalent length = Straight duct + 15 ft per 90 elbow + 25 ft for the cap

Worked example

A 12 x 14 ft master bath with a 9 ft ceiling, a toilet, a separate shower and a jetted tub. The duct is 25 ft with three elbows in 6 in. Area method: 12 x 14 = 168 sq ft, times 9 / 8 for the ceiling, which is 189 CFM. Fixture method: 50 for the toilet, 50 for the shower and 100 for the jetted tub, which is 200 CFM. The larger is 200, so the fan is the 250 CFM size.

Equivalent length is 25 + 3 x 15 + 25 = 95 ft, so a fan holding 0.25 in w.c. has 0.26 in w.c. per 100 ft to spend. A 6 in duct passes about 181 CFM at that rate and an 8 in duct about 387 CFM. The 6 in duct would throw away a fifth of the fan, so this bath gets 8 in duct. Two 110 CFM fans on separate 6 in runs also work, one over the shower and one over the tub.

Quick reference

Bathroom fan size by floor area and fixtures, at an 8 ft ceiling
Bathroom size	Area method	Fixtures	Fixture method	Fan to buy
5 x 8 ft (40 sq ft)	50 CFM	Toilet, tub and shower combo	100 CFM	110 CFM
6 x 9 ft (54 sq ft)	54 CFM	Toilet, tub and shower combo	100 CFM	110 CFM
8 x 10 ft (80 sq ft)	80 CFM	Toilet, tub and shower combo	100 CFM	110 CFM
10 x 12 ft (120 sq ft)	120 CFM	Toilet, separate shower and tub	150 CFM	150 CFM
12 x 14 ft (168 sq ft)	168 CFM	Toilet, shower, jetted tub	200 CFM	250 CFM
14 x 16 ft (224 sq ft)	224 CFM	Toilet, shower, jetted tub	200 CFM	250 CFM

Two sizing rules, and when each one applies

The area rule is 1 CFM for every square foot of floor, with 50 CFM as the floor, and HVI applies it to bathrooms up to 100 square feet. A 10 x 8 ft bath is 80 sq ft, so 80 CFM. Above 100 square feet the fixture rule takes over: 50 CFM each for a toilet, a shower and a bathtub, and 100 CFM for a jetted tub. The air that needs removing follows the fixtures rather than the floor. Run both, take the larger, and round up to a stock size. Fans are sold at 50, 70, 80, 110, 150, 190 and 250 CFM.

Sound decides whether the fan gets used. HVI publishes a sone rating measured at 0.1 in w.c. A 4 sone builder fan is the one people switch off; a 0.3 sone unit is the one that runs. A quiet 80 to 110 CFM fan typically runs $120 to $300 against $40 to $80 for the noisy one. Wire it to a timer switch or buy a humidity-sensing model. A shower puts moisture into the room faster than any fan removes it, and the room needs roughly twenty minutes of run time after the water stops. In a large bath, two smaller fans, one over the shower and one over the toilet, beat one big one in the middle of the ceiling.

The duct decides what you actually get

A fan is certified at 0.1 inches of water column. A real run costs more, and the label does not know about your attic. Count equivalent length rather than measured length: about 15 ft for each 90 degree elbow and 25 to 40 ft for the cap and its damper. A 10 ft run with two elbows is really about 65 ft. At that length a 4 in duct passes about 76 CFM, which strangles a 110 CFM fan. Going to 6 in raises it to about 221 CFM and the fan becomes the limit again. Four inches is the practical minimum, and 3 in duct, still found on 1970s fans, is a dead end. A 50 CFM fan on 20 ft of 3 in flex delivers about 26 CFM.

Where the duct goes matters as much as its size. IRC M1501.1 requires the exhaust to terminate outdoors, so use a roof cap or a wall cap with a damper. A soffit vent feeds the air straight back into the attic, and a loose end just wets the insulation. Insulate the duct and keep the run short and pitched slightly down toward the cap. Warm wet air moving through a 20 F attic condenses inside the pipe and runs back into the housing. Rigid metal beats flex where it fits; flex at the same diameter carries about 1.5 times the friction and has to be pulled tight with no sags to collect water.

What this calculator leaves out

This gives airflow and duct size, not the install. Not included: the fan itself, typically $40 to $300 depending on the sone rating. Nor the roof or wall cap and damper, insulated duct and clamps, or a timer or humidity sensing switch. The wiring is extra too, and a housing over a tub or shower has to be listed for that location and GFCI protected (NEC 410.10(D)). Nor the air sealing and fire caulking around the housing that IRC N1102.4 expects, drywall patching, or a permit where the circuit is new, typically $50 to $250. It also does not size whole-house ventilation under IRC M1505.3, which a continuously running bath fan is sometimes asked to cover on its own.

Code limits that apply

IRC M1505.4checked
Bathroom local exhaust of 50 cfm intermittent or 20 cfm continuous
If you miss it: A bath with no working exhaust and no openable window does not pass the final inspection
IRC R303.3
A bathroom needs an openable window or mechanical exhaust discharging outdoors
If you miss it: Moisture stays in the room, the paint peels off the ceiling and mold shows at the corners
IRC M1501.1
Exhaust ducts terminate outdoors and are fitted with a backdraft damper
If you miss it: Shower moisture frosts the roof sheathing and soaks the insulation when it thaws
IRC M1505.2
Exhaust air is not recirculated to another room and needs a path for replacement air
If you miss it: A fan with no makeup path stalls against a closed door and delivers a fraction of its rating
IECC R403.6.1
Fan efficacy: 1.4 cfm per watt under 90 cfm, 2.8 cfm per watt at 90 cfm and above
If you miss it: An old shaded-pole fan misses the table and the energy inspection on a new build fails

Provision	Limit	If you miss it
IRC M1505.4checked	Bathroom local exhaust of 50 cfm intermittent or 20 cfm continuous	A bath with no working exhaust and no openable window does not pass the final inspection
IRC R303.3	A bathroom needs an openable window or mechanical exhaust discharging outdoors	Moisture stays in the room, the paint peels off the ceiling and mold shows at the corners
IRC M1501.1	Exhaust ducts terminate outdoors and are fitted with a backdraft damper	Shower moisture frosts the roof sheathing and soaks the insulation when it thaws
IRC M1505.2	Exhaust air is not recirculated to another room and needs a path for replacement air	A fan with no makeup path stalls against a closed door and delivers a fraction of its rating
IECC R403.6.1	Fan efficacy: 1.4 cfm per watt under 90 cfm, 2.8 cfm per watt at 90 cfm and above	An old shaded-pole fan misses the table and the energy inspection on a new build fails

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Duct dumped into the attic instead of a roof or wall cap: the moisture frosts the sheathing and drips back through the ceiling in spring.
A 50 CFM fan on 20 ft of 3 in flex delivers about 26 CFM, half the IRC M1505.4 minimum, and the mirror stays fogged.
Flex left sagging between joists: every low spot collects condensate, and the fan gurgles before it stops moving air.
Uninsulated duct in a cold attic condenses inside and runs the water back down into the fan housing and the ceiling.
Fan wired to the light switch runs three minutes; a shower needs roughly twenty after, so fit a timer or a humidity-sensing model.
Buying the 4 sone builder fan to save typically $40 to $150: it is loud enough that nobody uses it, so the room never dries.

Key facts

IRC M1505.4 requires local exhaust in a bathroom at 50 cfm intermittent or 20 cfm continuous, and the air has to go outdoors.
The HVI area rule is 1 CFM per square foot of floor for bathrooms up to 100 square feet, with a 50 CFM minimum.
The HVI fixture rule allows 50 CFM each for a toilet, shower or bathtub and 100 CFM for a jetted tub. It is the method HVI uses above 100 square feet.
Bath fans are certified by HVI at 0.1 inches of water column, while a real duct run costs about 0.25 in w.c. Delivered airflow is always below the label.
2021 IECC Table R403.6.1 requires 1.4 cfm per watt for bathroom fans under 90 cfm and 2.8 cfm per watt at 90 cfm and above.

Frequently asked questions

What size exhaust fan do I need for a bathroom?

Take the larger of 1 CFM per square foot of floor and the HVI fixture allowance of 50 CFM each for a toilet, shower or tub. A 10 x 8 ft bath with a toilet and a tub-shower comes to 80 CFM by area and 100 CFM by fixtures, so buy the 110 CFM fan.

Is a 50 CFM bathroom fan enough?

It meets the IRC M1505.4 minimum and suits a powder room or a bath up to 50 square feet. Anything larger, or any bath with both a separate tub and a shower, needs more. Remember the 50 CFM is the label; on 20 ft of 3 in duct the fan delivers about half that.

Can a bathroom fan vent into the attic?

No. IRC M1501.1 requires exhaust to terminate outdoors, through a roof cap or a wall cap with a backdraft damper. Air dumped into an attic condenses on the cold sheathing, frosts over in winter and drips onto the insulation in spring.

What size duct does a bathroom fan need?

Four inches is the practical minimum and 6 in is what a 110 CFM or larger fan wants on any run past about 15 ft of equivalent length. A 110 CFM fan on 4 in duct over a 65 ft equivalent run delivers about 76 CFM; the same fan on 6 in delivers its full rating.

Sources and references

Chapter 15 Exhaust Systems, Sections M1501 termination and M1505 mechanical ventilation, 2021 IRC (ICC)
Bathroom Exhaust Fans: sizing, sound and duct guidance (U.S. Department of Energy, Building America Solution Center)
HVI Certified Products Directory: certified airflow and sone ratings (Home Ventilating Institute)
A Brief Guide to Mold, Moisture and Your Home (indoor humidity 30 to 50%) (EPA)

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