What size bathroom exhaust fan do I need?

50 cfm if it runs intermittently, or 20 cfm if it runs continuously, under 2021 IRC Table M1505.4.4. The size of the bathroom does not change the minimum. Choose a fan labeled at or above that rate, because M1505.3 requires the airflow to be listed and labeled.

How many cfm does a range hood need?

The residential code's minimum for kitchen exhaust is 100 cfm intermittent or 25 cfm continuous. That is the floor for any kitchen, not a sizing rule for a particular range, and some jurisdictions amend it: Washington requires a minimum capture efficiency or 160 cfm for an electric range and 250 cfm for a combustion range.

How do I calculate cfm for a room?

Multiply length, width and ceiling height to get cubic feet, multiply by the air changes per hour you want, and divide by 60. A 12 by 10 ft room with an 8 ft ceiling at 6 air changes an hour needs 96 cfm.

How much whole-house ventilation does a house need?

0.01 cfm per square foot plus 7.5 cfm per bedroom plus one, running continuously, by Equation 15-1. A 2,000 sq ft, three-bedroom house needs 50 cfm, or 60 cfm from Table M1505.4.3(1), and the code accepts either. A fan that runs half the time must move twice that.

Can the bathroom fan be the whole-house ventilation?

Yes. M1505.4.1 permits local exhaust fans to serve as the whole-house system, provided the fan meets the whole-house rate and has the manual override control M1505.4.2 requires.

Home › HVAC › Exhaust Fan CFM Calculator

Exhaust Fan CFM Calculator

2021 IRC M1505.4.3 and M1505.4.4

A bathroom fan needs 50 cfm if it runs when someone switches it on, or 20 cfm if it never stops. The room size does not enter into it. The whole house is a different number again, set by floor area and bedrooms.

Exhaust and ventilation airflow from the 2021 IRC

What are you sizing

How many of these rooms (bathrooms and kitchens)

House floor area (whole house)

sq ft

Bedrooms (whole house)

Fan run time (whole house, 25 to 100)

Room length (by air changes)

Room width (by air changes)

Ceiling height (by air changes)

Air changes per hour (your target)

Minimum airflow at the fan. A long duct run delivers less than the fan's label.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Airflow the code asks for

Worked example

A 2,000 sq ft house with three bedrooms, ventilated by a fan that runs half of every four hours.

Equation 15-1. 0.01 × 2,000 + 7.5 × (3 + 1) = 20 + 30 = 50 cfm continuous.
Table M1505.4.3(1). 1,501 to 3,000 sq ft with 2 to 3 bedrooms reads 60 cfm. The code accepts either, so the equation's 50 is the minimum.
Intermittent operation. At 50 percent run time Table M1505.4.3(2) gives a factor of 2, so the fan must move 100 cfm while it runs.

The table and the equation disagree by 10 cfm on an ordinary house, and the code lets you use whichever you prefer. A fan that runs half the time has to be twice the size, which is why an intermittent whole-house fan is usually the bathroom fan upsized rather than a new one.

The formula

Three rules from IRC Section M1505, and one piece of arithmetic for everything else:

whole house cfm = 0.01 × floor area + 7.5 × (bedrooms + 1) | room cfm = volume × ACH / 60

Table M1505.4.4: kitchens 100 cfm intermittent or 25 continuous; bathrooms and toilet rooms 50 intermittent or 20 continuous
Equation 15-1: the continuous whole-house rate, or Table M1505.4.3(1) instead
Table M1505.4.3(2): run-time factor: 4 at 25%, 3 at 33%, 2 at 50%, 1.5 at 66%, 1.3 at 75%, interpolated between
ACH: air changes per hour, a target you choose; 60 turns hours into minutes

The local exhaust rates do not scale with the room. Only the air change arithmetic does.

Where the numbers come from

The tables are the 2021 International Residential Code, Section M1505. ICC's own code site does not allow automated reading, so the text used here is the City of Scottsdale's reprint of Section M1505, which marks its one local amendment (M1505.2, recirculation of air) and prints Tables M1505.4.3(1), M1505.4.3(2) and M1505.4.4 unchanged.

Your jurisdiction may amend them. Washington, for one, replaced the kitchen row with a range hood capture efficiency rule and 5 air changes per hour for enclosed kitchens. Check the edition and amendments your building department enforces.

Intermittent or continuous, and why the numbers are so far apart

A fan that runs only when the light is on has to clear the room quickly, so the code asks for 50 cfm in a bathroom and 100 cfm in a kitchen. A fan that never stops only has to keep up, so the same rooms need 20 and 25 cfm. The continuous rates are 40 percent and 25 percent of the intermittent ones.

Neither figure depends on the size of the room. A calculator that multiplies bathroom floor area by one cfm per square foot is answering a different question, a rule of thumb from fan makers, and the code does not say it.

Air changes per hour for any other room

For a workshop, a garage, a basement or a room with no code rate, airflow is the room's volume times the air changes per hour you want, divided by 60 to turn hours into minutes. A 12 by 10 ft room with an 8 ft ceiling is 960 cu ft, and 6 air changes an hour is 960 × 6 / 60 = 96 cfm.

The air change target is the one number on this page that is yours rather than the code's. The residential code sets none for an ordinary room.

Frequently asked questions

What size bathroom exhaust fan do I need?: 50 cfm if it runs intermittently, or 20 cfm if it runs continuously, under 2021 IRC Table M1505.4.4. The size of the bathroom does not change the minimum. Choose a fan labeled at or above that rate, because M1505.3 requires the airflow to be listed and labeled.
How many cfm does a range hood need?: The residential code's minimum for kitchen exhaust is 100 cfm intermittent or 25 cfm continuous. That is the floor for any kitchen, not a sizing rule for a particular range, and some jurisdictions amend it: Washington requires a minimum capture efficiency or 160 cfm for an electric range and 250 cfm for a combustion range.
How do I calculate cfm for a room?: Multiply length, width and ceiling height to get cubic feet, multiply by the air changes per hour you want, and divide by 60. A 12 by 10 ft room with an 8 ft ceiling at 6 air changes an hour needs 96 cfm.
How much whole-house ventilation does a house need?: 0.01 cfm per square foot plus 7.5 cfm per bedroom plus one, running continuously, by Equation 15-1. A 2,000 sq ft, three-bedroom house needs 50 cfm, or 60 cfm from Table M1505.4.3(1), and the code accepts either. A fan that runs half the time must move twice that.
Can the bathroom fan be the whole-house ventilation?: Yes. M1505.4.1 permits local exhaust fans to serve as the whole-house system, provided the fan meets the whole-house rate and has the manual override control M1505.4.2 requires.

Check these numbers yourself

2021 IRC Section M1505, Mechanical Ventilation, reprinted in Whole-House Mechanical Ventilation OptionsCity of Scottsdale, Arizona, rev. 3/10/23. Read 2026-09-15. Tables M1505.4.3(1), M1505.4.3(2) and M1505.4.4 and Equation 15-1; the amendment to M1505.2 is marked as Scottsdale's.
WAC 51-51-1505, Section M1505 Mechanical ventilationWashington State Legislature, effective March 16, 2024. Read 2026-09-15. Source of the Washington kitchen amendment quoted in the FAQ.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-15. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

These are code minimums at the fan. A fan's label is measured at a stated static pressure, and duct length, elbows and the termination cap reduce what it delivers. Exhaust to the outdoors, never into an attic or crawl space (M1505.2).