How to size a bathroom fan
Measure the room's length and width. That is all you need for most bathrooms, because HVI's rule for a room up to 100 square feet is simply 1 CFM per square foot, about eight air changes an hour. A room over 100 square feet goes by what is in it: 50 CFM each for a toilet, a shower and a tub, and 100 for a jetted tub.
Check the ceiling too. HVI says rooms with a ceiling above 8 feet may need more ventilation, and gives no figure. We show a check that uses its eight air changes against the room's actual volume, which is our own derivation. If that figure is higher, lean toward it. Also check the duct, as long or bent runs reduce what the fan can actually deliver.
When you shop, read the label for the CFM and the sones. HVI says a fan should be rated at 1.0 sone or less for quiet ventilation, and warns that inflated ratings are common on fans that are not HVI-certified.
A worked example
HVI prints two examples, one for each method, and the calculator above reproduces both. These lines come from the same formula.
HVI's two printed examples
- A 7 by 10 ft bathroom: 7 × 10 = 70 CFM
- A bathroom over 100 sq ft with a tub, a shower and a toilet: 50 × 3 = 150 CFM
In the second case, HVI says one 150 CFM fan works, or three 50 CFM fans, one by each fixture. Separate fans clear steam closer to the source, though they need more ducting. The code's own floor is 50 CFM for a fan run when needed, or 20 CFM for one that runs all the time. Planning the whole room? See the bathroom remodel cost calculator.
Bathroom fan size by room
All with an 8 foot ceiling, at HVI's methods. Note the jump where a room crosses 100 square feet. A big bath is no longer about its area.
| Bathroom | Area | HVI recommends |
|---|---|---|
| Powder room, 5 × 4 ft | 20 sq ft | 50 CFM |
| Small bath, 8 × 5 ft | 40 sq ft | 50 CFM |
| Standard bath, 10 × 7 ft | 70 sq ft | 70 CFM |
| Larger bath, 10 × 8 ft | 80 sq ft | 80 CFM |
| Main bath, 12 × 10 ft: toilet, shower, tub | 120 sq ft | 150 CFM |
| Spa bath, 14 × 12 ft: toilet, shower, jetted tub | 168 sq ft | 200 CFM |
Mistakes that leave a bathroom damp
- Venting into the attic. It moves the moisture to where it can do the most harm. Duct it outdoors.
- Trusting the box through a long duct. Code ratings are at 0.25 inch water column. A long or bent run can deliver less.
- Turning it off too soon. HVI says to leave it on for 20 minutes after you finish. A timer switch makes this painless.
- Choosing a loud fan. People do not run what they cannot stand. Pick 1.0 sone or less.
- A separate toilet room with no fan. Enclosed fixtures may need their own, so count them.
Questions
What CFM bathroom fan for an 8x10 bathroom?
An 80 CFM fan for an 8 by 10 bathroom, at HVI's 1 CFM per square foot for an 8 foot ceiling. With a taller ceiling, more may be needed. A 7 by 10 bathroom needs 70 CFM, as HVI's own example says.
How many CFM for a bathroom fan with a shower?
No more than the room's area up to 100 square feet, with nothing extra for the shower. Over 100 square feet, HVI counts 50 CFM per toilet, shower and tub, and 100 for a jetted tub. A larger bath with a toilet, a shower and a tub needs 150 CFM.
What is the minimum CFM for a bathroom fan?
50 CFM intermittent or 20 CFM continuous, under 2021 IRC section M1505.4.4 and 2024 IRC M1505.5. That is at a static pressure of 0.25 inch water column, and it is not a free-air figure. Your local code may differ.
Does a bigger bathroom need more than 1 CFM per square foot?
Often yes. Above 100 square feet, HVI drops the area rule and adds up fixtures. Rooms over 100 square feet may also split the total across several fans, for example one over the shower and another over the toilet.
How long should a bathroom fan run?
About 20 minutes after you finish, as HVI recommends, so that damp air clears out. A timer switch helps. A fan rated at 1.0 sone or less is quiet enough that people leave it running.
When do I need a professional?
You need a licensed electrician for new wiring, especially over a tub or shower, and a professional for any new duct through a roof or wall. A permit may be required. Ask your building department what applies where you live.
This bathroom fan size is a guide using HVI's method and 2021 IRC section M1505.4.4, a minimum at 0.25 inch of static pressure. Local codes differ, so your building department and inspector decide. Long or bent runs cut the airflow, and venting into an attic can cause mold, so take it outdoors. Wiring over a shower or tub has its own rules.
Sources and review
- Bathroom exhaust fans, Home Ventilating Institute, no date shown, read November 2026.
- Bathroom ventilation, Home Ventilating Institute, no date shown, read November 2026.
- Chapter 15 exhaust systems, 2021 IRC, International Code Council, 2021, search excerpt.
- Chapter 15 exhaust systems, 2024 IRC, International Code Council, 2024, search excerpt.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
A smaller bathroom
CFM = Length × Width, and at least 50
- Length, Width
- In feet, at 1 CFM per square foot
Worked through: 7 × 10 = 70 CFM
A bigger bathroom
CFM = 50 × (Toilets + Showers + Tubs) + 100 × Jetted tubs
- 50, 100
- HVI's CFM per fixture
Worked through: 50 × 3 = 150 CFM
Check by room volume
CFM = Length × Width × Height × 8 ÷ 60
- 8
- HVI's eight air changes an hour
Worked through: 10 × 8 × 8 × 8 ÷ 60 = 85.33 CFM
The constants, and where they come from
CFM_PER_SQ_FT
Published figureCFM per square foot, for a bathroom up to 100 square feet
Applies to: Bathrooms with an 8 foot ceiling
Source: Bathroom ventilation, Home Ventilating Institute, no date on the page, read 2026-11.
HVI's recommendation, which it says is about eight air changes an hour. HVI's pages show a copyright date and no publication date.
AREA_LIMIT_SQ_FT
Published figureThe area where HVI's method changes, in square feet
Applies to: HVI's two methods
Source: Bathroom ventilation, Home Ventilating Institute, no date on the page, read 2026-11.
HVI's pages differ on exactly 100, and we treat that as the area method.
MINIMUM_CFM
Published figureSmallest fan, and the code minimum for intermittent use, in CFM
Applies to: Places using the 2021 IRC, at 0.25 inch water column. Local codes differ
Source: Chapter 15 exhaust systems, 2021 IRC, International Code Council, published 2021, read 2026-11. This source is about 5 years old, and it is the one the figure rests on.
Also from: Bathroom exhaust fans, Home Ventilating Institute, no date on the page, read 2026-11.
Also from: Chapter 15 exhaust systems, 2024 IRC, International Code Council, published 2024, read 2026-11.
2021 IRC M1505.4.4, and 2024 IRC M1505.5, read from an ICC search excerpt and Seattle's reproduction, since ICC's reader returned no text. HVI recommends 50 for rooms of 50 square feet and under, though another of its pages shows 40 for a 40 square foot room.
CFM_PER_FIXTURE
Published figureCFM for a toilet, a shower or a tub, in a room over 100 square feet
Applies to: Bathrooms over 100 square feet
Source: Bathroom exhaust fans, Home Ventilating Institute, no date on the page, read 2026-11.
HVI's fixture table. It also allows the total to be split across several fans.
CFM_PER_JETTED_TUB
Published figureCFM for a jetted tub, in a room over 100 square feet
Applies to: Bathrooms over 100 square feet
Source: Bathroom exhaust fans, Home Ventilating Institute, no date on the page, read 2026-11.
From the same HVI fixture table.
CONTINUOUS_CFM
Published figureContinuous rate that can replace 50 CFM intermittent
Applies to: Fans that run all the time
Source: How much ventilation do I need?, Home Ventilating Institute, no date on the page, read 2026-11.
Also from: Chapter 15 exhaust systems, 2021 IRC, International Code Council, published 2021, read 2026-11.
HVI and the code tables both state 20 CFM continuous.
AIR_CHANGES_PER_HOUR
Published range, narrowedAir changes an hour, for the high-ceiling check
Applies to: Rooms with a ceiling above 8 feet, as a guide
Source: Bathroom ventilation, Home Ventilating Institute, no date on the page, read 2026-11.
HVI says tall ceilings may need more with no figure. Applying its eight air changes to the room's volume is our own derivation.
MINUTES_PER_HOUR
DefinitionalMinutes in one hour
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Sixty minutes make an hour by definition, which turns air changes per hour into a rate per minute.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the bathroom fan CFM calculator
- 1
Measure the length and width of the bathroom
- 2
Measure the ceiling height
- 3
If it is over 100 square feet, count the fixtures
- 4
Read the CFM to look for, and the code minimum
- 5
Check the fan's sones and whether it is HVI-certified
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Up to 100 square feet goes by area, and above that by fixture, as HVI sets out
- An 8 foot ceiling. Above that, HVI says more air may be needed, and a volume check is shown
- No correction for duct length, as we found no primary method. Long or bent runs need a stronger fan
- A rating is the fan's listed figure. Real delivered flow depends on your duct
Pro tips
- Look for HVI-certified fans, as HVI warns that others may be inflated
- Pick 1.0 sone or less, so that people actually use it
- Keep the duct short and straight, with as few elbows as you can