Bathroom exhaust fan CFM calculator
Size a bathroom fan at 1 CFM per square foot of floor, with at least 50 CFM, for baths under 100 square feet. Bigger baths add 50 CFM per toilet, shower and tub, and 100 per jetted tub. An 8 x 10 ft bath needs about 80 CFM, ducted outdoors.
Updated · By the Interior Redesign AI editorial team · Sources cited on the page
Recommended fan size
80 CFM
1 CFM per sq ft for 80 sq ft, minimum 50 (HVI).
| Floor area | 80 sq ft |
|---|---|
| Area rule (1 CFM per sq ft, min 50) | 80 CFM |
| Fixture rule (50 each, 100 jetted tub) | n/a under 100 sq ft |
| IRC minimum (Table M1505.4.4) | 50 CFM or 20 continuous |
| Recommended | 80 CFM |
- CheckDuct too long: a 4 in flex duct at 80 CFM allows 0 ft after elbows. Shorten it, cut elbows or go up a size.
- OKVent it outdoors: the IRC (M1505.2) bans bath exhaust into an attic or crawl space.
HVI gives no ceiling-height adjustment, so the 8 ft ceiling does not change the result. Buy a fan whose CFM is rated at 0.25 in of static pressure, as the IRC requires.
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Key numbers
- Under 100 sq ft (HVI)
- 1 CFM per sq ft of floor area, minimum 50 CFM
- Over 100 sq ft (HVI)
- 50 CFM each per toilet, shower and tub; 100 CFM per jetted tub
- IRC minimum (2021, Table M1505.4.4)
- 50 CFM intermittent or 20 CFM continuous, rated at 0.25 in water column; the minimum wherever a fan is installed or required (R303.3 lets one replace an openable window)
- Where it vents
- Directly outdoors, never into an attic or crawl space (IRC M1505.2)
- Duct length (IRC Table M1504.2)
- Limited by duct type, diameter and fan rating; deduct 15 ft per elbow
- Enclosed toilet (HVI)
- Give it its own fan
Worked example
An 8 × 10 ft hall bath with a toilet and shower
80 sq ft, 8 ft ceiling, 10 ft of 4 in flex duct with one elbow to a wall cap.
- Area rule: 8 × 10 = 80 sq ft × 1 CFM = 80 CFM (above the 50 CFM minimum).
- The bath is under 100 sq ft, so the area rule decides.
- Duct: 4 in flex at the 80 CFM column allows 4 ft in Table M1504.2, which is 0 ft after one elbow.
- 10 ft is too long for that duct, so step up to 5 in flex or 4 in smooth-wall duct, which allow more length.
Buy a fan rated at least 80 CFM at 0.25 in static pressure, and use 5 in flex or 4 in smooth-wall duct for a 10 ft run.
How we calculate
Under 100 square feet we use the Home Ventilating Institute's rule: 1 CFM per square foot of floor, never less than 50 CFM. Over 100 square feet we switch to HVI's fixture method and add 50 CFM for each toilet, shower and bathtub and 100 CFM for a jetted tub. If you leave the fixtures blank for a big bath, we fall back to the area rule and say so.
HVI publishes no ceiling-height adjustment for bathrooms, so ceiling height is shown for reference only and does not change the result.
The duct check reads the 2021 IRC Table M1504.2: the maximum duct length for your duct type, diameter and fan rating, less 15 feet for every elbow. We round the fan up to the next table column (50, 80, 100, 125, 150, 200, 250 or 300 CFM), which is the conservative reading. The table assumes the fan is rated at 0.25 inch of static pressure.
Estimates, sources
Planning estimates and published guidelines, not quotes, designs or code rulings. Figures were checked on each source on the date shown. Local prices, your home and your jurisdiction's adopted code decide the real answer.
- How Much Ventilation Do I Need? (Home Ventilating Institute (HVI), as of Oct 26, 2026): bathrooms less than 100 sq ft: 1 cfm per sq ft of floor area, minimum 50 cfm; more than 100 sq ft: add 50 cfm per toilet, shower and bathtub, 100 cfm per jetted tub; an enclosed toilet should have its own fan; 20 cfm continuous may replace a 50 cfm intermittent fan; no ceiling-height adjustment is given
- 2021 International Residential Code, Chapter 15 Exhaust Systems (M1504.2, M1505) (International Code Council (ICC), as of Oct 26, 2026): Table M1505.4.4: bathrooms and toilet rooms 50 cfm intermittent or 20 cfm continuous, rated at 0.25 in water column; M1505.2: exhausted directly outdoors, never into an attic or crawl space; Table M1504.2: maximum duct length by duct type, diameter and fan rating, less 15 ft per elbow
- 2021 International Residential Code, Chapter 3 (R303.3, R307.1, Figure R307.1, R307.2) (International Code Council (ICC), as of Oct 26, 2026): Figure R307.1: 15 in from toilet centerline to a side wall, 21 in clearance in front of the toilet and lavatory, 24 in clearance in front of the shower opening, 30 in shower minimum; R307.2: walls above tubs with shower heads and in showers finished with a nonabsorbent surface to at least 6 ft above the floor; R303.3: a bathroom needs 3 sq ft of window (half openable) unless it has artificial light and a local exhaust system sized per M1505
- Bathroom Planning Guidelines with Access Standards (National Kitchen & Bath Association (NKBA), as of Oct 26, 2026): recommended: 30 in clear floor space in front of every fixture; 18 in from toilet or bidet centerline to any fixture, wall or obstacle; 20 in from lavatory centerline to a side wall; 36 in between two lavatory centerlines; shower interior at least 36 x 36 in; lavatory height 32-43 in; 32 in clear door opening (code references based on the 2015 IRC)
Bathroom fan CFM by room size
For baths under 100 square feet the floor area sets the fan size. Above that, the fixtures do: a large primary bath with a toilet, a shower and a jetted tub needs about 200 CFM by HVI's method.
A 132 sq ft bath with those three fixtures comes to 200 CFM. On 6 in smooth-wall duct with two elbows, Table M1504.2 allows 23 ft at the 200 CFM column.
| Bathroom | Floor area | Fan size |
|---|---|---|
| 5 × 8 ft hall bath | 40 sq ft | 50 CFM (minimum) |
| 7 × 10 ft bath | 70 sq ft | 70 CFM |
| 9 × 11 ft bath | 99 sq ft | 99 CFM |
| 12 × 11 ft, toilet + shower | 132 sq ft | 100 CFM |
| 12 × 11 ft, toilet + shower + jetted tub | 132 sq ft | 200 CFM |
Why the duct matters as much as the fan
A fan's CFM rating is measured against a set static pressure. A long, narrow or kinked duct adds resistance and the fan moves less air than the box says. That is why the IRC limits duct length: a 4 in flex duct is not allowed at all for a fan rated 100 CFM or more, while a 6 in smooth-wall duct has no length limit up to 100 CFM.
If your run is longer than the table allows, shorten it, remove elbows, switch to smooth-wall duct or go up a diameter. The exception in section M1504.2 allows any length if the fan maker's design criteria are met or the installed airflow is measured.
How to use this tool
Measure the floor
Room length and width. Enter the fixtures too if the bath is over 100 sq ft.
Describe the duct
Flex or smooth-wall, its diameter, its length to the outside and the number of elbows.
Read the size
The recommended CFM, how it was worked out, and whether the duct passes the IRC table.
Buy on the rating
Pick a fan rated at or above the result at 0.25 in static pressure, vented outdoors.
Questions
What size exhaust fan do I need for my bathroom?
For a bath under 100 sq ft, 1 CFM per square foot with at least 50 CFM: a 5 x 8 ft bath needs 50 CFM, an 8 x 10 ft bath 80 CFM. Above 100 sq ft, add 50 CFM per toilet, shower and tub and 100 per jetted tub.
What is the code minimum for a bathroom fan?
The 2021 IRC requires 50 CFM intermittent or 20 CFM continuous for a bathroom or toilet room without an openable window, rated at 0.25 inch of static pressure and exhausted directly outdoors. Check your local code; many places adopt the IRC with changes.
Can a bathroom fan vent into the attic?
No. The IRC (M1505.2) says bath exhaust must go directly outdoors and must not discharge into an attic, crawl space or other space inside the building.
Does ceiling height change the CFM I need?
Not in HVI's published bathroom rule, which is based on floor area or fixtures. This calculator shows ceiling height for reference only.
Can I use 3 inch duct for a bathroom fan?
Under IRC Table M1504.2, 3 in duct is only allowed for a 50 CFM fan on smooth-wall duct up to 5 ft. Most fans need 4 in or larger; check the table for your fan rating.
Is a bigger bathroom fan always better?
Not if the duct cannot carry the air. An oversized fan on a long 4 in flex duct moves far less than its rating and runs louder. Size the fan to the rule and the duct to the fan.
Does a toilet room need its own fan?
HVI recommends that an enclosed toilet have its own exhaust fan. Under the IRC, a toilet room without an openable window needs the same 50 CFM intermittent or 20 CFM continuous exhaust.
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