A bathroom exhaust fan is sized in cubic feet per minute (CFM), and the industry rule is simple: for a bathroom up to 100 sq ft, about 1 CFM per square foot of floor, never less than 50 CFM; for a larger bathroom, add up the fixtures. The calculator below applies the Home Ventilating Institute (HVI) guidance to your room, checks the air changes per hour for tall ceilings, and the chart covers common bathroom sizes. Further down: how to read the airflow and sones on the box, what ENERGY STAR requires, and where the fan should go.
Bathroom exhaust fan CFM calculator
Fixtures only count for rooms over 100 sq ft. For a smaller bathroom the floor area decides, whatever is in it.
Bathroom fan CFM chart by room size
| Bathroom | Floor area | Fan size (HVI) | Air changes per hour, 8 ft ceiling | For 8 air changes at a 9 ft ceiling |
|---|---|---|---|---|
| 5 x 7 ft | 35 sq ft | 50 CFM | 10.7 | 42 CFM |
| 5 x 8 ft | 40 sq ft | 50 CFM | 9.4 | 48 CFM |
| 5 x 9 ft | 45 sq ft | 50 CFM | 8.3 | 54 CFM |
| 6 x 8 ft | 48 sq ft | 50 CFM | 7.8 | 58 CFM |
| 6 x 9 ft | 54 sq ft | 54 CFM | 7.5 | 65 CFM |
| 7 x 9 ft | 63 sq ft | 63 CFM | 7.5 | 76 CFM |
| 7 x 10 ft | 70 sq ft | 70 CFM | 7.5 | 84 CFM |
| 8 x 10 ft | 80 sq ft | 80 CFM | 7.5 | 96 CFM |
| 8 x 12 ft | 96 sq ft | 96 CFM | 7.5 | 116 CFM |
| 9 x 11 ft | 99 sq ft | 99 CFM | 7.5 | 119 CFM |
| 10 x 10 ft | 100 sq ft | 100 CFM | 7.5 | 120 CFM |
The 50 CFM floor is why a small powder room or a 5 x 7 ft bathroom still gets a 50 CFM fan. HVI’s bathroom page puts it slightly differently, with a worked example in which a 40 sq ft bathroom takes a 40 CFM fan, while its sizing table gives 50 CFM as the minimum. We follow the table and use 50 CFM as the minimum throughout; the larger fan also leaves some margin for airflow lost in the duct.
Why 1 CFM per sq ft? HVI bases the rule on about eight air changes per hour, which is what 1 CFM per sq ft gives in a room with an 8 ft ceiling: an 8 x 10 ft bathroom holds 640 cu ft of air, and 80 CFM replaces it in 8 minutes. If your ceiling is higher, or vaulted, the same rule gives fewer air changes, so size up using the last column of the chart or the calculator.
Bathrooms over 100 sq ft: add up the fixtures
| Fixture | Exhaust rate (HVI) |
|---|---|
| Toilet | 50 CFM |
| Shower | 50 CFM |
| Bathtub | 50 CFM |
| Jetted tub | 100 CFM |
| Example bathroom | Area | Fixture total | How to split it |
|---|---|---|---|
| 12 x 10 ft, toilet, tub/shower combination | 120 sq ft | toilet 50 + tub 50 = 100 CFM | One fan over the tub, or one central fan |
| 12 x 12 ft, toilet, separate shower and tub | 144 sq ft | 50 + 50 + 50 = 150 CFM | Fans over the shower and tub, or one 150 CFM fan |
| 14 x 12 ft, enclosed toilet room, shower, jetted tub | 168 sq ft | 50 + 50 + 100 = 200 CFM | A fan in the toilet room plus one or two over the wet areas |
| 20 x 12 ft, enclosed toilet, shower, tub (HVI’s example) | 240 sq ft | 50 + 50 + 50 = 150 CFM | HVI: three 50 CFM fans, or one 150 CFM fan |
HVI gives two ways to deliver the total: several smaller fans, each placed where the moisture is made (over the shower, over the tub, in the toilet room), or one larger fan in a central spot that pulls air across the whole room. Several fans put the exhaust where it is needed and only run when that part of the room is used; one fan is cheaper to install but needs to be placed so it does not just pull air straight from the door.
Reading the box: airflow at 0.1 and 0.25 in. w.g.
Fan airflow is measured against a resistance called static pressure, given in inches of water gauge (in. w.g.). The number printed largest on most boxes is the airflow at 0.1 in. w.g., which is a short, smooth duct. A real installation, with a few feet of flexible duct, an elbow or two and a roof or wall cap with a damper, usually pushes the fan harder, and its airflow falls. ENERGY STAR deals with this directly: a certified bathroom fan must still deliver at least 70% of its 0.1 in. airflow at 0.25 in. w.g. So a fan sold as 80 CFM that is ENERGY STAR certified keeps at least 56 CFM at the higher pressure; a non-certified fan may lose more.
If the box lists airflow at 0.25 in. w.g., size from that number. If it does not, and the duct run is long, choose the next size up. Keep the duct the diameter of the fan outlet or larger, keep it short, and avoid crushed or sagging flexible duct, which adds resistance quickly.
Sones and ENERGY STAR
| Fan type | Minimum efficacy (CFM per watt) | Maximum sound (sones) |
|---|---|---|
| Bathroom and utility room fans, 10 to 89 CFM | 2.8 | 2.0 |
| Bathroom and utility room fans, 90 to 200 CFM | 3.5 | 2.0 |
| Bathroom and utility room fans, 201 to 500 CFM (max speed) | 4.0 | 3.0 |
| In-line fans (single and multi-port) | 3.8 | no limit listed |
Sones measure loudness as people hear it, and the scale is linear: a 2 sone fan sounds about twice as loud as a 1 sone fan. ENERGY STAR caps bathroom fans up to 200 CFM at 2.0 sones, and HVI describes fans meant to run continuously as typically under 1.5 sones. Quiet matters in practice: a loud fan may get switched off before the moisture has cleared. Efficacy is airflow per watt of electricity; a fan that runs continuously or for long periods uses noticeably less power at the ENERGY STAR levels.
Where to put the fan and how long to run it
- Over or near the shower or tub. HVI says exhaust points are typically over or near the shower or tub, and in an enclosed toilet room. A fan directly over a shower must be one marked as suitable for that wet location; follow the fan maker’s wiring and installation instructions.
- Away from the supply air. Locate the fan away from where replacement air enters, so the air sweeps across the room instead of short-circuiting from the door to the fan.
- Leave a gap under the door. HVI calls for at least 3/4 in clearance under the bathroom door so that air can get in to replace what the fan removes.
- Run it 20 minutes after a shower. HVI recommends at least 20 minutes after each use (20 to 30 minutes in its living-areas guide). A timer switch or humidity-sensing control makes that automatic.
- Duct it outside. HVI’s guidance is for fans ducted to the outside. Moist air dumped into an attic or a soffit can condense on cold framing and sheathing, the kind of damage the fan is meant to prevent.
- Steam showers need their own fan in the steam room, switched on after use (HVI).
Intermittent or continuous ventilation
HVI allows a fan that runs all the time at a minimum of 20 CFM in place of a 50 CFM fan switched on when needed. Continuous ventilation suits tight houses where the bathroom fan doubles as whole-house ventilation: a very quiet, efficient fan runs at a low speed all day and boosts when the bathroom is used. If you go that way, check the efficacy and sound rating at the low speed, since that is where the fan spends most of its time.
Worked example: 8 x 10 ft bathroom with a 9 ft ceiling
Floor area 80 sq ft, under 100 sq ft, so the HVI rule gives 80 CFM. The room holds 8 x 10 x 9 = 720 cu ft; 80 CFM changes it 80 x 60 / 720 = 6.7 times an hour. To keep eight air changes the fan would need 720 x 8 / 60 = 96 CFM, so a 100 CFM fan is the better pick, especially with a 10 ft run of duct to a roof cap. Rated at 0.1 in. w.g., an ENERGY STAR fan of that size must still give at least 70 CFM at 0.25 in.
Building codes also regulate exhaust fans, duct termination and makeup air, and they vary by state and city. HVI’s figures are industry guidance; your building department decides what is required on your job.
Related calculators
- Range hood CFM calculator: kitchen hood airflow by cooktop width
- Whole house fan size calculator: cooling with outdoor air
- Toilet clearance guide: space around the toilet, vanity and shower
- Water heater size calculator: first-hour rating for the morning showers
Frequently asked questions
How many CFM do I need for a bathroom fan?
1 CFM per sq ft of floor area for bathrooms up to 100 sq ft, with a 50 CFM minimum (HVI). For larger bathrooms add 50 CFM for each toilet, shower and tub and 100 CFM for a jetted tub.
Is 50 CFM enough for a bathroom?
For a bathroom of 50 sq ft or less with an 8 ft ceiling, yes. An 8 x 10 ft bathroom needs about 80 CFM.
Is 80 or 110 CFM better for a bathroom?
Match the fan to the room: 80 CFM suits about 80 sq ft at an 8 ft ceiling; choose 110 for a room near 100 sq ft, a taller ceiling or a long duct run.
What sone rating is quiet for a bathroom fan?
ENERGY STAR allows up to 2.0 sones for fans up to 200 CFM. HVI describes continuous-duty fans as typically very quiet, under 1.5 sones.
How long should a bathroom fan run after a shower?
At least 20 minutes, according to HVI. A timer or humidity control switch makes this automatic.
Can a bathroom fan vent into the attic?
HVI’s guidance assumes the fan is ducted to the outside, and venting moist air into an attic risks condensation and mould on the framing. Check your local code for where the duct must end.
Sources
- Home Ventilating Institute (HVI), How Much Ventilation Do I Need? (HVI ventilation guide articles; read 25 October 2026).
- HVI, Bathroom Ventilation (small and large rooms, fixture table, worked examples, fan location and run time).
- HVI, Ventilating Living Areas (eight air changes per hour basis, ducting to the outside).
- US EPA, ENERGY STAR, Ventilating Fans Key Product Criteria: minimum efficacy, maximum sound levels and installed performance (read 25 October 2026).
Last reviewed: 25 October 2026. Numbers in the tables are our arithmetic from the sources listed; code sections are summarised, not quoted. How we check our work: methodology and corrections. Spotted an error? Please let us know.