Home Bathroom Exhaust Fan CFM Calculator
Bathroom Exhaust Fan CFM Calculator
Find the right bathroom exhaust fan size in CFM using both the square-footage method and the fixture-count method — the calculator checks both and recommends the larger figure, so the fan is never undersized.
Based on standard HVI/building-code sizing guidance. Always confirm against your local code for the final word.
Quick reference — bathroom fan CFM
By floor area: 50 sq ft → 50 CFM · 100 sq ft → 110 CFM · 150 sq ft → 150 CFM · 200 sq ft → 220 CFM
By fixtures: Toilet or shower alone → 50 CFM · Toilet + shower → 110 CFM · Toilet + shower + tub → 150 CFM · Add a jetted tub → 220 CFM
Use the calculator below for your exact bathroom — it checks both methods and takes the higher number.
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On this page: Calculator · Sizing methods · Noise (sone) guide · Duct & installation basics · Related calculators · FAQ
Sizing continuous whole-home ventilation instead of a single bathroom fan? See the HRV/ERV size calculator.
Size your bathroom exhaust fan
Floor area of the bathroom being ventilated.
Long or bendy runs need extra CFM headroom.
Fixtures in this bathroom
A combo shower/tub counts as one fixture — check either Shower or Standard bathtub, not both.
Changes the product suggestion only, not the CFM math.
How bathroom fan CFM sizing works
Two methods are used, and the safe practice — the one this calculator follows — is to check both and go with whichever number is larger:
1. Square-footage method
Multiply the bathroom’s floor area by 1 CFM per square foot, with a 50 CFM floor no matter how small the room is. A 65-square-foot bathroom needs about 65 CFM, rounded up to an 80 CFM fan. This method is the standard for smaller, single-fixture bathrooms.
2. Fixture-count method
Once a bathroom is 100 square feet or larger, or has multiple fixtures, floor area alone under-counts the ventilation load. Instead, add up CFM by fixture:
| Fixture | CFM |
|---|---|
| Toilet | 50 CFM |
| Shower | 50 CFM |
| Standard bathtub | 50 CFM |
| Combination shower/tub | 50 CFM (counts once) |
| Jetted / whirlpool tub | 100 CFM |
A powder room with just a toilet needs only 50 CFM. A full bath with toilet, shower, and a jetted tub needs 200 CFM — well above what the square-footage method alone would suggest for the same floor area.
Understanding sone (noise) ratings
CFM isn’t the only spec that matters — a powerful fan that’s too loud to run often just doesn’t get used. Sones measure perceived loudness on a roughly linear scale: double the sone rating and a fan sounds about twice as loud.
| Sone rating | Perceived loudness |
|---|---|
| Under 1.0 sone | Very quiet (“whisper-quiet”) |
| 1.0–2.0 sones | Normal for a mid-range bathroom fan |
| 2.0–3.0 sones | Noticeable but tolerable |
| 3.0+ sones | Loud — typical of older or budget fans |
A higher CFM rating doesn’t automatically mean louder — better-engineered fans move more air per sone than older designs, so it’s worth comparing both specs rather than CFM alone.
Duct & installation basics
- Vent to the outside, never the attic. Venting into an attic or soffit dumps moisture where it can cause mold and rot — the duct must terminate through a roof cap or wall cap outdoors.
- CFM rating is measured in free air. Duct length, elbows, and the termination cap all add resistance that reduces real airflow. Long runs (15+ ft) or multiple bends can meaningfully cut delivered CFM — size one tier up to compensate.
- Most codes require mechanical ventilation in bathrooms without an operable window, and increasingly require it regardless of window size — check your local building code.
- Humidity-sensor fans turn on automatically when they detect rising humidity, which is useful for consistent moisture control without remembering to flip a switch.
Frequently asked questions
What size bathroom exhaust fan do I need?
For bathrooms under 100 square feet, the standard rule is 1 CFM per square foot, with a 50 CFM minimum — an 80-square-foot bathroom needs an 80 CFM fan. For bathrooms 100 square feet or larger, or with multiple fixtures, size by fixture count instead: 50 CFM each for a toilet, shower, or standard tub, and 100 CFM for a jetted or whirlpool tub. Use the calculator above for a number that checks both methods and recommends the larger figure.
How many CFM per square foot does a bathroom fan need?
The standard rule of thumb is 1 CFM per square foot of bathroom floor area, with a 50 CFM minimum regardless of how small the room is. This square-footage method applies to bathrooms under 100 square feet; larger bathrooms are sized by fixture count instead, since floor area alone under-represents the ventilation load once multiple fixtures are involved.
Is it better to oversize a bathroom exhaust fan?
A modest amount of oversizing is fine and often wise — it lets the fan clear moisture faster and run at a lower, quieter speed if it has variable settings. Significant oversizing mainly wastes a little electricity and adds a small amount of noise; it doesn’t cause the damage that an oversized heating or cooling system can. When in doubt, round up to the next standard CFM size rather than down.
What does the sone rating on a bathroom fan mean?
Sones measure perceived loudness — roughly double the sone rating and a fan sounds about twice as loud. Fans rated under 1.0 sone are considered very quiet (whisper-quiet); 1.0 to 2.0 sones is normal for a mid-range bathroom fan; older or bargain fans often run 3 to 4 sones or louder. A higher CFM fan isn’t necessarily louder — better-engineered fans move more air per sone than older designs.
Do I need a continuous or intermittent bathroom fan?
Most homes use an intermittent fan — on a switch or humidity sensor, running only while showering or for a while after. Continuous low-flow ventilation (running around the clock at a low CFM) is a whole-home strategy under ASHRAE 62.2, more relevant to tight, well-sealed homes without much natural air leakage. If you’re sizing continuous whole-home ventilation rather than a single bathroom, see the HRV/ERV size calculator instead.
Does duct length affect the CFM a bathroom fan actually delivers?
Yes. A fan’s CFM rating is measured in free air with no ductwork attached; every foot of duct, each elbow, and the exterior vent cap all add static pressure that reduces real-world airflow. Long runs (over about 15–20 feet) or runs with several elbows can cut delivered CFM noticeably, so it’s common practice to size one tier above the calculated minimum when the duct run is long or bends more than once or twice.