Bathroom fan size calculator
Enter your bathroom’s dimensions and fixtures to get a target CFM rating, minimum code airflow, and the duct size to match. Results update as you type.
Bathroom specification
L × W × HFigures update automatically as you adjust the room or fixtures above.
Fan requirement
CFMSizing methods compared
The highest of the three methods sets the minimum; the recommended size adds a ~20% margin so the fan isn’t running flat-out.
| Fan output | Duct size | Max run |
|---|---|---|
| Up to 50 CFM | 4″ | 25 ft |
| 50–150 CFM | 6″ | 50 ft |
| 150–300 CFM | 8″ | 75 ft |
Bathroom Exhaust Fan CFM
Guessing at bathroom fan size is how you end up with a fan that’s either too weak to clear steam before it fogs the mirror in the next room, or so loud you avoid using it altogether. The bathroom fan size calculator above solves that by running your room’s dimensions and fixtures through the same three sizing methods contractors use — area, volume with air changes per hour (ACH), and fixture load — then giving you the higher of the three plus the duct size to match.
Below, we break down what CFM actually means, how each sizing method works, what U.S. building code requires as a minimum, and the mistakes that cause most bathroom fans to underperform even when the box says they shouldn’t.
What CFM means for a bathroom fan
CFM stands for cubic feet per minute — the volume of air a fan can move out of your bathroom every minute. A higher CFM rating means the fan clears humidity, odors, and steam faster. Size it too low and moisture lingers long enough to feed mold on grout and drywall; size it too high for a small room and you’re paying for noise and energy you don’t need.
How to use the calculator
- Measure your bathroom’s length, width, and ceiling height in feet and enter them.
- Check off every fixture in the room — toilet, shower, tub, jetted tub, or steam shower — since each adds its own airflow demand.
- Flag it if the bathroom has no window, since a fully enclosed room needs extra margin.
- Choose intermittent (switch-operated) or continuous (always-on) operation.
- Read your recommended CFM, minimum required CFM, matching duct size, and target noise level in the results panel.
Bathroom Ventilation Calculator, The three sizing methods, explained
Most online calculators use one method. This one checks all three and uses whichever number is highest, because any one method alone can under-size a fan for an unusual room.
Area method: 1 CFM per square foot of floor area, with a 50 CFM floor. Fast, but it ignores ceiling height and fixtures, so it tends to under-size larger or higher-ceilinged bathrooms.
Volume × ACH method: multiplies the room’s full cubic footage (length × width × height) by a target number of air changes per hour, then converts to CFM. This is the only method that accounts for ceiling height, which matters in bathrooms with vaulted or 9–10 foot ceilings.
Fixture method: adds up a fixed CFM allowance per fixture (toilet, shower, tub, jetted tub, steam shower). This is what typically pushes primary bathrooms with a soaking tub or steam shower above what the area alone would suggest.
| Method | Formula | Best for |
|---|---|---|
| Area | Floor area (sq ft) × 1 CFM, 50 CFM minimum | Quick estimate, standard 8 ft ceilings |
| Volume × ACH | (L × W × H × ACH) ÷ 60 | Vaulted or tall ceilings, continuous fans |
| Fixture load | Sum of per-fixture CFM allowances | Primary baths with a tub, jetted tub, or steam shower |
Building code minimums you still need to hit
Whatever the calculator recommends, your fan still has to clear the legal floor. Under the International Residential Code (IRC), a bathroom without an operable window needs mechanical exhaust rated at a minimum of 50 CFM for intermittent (switch-operated) fans, or 20 CFM for continuous fans, and the fan must be HVI-listed and duct directly to the outdoors — never into an attic, soffit, or wall cavity.
| Bathroom area | Code minimum CFM |
|---|---|
| Up to 100 sq ft | 50 CFM |
| 101–200 sq ft | 75 CFM |
| 201–300 sq ft | 90 CFM |
| Over 300 sq ft | 1 CFM per sq ft |
Some state and local codes go further: California’s Title 24 requires new-construction bathroom fans to run on a timer or humidity sensor rather than a plain on/off switch, and Washington State caps continuous fans at around 1.0 sone for noise. Check your local amendment before you buy — the IRC numbers are a national floor, not a ceiling.
Match the duct size to the fan
An undersized duct chokes airflow no matter how strong the fan is rated. As a rule of thumb:
- Up to 50 CFM → 4-inch duct, up to a 25 ft run
- 50–150 CFM → 6-inch duct, up to a 50 ft run
- 150–300 CFM → 8-inch duct, up to a 75 ft run
Every 90-degree elbow in the run effectively shortens your allowable duct length, and a fan’s box rating is measured with no duct attached at all — real-world delivered airflow after ducting and a wall or roof cap is often 20–35% lower than the printed number. That’s why sizing a notch above the bare minimum, as the calculator’s recommended figure does, holds up better once it’s installed.
Common bathroom fan sizing mistakes
- Sizing off floor area alone and skipping ceiling height, which under-sizes rooms with 9–10 ft or vaulted ceilings.
- Ignoring a jetted tub or steam shower, both of which add significant moisture load beyond a standard shower.
- Running long duct with multiple elbows and expecting the fan’s rated CFM to show up at the vent cap.
- Choosing continuous operation without picking a low-sone fan rated for 24/7 use — a loud intermittent fan left running around the clock is a fast way to stop using it.
- Venting into an attic or soffit instead of straight outdoors, which fails inspection and dumps moisture where it causes rot.
Frequently asked questions
What size fan do I need for a small bathroom (5×8 ft)?
A typical 5×8 ft bathroom with a toilet and shower needs roughly 100–120 CFM once you add the standard sizing margin, well above the 50 CFM code floor, and pairs with a 6-inch duct.
Is a bigger CFM fan always better?
No. Oversizing wastes energy, adds noise, and can even depressurize a tightly sealed bathroom enough to cause backdrafting from other combustion appliances in the home. Match the fan to the calculated requirement rather than buying the highest CFM available.
Can I use one fan for a bathroom with both a shower and a separate jetted tub?
Yes, as long as it’s sized for the combined fixture load — that’s exactly what the fixture method in the calculator accounts for, rather than sizing for the shower alone.
What’s the difference between intermittent and continuous fan sizing?
Intermittent fans run on a switch while the bathroom is in use and need to clear moisture in a short window, so they’re sized to a higher CFM (50 CFM code minimum). Continuous fans run at low speed around the clock as part of whole-house ventilation, so they’re sized lower (20 CFM minimum) but need a quiet, continuous-duty rating.
Do I need a bigger fan just because my bathroom has no window?
Yes — a fully enclosed bathroom relies entirely on the fan to clear moisture, so most sizing guidance adds roughly 10% over a bathroom that has a window to open as backup.