How long should a bathroom fan run after a shower?

The required time depends on fan delivery, room volume, incoming-air humidity and how wet the surfaces are. Use the calculated air-only time plus a practical wet-surface allowance.

Should the fan run during the shower?

Usually yes. Removing moisture while it is generated lowers the peak humidity and reduces the after-run time.

What fan airflow should I enter?

Enter measured delivered airflow when available. A fan's label rating can be reduced by duct length, bends, grilles, dirt and poor makeup-air paths.

What does the coldest-surface margin mean?

A positive margin means the selected surface is warmer than the post-shower dew point. A negative margin means condensation is possible.

Is visible steam the same as high room humidity?

Not exactly. Steam near the shower is local condensed mist. The calculator estimates the average well-mixed room-air humidity.

Bathroom Fan Runtime & Shower Humidity Calculator

Q: Why can the target humidity be unattainable?

If incoming air contains more moisture than the selected target condition, exhaust ventilation alone cannot reach that target at the entered bathroom temperature.

What this calculator does

The tool converts temperature and relative humidity into humidity ratio, applies a room moisture balance during the shower, then models exponential moisture removal by the exhaust fan. An optional drying allowance accounts approximately for wet towels, grout, glass and other surfaces that keep releasing moisture after the shower ends.

How to use this bathroom fan calculator

1Measure the bathroom

Enter room dimensions and temperature. The calculator converts them into room air volume.

2Choose the humidity method

Use measured post-shower RH for the most practical result, or estimate moisture when no hygrometer is available.

3Enter real fan delivery

Use measured airflow when possible. Otherwise reduce the fan label rating for duct bends, grilles and restricted makeup air.

4Set a target and surface

Read the air-only time, recommended after-run and whether the selected cold surface may collect condensation.

Best practical method: place a humidity meter away from direct spray, run the fan during the shower, and use “Measured post-shower RH” after the reading stabilizes.

Inputs

Bathroom examples

Bathroom length m

Bathroom width m

Ceiling height m

Bathroom temperature °C

Initial bathroom RH %

Target RH %

Choose a practical target above the humidity of incoming replacement air.

Incoming-air temperature °C

Incoming-air RH %

Usually adjacent indoor air entering through the door undercut or open door.

Rated fan airflow m³/h

Installed airflow factor %

Allows for duct length, bends, grille resistance and poor makeup-air paths.

These factors are rough screening choices. A measured airflow is much more reliable.

Shower duration min

Airborne moisture release g/min

Illustrative source term. Use measured post-shower RH when possible.

Fan operation during shower

Wet-surface drying allowance min

Added after the air-only model reaches target RH.

Coldest surface temperature °C

Example: window edge, external wall corner or mirror.

Results

✓

Fan runtime appears practicalThe selected fan can reduce the modelled humidity to the target.

Post-shower RH

77.6%

Estimated room-air value

Recommended after-run

15.2 min

Air-only time plus surface allowance

Air-only time to target

5.2 min

Ideal well-mixed decay

Delivered fan airflow

80.0 m³/h

6.67 air changes per hour

Post-shower dew point

18.9 °C

Condensation possible on colder surfaces

HVI airflow screen

85.0 m³/h

Small-bathroom sizing reference

Airborne moisture added

150 g

Estimate mode only

Estimated condensation

0 g

Excess above room-air saturation

Coldest-surface margin

—

Surface temperature minus dew point

Bathroom humidity timeline

The chart shows the shower phase, the modelled fan decay and the selected RH target. The added drying allowance keeps the fan running beyond the ideal air-only target time.

Shower periodRoom RHTarget RH

Scroll the plot horizontally for readable labels.

Worked example: a typical small bathroom

Inputs

12 m³ room, 80 m³/h delivered airflow, 10-minute shower and 60% target RH.

Model result

The air-only decay takes about five minutes; adding ten minutes for wet surfaces gives roughly a 15-minute after-run.

Real-world check

If mirrors remain wet or RH stays high, verify duct discharge, clean the grille and measure actual airflow.

What to do with the result

01

Run the fan during the shower

Removing moisture at the source usually lowers the peak humidity and shortens the required after-run.

02

Keep it running after the shower

Use the calculated air-only time plus a practical allowance for wet surfaces. A timer or humidity control avoids relying on memory.

03

Check the real airflow

A dirty grille, long flexible duct, tight door or blocked exterior hood can reduce delivered airflow well below the fan label rating.

04

Vent directly outdoors

Do not discharge bathroom moisture into an attic, ceiling void, crawlspace or another indoor room.

Interpretation and limitations

Measured RH is preferred. Shower moisture generation depends strongly on water temperature, spray pattern, enclosure, room surfaces and user behaviour.

A high calculated runtime can indicate low delivered airflow, humid replacement air, a very small bathroom, or a low target RH. If the incoming-air humidity ratio is already above the target condition, exhaust ventilation alone cannot reach the selected target.

Frequently Asked Questions

Practical questions about fan runtime, humidity, airflow and condensation risk.