Bathroom Exhaust Fan Calculator
Estimate bathroom exhaust airflow from room volume and target air changes, then account for duct loss, fan quantity, energy use and cost.
Default project preview
85 CFM
Recommended fans
1
Room volume
640 ft³
Effective airflow
88 CFM
Estimated actual ACH
8.3
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Bathroom length
Enter this project assumption in ft. The current imperial default is 10 ft.
Bathroom width
Enter this project assumption in ft. The current imperial default is 8 ft.
Ceiling height
Enter this project assumption in ft. The current imperial default is 8 ft.
Target air changes
Enter this project assumption in ACH. The current imperial default is 8 ACH.
Rated airflow per fan
Enter this project assumption in CFM. The current imperial default is 110 CFM.
Duct and grille allowance
Enter this project assumption in %. The current imperial default is 20 %. Minimum: 0. Maximum: 50.
Power per fan
Enter this project assumption in W. The current imperial default is 30 W. Minimum: 0.
Operating time
Enter this project assumption in hours/day. The current imperial default is 1 hours/day. Minimum: 0. Maximum: 24.
Electricity price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
Price per fan
Enter this project assumption in a unitless value. The current imperial default is 95. Minimum: 0.
How to read the outputs
Recommended fans
110 CFM rated each. Default preview: 1.
Room volume
rectangular room. Default preview: 640 ft³.
Effective airflow
loss allowance applied. Default preview: 88 CFM.
Estimated actual ACH
with rounded fan count. Default preview: 8.3.
Daily energy
1 operating hours. Default preview: 0.03 kWh.
Fan and daily energy cost
fans / electricity. Default preview: $95.00 / $0.01.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Required airflow = room volume × target ACH ÷ 60 for CFM, or room volume × ACH for m³/h. Fan count uses rated airflow after the duct-loss allowance.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate preliminary bathroom exhaust airflow from room area, volume or fixture-based rules while keeping fan selection tied to duct resistance and local requirements.
Calculation steps
- 1Measure room area and ceiling height.
- 2Apply the selected airflow method.
- 3Estimate duct length and fittings separately.
- 4Choose a fan from its performance curve at expected static pressure.
Worked numerical example
An 8 ft × 10 ft bathroom suggests 80 cfm using 1 cfm per ft². A long duct with multiple elbows may require a fan rated above 80 cfm to deliver that airflow in service.
Purchasing and rounding
Purchase based on delivered airflow, sound rating, controls and listed installation location.
Interpret the result
The calculator is preliminary; code, make-up air and whole-house ventilation interactions require verification.
Page-specific assumptions
- One bathroom zone.
- Entered dimensions are finished dimensions.
- Duct losses are evaluated separately.
Common mistakes
- Choosing by free-air cfm only.
- Using undersized flexible duct.
- Terminating in an attic.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Bathroom Exhaust Fan Calculator questions
Why is installed airflow lower than the fan rating?+
Ducts, elbows, grilles and exterior terminations create resistance that can reduce delivered airflow.
Should the fan run after a shower?+
Post-run time can help remove remaining moisture, but controls and duration depend on the room and local guidance.
Can the fan discharge into the attic?+
Generally exhaust should terminate outdoors through an approved route, not into an enclosed building cavity.