HVAC
How to Size and Estimate a Bathroom Exhaust Fan
Contractor method for sizing a bathroom exhaust fan by room volume and estimating duct, wiring and unit cost.
A bathroom exhaust fan is sized by the room volume and the air changes per hour. The cost is the unit, duct and wiring.
THE SHORT ANSWER
How do you size a bathroom exhaust fan?
Multiply length by width by height for the volume, then apply the air change rate for the required flow. An 8 x 5 x 8 ft bathroom is 320 cu ft, and 8 changes/hr needs about 43 CFM, or 50 minimum.
Find the room volume
Multiply the room length, width and ceiling height. A 8 x 5 x 8 ft bathroom is 320 cu ft.
- Room length
- Room width
- Ceiling height
- Air changes/hr
Compute the required flow
For bathrooms, a common target is 8 air changes per hour. Convert volume and changes into CFM, then round up to a standard fan size.
Add duct and wiring
A ducted fan adds the duct run cost. Wiring and installation add a labor line.
Total and validate
320 cu ft at 8 changes/hr needs ~43 CFM, so a 50 CFM fan. Unit, duct and labor make the total. Use the Bathroom Exhaust Fan Sizing and Installation Calculator.