How many CFM does a bathroom exhaust fan need?
A bathroom local exhaust fan must move at least 50 cfm if intermittent, or 20 cfm if run continuously (IRC Table M1507.4).
When a bathroom or toilet room is served by a mechanical exhaust fan, the IRC sets the minimum airflow at 50 cfm for an intermittent (on-demand) fan, or 20 cfm for a fan operated continuously.
The exhaust air has to be ducted to the outdoors. It cannot dump into an attic, soffit, crawlspace, or other concealed building space.
These figures are minimums for the local exhaust point itself and are separate from any whole-house ventilation rate the dwelling must also meet.
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