Bathroom Fan Size Calculator

Bathroom exhaust fan sizing is based on room volume and the number of air changes per hour needed.

For bathrooms up to 100 sqft: CFM = 1 CFM per square foot of floor area (minimum 50 CFM)

For bathrooms over 100 sqft: CFM = (Length × Width × Height × 8 air changes) / 60

Additional CFM for fixtures:

• Jetted tub: +50 CFM
• Separate shower: +50 CFM
• Enclosed toilet: +50 CFM

Sone ratings (noise):

• 0.3–1.0 sones — Very quiet
• 1.0–2.0 sones — Normal conversation level
• 2.0–4.0 sones — Noticeable noise

Duct length considerations:

• Add 1 CFM per foot of duct for runs over 6 feet
• Add 15 CFM per 90° elbow
• Use rigid duct over flex for better airflow

The IRC building code requires 50 CFM intermittent or 20 CFM continuous ventilation for bathrooms.