A bathroom ventilation calculator determines how many CFM (cubic feet per minute) your exhaust fan must move to comply with ASHRAE ventilation standards and keep moisture under control.
Room Size
Fixtures Present
How to Size a Bathroom Exhaust Fan
Proper bathroom ventilation removes moisture that causes mold, mildew, peeling paint, and structural damage. The ASHRAE 62.2 standard provides minimum ventilation requirements for residential bathrooms.
HVI Fan Sizing Rule
For bathrooms under 100 sq ft: plan 1 CFM per square foot. For bathrooms with multiple fixtures: add 50 CFM for each toilet and shower, and 100 CFM for jetted tubs. This calculator applies both rules and uses the higher result.
Installation Considerations
Duct length and bends reduce effective airflow. For every 90-degree duct bend, derate the fan by about 5 CFM. For ducts over 10 feet, size up one fan class. Always vent to the outside — venting to the attic causes moisture damage.
FAQ
How many CFM do I need for my bathroom?
ASHRAE recommends 1 CFM per square foot for bathrooms up to 100 sq ft. For bathrooms with multiple fixtures, add 50 CFM for each toilet, 50 CFM for each tub or shower. This calculator applies the correct standard based on your room.
What is CFM?
CFM stands for cubic feet per minute — the volume of air the fan moves per minute. Higher CFM means more ventilation, which is important for removing moisture, odors, and preventing mold growth.
What is a sone rating?
Sones measure fan noise level. 1 sone = quiet library, 2 sones = normal conversation, 4 sones = normal office. For bedrooms and bathrooms, look for fans rated under 2 sones for quiet operation.
Should I get a fan with more CFM than required?
Yes, sizing up 10-20% is recommended. Duct bends and length reduce effective airflow, and it's better to over-ventilate than under-ventilate.
Is this calculator free?
Yes, completely free with no signup required.