Ventilation Rate Calculator
Find the whole-house mechanical ventilation rate from ASHRAE 62.2, with air changes per hour and the fan size for intermittent running.
Why tight houses need fans
Older houses get fresh air through leaks around windows, doors and floors. Modern houses built to tight energy codes leak far less, which saves heating and cooling but can let moisture, cooking smells, carbon dioxide and pollutants from furniture and cleaning products build up. The answer is deliberate mechanical ventilation: an exhaust fan, a supply fan or a balanced system with heat recovery, running at a known rate. In North America the rate is set by ASHRAE Standard 62.2, which many building codes adopt.
The 62.2 formula
The 2016 edition of ASHRAE 62.2 sets the total required ventilation rate as 0.03 CFM per square foot of conditioned floor area plus 7.5 CFM per occupant, with occupants assumed to be the number of bedrooms plus one. A 2,000 ft² house with three bedrooms needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM. With 8 ft ceilings that is 0.34 air changes per hour, a complete change of air roughly every three hours. The older 2010 edition used 0.01 CFM per square foot, giving only 50 CFM for the same house, and many local codes still reference it, so check which version applies.
Infiltration credit and intermittent fans
The 2016 standard lets you subtract part of the house's measured natural leakage, from a blower door test, from the required fan rate, so leakier houses need smaller fans. This calculator gives the total requirement before that credit. Fans do not have to run continuously: a fan that runs half of each hour must deliver twice the rate while on, 180 CFM for the example house. Long off periods reduce effectiveness, so continuous low-speed operation is generally preferred where the fan is quiet enough.
Common mistakes
Bathroom and kitchen exhaust fans are often counted as whole-house ventilation when they run only briefly; local exhaust is a separate requirement in 62.2, typically 50 CFM intermittent for a bathroom and 100 CFM for a kitchen range hood. Another common problem is a fan whose real airflow is far below its rating, because of long, crushed or undersized ducts. A rated 80 CFM fan on a 3 in. duct may deliver half that. Measuring delivered airflow with a flow hood or a simple bag test is the only way to know.
How to use it
- Enter the conditioned floor area in square feet and the number of bedrooms.
- Enter the average ceiling height to see the equivalent air changes per hour.
- Enter the share of each hour the fan runs, 100% for continuous operation.
- Read the required rate and, for intermittent running, the fan airflow needed while it is on.
Questions from the job site
How much mechanical ventilation does a house need?
Under ASHRAE 62.2-2016, 0.03 CFM per square foot of floor area plus 7.5 CFM for each occupant, counted as bedrooms plus one. A 2,000 ft² three-bedroom house needs 90 CFM continuously, before any credit for measured natural air leakage.
What is the difference between ASHRAE 62.2-2010 and 2016?
The 2010 edition used 0.01 CFM per square foot plus 7.5 CFM per occupant; the 2016 edition raised the area term to 0.03 and added a credit for measured infiltration. The 2016 rate is higher for most houses, but leaky houses can subtract part of their natural leakage.
Do bathroom fans count as whole-house ventilation?
Only if they are designed and controlled to run at the required rate, usually continuously or on a timer. Ordinary bathroom fans switched on for a few minutes are local exhaust, which 62.2 requires separately: typically 50 CFM intermittent for bathrooms and 100 CFM for kitchens.
Can a ventilation fan run part of the time?
Yes, if it runs at a correspondingly higher rate. A fan running 50% of each hour needs twice the continuous rate while on. Very long off periods reduce effectiveness, so continuous or frequent short cycles are preferred, ideally with a quiet fan.
Disclaimer: HVAC results are estimates from simplified methods and the values you enter. They are not a Manual J, S or D design and must not replace one for equipment selection or permits. Gas, electrical and refrigerant work must be carried out by licensed technicians to local codes, and combustion appliances need working carbon monoxide alarms.