About this tool
The Whole-House Ventilation Calculator is a free, in-browser tool that returns the continuous mechanical ventilation airflow a home needs to maintain healthy indoor air quality under the ASHRAE 62.2 residential standard. The whole-house fan flow is Qfan (CFM) = 0.03 × conditioned floor area (sq ft) + 7.5 × (number of bedrooms + 1). The area term scales with the volume of the house, and the bedroom term is a proxy for the number of occupants (bedrooms + 1 assumes two people in the primary bedroom and one in each additional room).
All of the math runs locally in your browser, so nothing you type is uploaded. The calculator shows the two terms of the equation separately so you can see how floor area and occupancy each contribute, and it converts the required flow to cubic metres per hour for metric users. The base 62.2-2019 equation is used without an infiltration credit, which is the conservative choice; tighter or looser envelopes can adjust the target, but the base figure is a safe design starting point for a builder or inspector.
Whole-house ventilation is separate from local exhaust. ASHRAE 62.2 also calls for bathroom exhaust of 50 CFM intermittent (or 20 CFM continuous) and kitchen exhaust of 100 CFM intermittent (or 25 CFM continuous). If a fan runs intermittently rather than continuously, its effective flow over the on/off cycle must still meet the Qfan target. Use this figure to size a supply, exhaust, or balanced ventilation system and to document compliance.