How much fresh air does your house need?
Enter the floor area and bedrooms, get the ASHRAE 62.2 airflow in CFM and how long a fan must run.
ASHRAE counts bedrooms plus one person, and never fewer than one bedroom: a studio counts as 1.
A fan size gives the hours a day it must run if it cycles; a kitchen area gives the continuous 5 air changes an hour in cfm. Check the ASHRAE 62.2 edition your local code has adopted.
How much ventilation does a house need under ASHRAE 62.2?
ASHRAE Standard 62.2 sets the whole-dwelling rate as Qtot = 0.03 × floor area (ft²) + 7.5 × (bedrooms + 1), in cfm, counting at least one bedroom. In SI it is 0.15 × floor area (m²) + 3.5 × (bedrooms + 1) in L/s. A 2,000 ft² house with 3 bedrooms needs 0.03 × 2,000 + 7.5 × 4 = 90 cfm, about 42 L/s, which in a house with 8 ft ceilings is roughly 0.34 air changes an hour. ASHRAE's own table gives the same 90 cfm for 1,501–2,000 ft² and 3 bedrooms. The 0.03 floor-area coefficient replaced the 0.01 of the 2010 edition (Addendum r to that edition).
Which edition of ASHRAE 62.2 applies, and is it required?
ASHRAE 62.2 is a voluntary standard until a building code, energy program or contract adopts a particular edition, and ASHRAE has published new editions in 2016, 2019, 2022 and 2025. The equation here is the one printed in ASHRAE's 2013-era addenda and used with the 2019 edition; later editions add provisions this calculator does not model. Check which edition your local code or program has adopted, and with your building department which parts apply to your home.
How many hours a day must an intermittent ventilation fan run?
When the system comes on at least once every 3 hours, ASHRAE counts the time-average airflow, so the fraction of time on is the required rate divided by the fan's rate. A 150 cfm fan serving a 90 cfm requirement runs 90 ÷ 150 = 60% of the time: 14.4 hours a day, or 36 minutes of every hour. Longer cycles need a ventilation-effectiveness adjustment from the standard's tables, which this calculator does not apply, and a fan smaller than the requirement cannot comply even running all day.
How do bathroom and kitchen fans fit in?
ASHRAE 62.2 sets local exhaust minimums separately: a bathroom fan of 50 cfm on demand or 20 cfm continuous, and in the kitchen a 100 cfm vented range hood on demand, or 300 cfm (or a capacity of 5 air changes an hour) for other kitchen fans, or 5 air changes an hour continuous in an enclosed kitchen. A local fan can also count toward the whole-house rate when it is used that way. For sizing a bathroom fan by area or a range hood by cooktop width, use the CFM calculator.
What is the infiltration credit, and does this calculator include it?
If a blower door test shows how leaky the house is, ASHRAE 62.2 lets the air that leaks in count toward the total, so the fan only has to make up the rest; for a detached house the credit was capped at two-thirds of Qtot in the 2013 edition. The method changed in later editions, so this calculator does not compute it and shows the full Qtot. Free 62.2 calculators are listed in DOE's Building America Solution Center.
Sources: ANSI/ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings — Addendum m to the 2013 edition (equations 4.1a and 4.1b, Table 4.1a, infiltration credit), Addendum u to the 2010 edition (intermittent ventilation), Addendum h to the 2019 edition (Tables 5-1 and 5-2), all free at ashrae.org; R. Karg, “ASHRAE 62.2-2016” (Efficiency Maine). Bathroom fans and range hoods by HVI: CFM calculator. Cleaning the air in one room: air purifier size calculator. Roof and attic vents: attic ventilation calculator. How we check our calculators.