CFM from air changes, and whole-home ventilation
Enter a room's size and the air changes per hour you want to get CFM, or the CFM you have to get ACH. Then check the ASHRAE 62.2 ventilation rate for the whole home.
The air change rate depends on the job: use the rate in the design, the equipment instructions or the code that applies. ASHRAE 62.2's total rate counts bedrooms as at least 1, and the fan rate can be lower if the home's leakage is measured and an infiltration credit is taken; this calculator doesn't take that credit. Kitchen and bath exhaust are separate requirements. Your local code may use a different table (the IRC has its own), and it governs.
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CFM = volume × ACH ÷ 60, because ACH is per hour and CFM is per minute. ASHRAE 62.2-2022 total dwelling-unit rate: Qtot = 0.03 × floor area + 7.5 × (bedrooms + 1). An estimate for planning; size equipment and ducts from the design and the code your area has adopted.
CFM = room volume in cubic feet × air changes per hour ÷ 60. A 14 × 12 ft room with an 8 ft ceiling is 1,344 ft³, so 6 air changes an hour take 134 CFM. For whole-home ventilation, ASHRAE 62.2-2022 sets 0.03 CFM per square foot plus 7.5 CFM per bedroom plus one: 90 CFM for a 2,000 sq ft, three-bedroom home.
How to use the CFM calculator
Size the room
Length, width and ceiling height in feet.
Enter ACH or CFM
The air changes per hour you want, or the airflow you measured, to get the other.
Check ventilation
Floor area and bedrooms for the ASHRAE 62.2-2022 whole-home rate.
What the CFM calculator does
Both directions
CFM from ACH, and ACH from CFM, for any room.
ASHRAE 62.2-2022
Qtot = 0.03 × floor area + 7.5 × (bedrooms + 1), with bedrooms counted as at least 1.
Steps shown
Volume, airflow and the ventilation formula worked out in a table.
Honest limits
Says when infiltration credit, local exhaust or a local code rate changes the answer.
- Room airflow
- CFM = volume (ft³) × ACH ÷ 60
- Air changes
- ACH = CFM × 60 ÷ volume
- Whole-home rate
- ASHRAE 62.2-2022: Qtot = 0.03 Afloor + 7.5 (Nbr + 1), Nbr at least 1
- Not included
- Infiltration credit, kitchen and bath exhaust, duct sizing, Manual J/D
CFM Calculator FAQ
How do I calculate CFM from air changes per hour?
Multiply the room's volume in cubic feet by the air changes per hour you want and divide by 60. A 1,344 ft³ room at 6 ACH needs 1,344 × 6 ÷ 60 = 134.4 CFM.
How do I find ACH from CFM?
Multiply CFM by 60 and divide by the room volume. 200 CFM in a 1,344 ft³ room is 200 × 60 ÷ 1,344 = 8.9 air changes per hour.
How much ventilation does a house need under ASHRAE 62.2?
ASHRAE 62.2-2022 sets a total rate of 0.03 CFM per square foot of floor area plus 7.5 CFM per bedroom plus one, counting at least one bedroom. A 2,000 sq ft, three-bedroom home needs 60 + 30 = 90 CFM.
Is the 62.2 rate the fan size?
Not always. The standard lets you take credit for measured infiltration, which can lower the fan rate, and kitchen and bathroom exhaust are separate requirements. The code your area adopted may also use its own table, and it governs.
What ACH should I design for?
It depends on the space and the standard it falls under; there's no single number for every room. Use the rate in the design documents, the equipment instructions or the code that applies, then use this to turn it into CFM.
Where the numbers come from
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