How to use this calculator
- Enter room floor area. Use the floor area served by the supply, return, exhaust or recirculation airflow.
- Enter ceiling height. Use average clear height if the room has a sloped or irregular ceiling.
- Set target ACH. Enter the air-change rate you are screening against.
- Enter design airflow. Use the measured or design airflow to check actual ACH and minutes per air change.
How it works
Room volume is floor area times ceiling height:
V = A · H
Air changes per hour compare the hourly airflow to that room volume:
ACH = Q · 3600 / V
with Q in m³/s and V in m³. The required airflow for a
target ACH is the same relation rearranged:
Q_req = V · ACH / 3600
In imperial units, the equivalent shortcut is ACH = CFM · 60 / ft³.
The calculator keeps the math in canonical SI units and converts the displayed
airflow to m³/h or CFM with the unit toggle.
Worked example
Verified against the live calculator
A 20 m² room with a 2.7 m ceiling has a volume of
54 m³. For a target of 6 ACH, the required airflow is
54 × 6 / 3600 = 0.09 m³/s, or about 324 m³/h
(191 CFM). If the measured airflow is 0.10 m³/s,
the room gets about 6.7 ACH, or one air change every
9 minutes.
Frequently asked questions
How do you calculate air changes per hour?
ACH = airflow × 3600 / room volume when airflow is in m³/s and volume is in m³. In imperial terms, ACH = CFM × 60 / room volume in ft³.
How much CFM do I need for a target ACH?
Required airflow = room volume × target ACH / 60 when volume is in ft³ and airflow is in CFM. For SI units, required airflow = room volume × target ACH / 3600 in m³/s.
Is ACH the same as outdoor ventilation rate?
No. ACH is a room air-change rate. Outdoor-air code calculations often depend on occupancy, floor area, zone ventilation effectiveness and system type, so use the applicable ASHRAE 62.1/62.2 or local-code method for compliance.
Can I use ACH for exhaust rooms?
Yes as a first-pass exhaust or dilution screen, as long as the airflow and room volume match the actual served space. Final designs still need pressure relationships, makeup air, capture effectiveness and code requirements.
Method & assumptions
- Assumes the entered airflow serves the same room volume entered above.
- ACH does not prove good mixing, capture, ventilation effectiveness or pollutant removal by itself.
- For occupied-building outdoor air, use the applicable ASHRAE 62.1, ASHRAE 62.2 or local-code ventilation method.