Formula & example
ACH=60·CFM/Vft³; CFMtarget=ACHtarget·Vft³/60.
A 20×15×8 ft room at 200 CFM has 2,400 ft³ volume and 5 ACH.
HVAC engineering calculator · Updated Sep 17, 2026
Air Changes per Hour Calculator — ACH, CFM & Room Volume
Calculate ACH from room dimensions and airflow, plus the CFM and m³/h required for a target air-change rate.
Inputs
Result
Room volume 2,400 ft³
Current ACH 5 /h
Minutes per air change 12 min
Required airflow @ target 240 CFM
Required metric airflow 407.7626 m³/h
Assumptions & limits
- Rectangular room volume is used.
- Entered airflow is treated as delivered airflow; duct/static losses are not inferred.
Frequently asked questions
What does this Air Changes per Hour Calculator calculate?
Calculate ACH from room dimensions and airflow, plus the CFM and m³/h required for a target air-change rate.
Can I use SI and US customary units?
Yes. The calculator exposes the unit basis in the inputs and reports useful cross-unit outputs where the engineering workflow benefits from them.
Is this a replacement for HVAC design software?
No. It is a transparent deterministic engineering calculator for sizing, diagnostics and cross-checks. Final equipment, duct and refrigeration decisions should use measured conditions, manufacturer data and applicable codes or standards.