Work out air changes per hour instantly with clear inputs, formula shown and shareable results.
Air changes per hour compares the volume of air delivered in an hour with the volume of the room itself: ACH = CFM x 60 / room volume. It is the standard yardstick for ventilation effectiveness because it is independent of room size, so a small bathroom and a large workshop can be judged on the same scale.
Air changes
ACH = CFM x 60 / volume ft³
Turnover time
Minutes per change = 60 / ACH
ACH = CFM x 60 / volume ft³. Air changes per hour compares the volume of air delivered in an hour with the volume of the room itself: ACH = CFM x 60 / room volume.
Typical guidance is 6-8 ACH for kitchens, 8-10 for bathrooms, 4-6 for offices and 10 or more for workshops handling fumes. Local codes take precedence.
This calculator takes 4 inputs: Supply airflow, Room length, Room width, Ceiling height. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.