Air changes per hour (ACH) calculator
Air changes per hour tells you how many times a fan replaces all the air in a room every hour — the real measure of whether ventilation is keeping up with moisture. Enter the room and the airflow.
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (Standard ventilation formula: ACH = CFM × 60 ÷ room volume). Last verified July 2026 — our methodology.
Air changes per hour
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- Room volume
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- Minutes per change
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- Verdict
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ACH = airflow in CFM × 60 ÷ the room's volume in cubic feet. Typical targets: a bathroom wants about 8 air changes per hour, a kitchen 7–8, and a damp basement or laundry roughly 4–6. Higher airflow or a smaller room means more changes and faster moisture removal.
Why air changes matter more than raw CFM
A 100 CFM fan clears a tiny powder room in seconds but barely stirs a large basement — the same airflow gives very different results depending on the room's size. Air changes per hour normalizes for that, which is why building codes and thebathroom fan sizingrules are built around it. If ventilation alone can't keep a space under the healthy 60% humidity ceiling, a right-sized dehumidifierremoves the moisture directly instead of just moving air around.