# Air Changes per Hour (ACH, n = Q/V) Compute the air change rate (ACH / n) of a room from the outdoor supply airflow Q and the room volume V: n = Q/V (1/h). Three modes: solve ACH (given Q and V), solve airflow (given n and V), or solve volume (given n and Q). Flow in m³/s/m³/h/CFM, volume in m³/ft³/L. Also reports the well-mixed single-zone purge time to reach a target residual fraction ε (default 1%): t = −ln(ε)/n hours. > Canonical page: https://elysiatools.com/en/tools/air-changes-per-hour - **Category:** Math & Numbers - **Keywords:** air changes per hour, ACH, air change rate, ventilation, outdoor air, purge, decay, well-mixed, infiltration, HVAC ## Overview Calculate air changes per hour (ACH) from outdoor airflow and room volume, or solve for the required airflow or room volume. The tool supports m³/s, m³/h, CFM, m³, ft³, and litres, and reports well-mixed purge time for a selected residual fraction. ## Inputs - **Mode** (select): Solve ACH n (needs Q and V), solve Air Flow (needs n and V), or solve Volume (needs n and Q). - **Air Flow Rate Q** (number): Outdoor / supply volumetric airflow Q. Enter in the selected Flow Unit. Required unless solving flow. - **Flow Unit** (select) - **Room Volume V** (number): Room (conditioned space) volume V. Enter in the selected Volume Unit. Required unless solving volume. - **Volume Unit** (select) - **Air Changes per Hour n (ACH)** (number): Air-change rate n (1/h). Required when solving flow or volume; ignored when solving ACH. - **Purge Target (residual fraction)** (number): Residual contaminant fraction ε after purge, between 0 and 1 (exclusive of 0). Default 0.01 (1%). The well-mixed purge time is t = −ln(ε)/n hours. - **Decimal Places** (number) ## When to use - Find the ACH of a room from its outdoor supply airflow and volume. - Calculate the airflow needed to achieve a target ACH in a room. - Estimate well-mixed purge time to reach a chosen residual fraction. ## How it works - Choose whether to solve ACH, airflow, or room volume. - Enter the known airflow, room volume, or ACH values and select their units. - The calculator applies n = Q/V, converting airflow and volume to compatible units. - It calculates purge time with t = −ln(ε)/n and returns the results as JSON. ## Use cases - Check ventilation rates for rooms, buildings, and HVAC calculations. - Size outdoor airflow for a target air-change rate when room volume is known. - Estimate the time required to reduce a well-mixed contaminant to a selected residual level. ## Frequently asked questions ### What is the ACH formula? ACH is calculated as n = Q/V, where Q is airflow and V is room volume. ### Which calculation modes are available? You can solve for ACH from airflow and volume, airflow from ACH and volume, or volume from ACH and airflow. ### Which airflow units are supported? The tool supports m³/s, m³/h, and CFM. ### Which room volume units are supported? You can enter volume in m³, ft³, or litres. ### How is purge time calculated? For a well-mixed single zone, purge time is t = −ln(ε)/n hours, where ε is the target residual fraction. ## Related tools - [Absolute Value Calculator](https://elysiatools.com/en/tools/absolute-value-calculator): Calculate the absolute value of positive, negative, or decimal numbers and explain the sign relationship - [Bearing Life L10 Calculator](https://elysiatools.com/en/tools/bearing-life-l10): Calculate the basic rating life of a rolling bearing: L10 = (C/P)^p, with p = 3 for ball bearings and p = 10/3 for roller bearings. 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