Work out how many times a room’s air is replaced each hour from its volume and airflow — or the airflow you need to hit a target air change rate.
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Start free trialLength times width times height gives the room volume in cubic metres — the amount of air to be replaced.
Air changes per hour is the airflow (in m³/h) divided by the room volume. One air change means the whole room’s air is replaced once.
Enter an airflow to find the resulting ACH, or a target ACH to find the airflow the fan or system must deliver.
Air changes per hour (ACH) is how many times the entire volume of air in a room is replaced with fresh or conditioned air each hour. It’s a common way to specify ventilation — an ACH of 6 means the room’s air is fully replaced six times an hour.
It depends on the use. Offices and living spaces might need only a few air changes an hour, while kitchens, bathrooms, workshops and spaces with high occupancy or contaminants need many more. Ventilation standards such as AS 1668.2 set the requirements for different spaces.
Multiply litres per second by 3.6 to get cubic metres per hour, or divide m³/h by 3.6 to get L/s. This tool handles the conversion so you can enter airflow in L/s and read the air change rate directly.
Not always. ACH counts total air movement, which may include recirculated air. Minimum fresh (outdoor) air rates are usually specified per person or per floor area — see our ventilation calculator for that.
This calculator is a general engineering aid. Required air change and ventilation rates for a space are set by standards such as AS 1668.2 and depend on the use and occupancy. Confirm requirements and design with a competent HVAC professional.
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