**Air changes per hour (ACH)** is a measure of airflow compared to a space's volume. Comparing the airflow to a space's volume is done to infer the number of times air within that space is replaced. Knowing the age of air within a space helps in understanding the systems effectiveness at removing contaminants from the space. As such, **ACH **is commonly used to defined requirements for healthcare and laboratory buildings.

The definition for **ACH **in imperial units uses airflow in CFM and volume in cubic feet:

ACH = (60*Airflow) / Volume

The definition for **ACH **in metric units uses airflow in L/S and volume in cubic meters:

ACH = (3.6*Airflow) / Volume

**ACH **will have a significant impact on the total energy use for a building due to the inclusion of volume in the definition.

*A useful rule is 1 CFM/SF = 6 ACH for a space with a 10ft ceiling*

This means that for any spaces with taller ceiling heights, the the equivalent CFM/SF will be higher. When modeling a building it's important to consider the impact of the volume on the airflow calculation.

**cove.tool** utilizes people and area based inputs for the mechanical airflow calculation. If using an ACH requirement for your project you can convert this to the area measures used, which are **CFM/SF** value (IP) or total **L/s/sqm **value (SI):

**ACH to CFM/SF**

CFM/SF = ACH * height (ft) / 60

**ACH to L/S/sqm**

L/S/sqm = ACH * height (m) / 3.6