The size of a single round-edged opening that a house would have if all the small random holes between the indoors and the outdoors were combined into one hole. Used in the U.S. and based on a pressure of 4 Pa. It is the same concept as equivalent leakage area, but is calculated differently.
Source: Understanding Ventilation: How to Design, Select, and Install Residential Ventilation Systems - published by The Healthy House Institute
