Frequently Asked Questions
What is an ERU?
ERU stands for Equivalent Residential Unit. The ERU was determined by measuring the impervious area for a representative random sample of single-family residential properties located within Butler County. We used aerial photography and Geographic Information System (GIS) data provided by the Butler County Auditor’s Office, along with a computer mapping software program. The average impervious area for those measured single-family residential properties was 4,000 square feet.