National Pollutant Discharge Elimination System (NPDES) permit writers often need to calculate low flow statistics for reasonable potential analyses and water quality-based effluent limitation (WQBEL) calculations or to confirm estimates provided by the permittee during the NPDES permit development process. The typical calculation procedures for low flow statistics are complex and cumbersome to execute by hand or with spreadsheet-based tools. However, several software applications that greatly simplify this process are available to permit writers. The purpose of this handbook is to help permit writers estimate low flow statistic values in a variety of situations using these free, publicly available tools.