This is volume three of a three-part study-the computer model with user's guide to estimate emissions from refinery process drains. APIDRAIN is a user-friendly WindowsTM-based software program operating under the Microsoft ExcelTM for WindowsTM environment. The model allows the user to sum up the emissions from a refinery process unit area or from the entire refinery. The model user can quickly and easily predict the contribution of process drain emissions to the total emission inventory of a refinery. Unit operators can use the output generated by the software for regulatory reporting according to the requirements of the Clean Air Act Amendments of 1990. The minimum system requirements for running APIDRAIN version 1.0 are PC 486 DX2 WindowsTM 3.11 platform, 8 MB RAM, and Windows 95TM/Windows NT. The user must have WindowsTM and ExcelTM installed on a personal computer to begin using the software. The APIDRAIN model is enhanced with automatic functions that enable the user to easily summarize important reporting information and to generate tabular emissions totals for both specific refinery process units and for the entire refinery. It is not necessary for the user to possess a rigorous understanding of ExcelTM to use APIDRAIN; only a few common principles of the WindowsTM operating environment are needed (such as point-and-click and navigation of tab and arrow keys).