HydrogeoEstimatorXL is a free software tool for the interpretation of flow systems based on spatial hydrogeological field data from multi-well networks. It runs on the familiar Excel spreadsheet platform. The program accepts well location coordinates and hydraulic head data, and returns an analysis of the area flow system in twodimensions based on a) a single best fit plane of the potentiometric surface and b) three-point estimators, i.e., well triplets assumed to bound planar sections of the potentiometric surface. The software produces graphical outputs including histograms of hydraulic gradient magnitude and direction, groundwater velocity (based on a site average hydraulic properties), as well as mapped renditions of the estimator triangles and the velocity vectors associated with them. Within the software, a transect can be defined and the mass discharge of a groundwater contaminant crossing the transect can be estimated. This kind of analysis is helpful in gaining an overview of a site’s hydrogeology, for problem definition, and as a review tool to check the reasonableness of other, independent calculations.