GeoCalculator is based on the familiar Windows Calculator but takes geological structural data for input and output. Calculations include most common structural geometrical calculations as well as drillhole calculations for oriented, partially oriented, and unoriented core. GeoCalculator accepts and outputs structural data using a wide variety of formatting conventions for referencing lines, planes, and directions in geological space. Because input and output conventions can be set separately, GeoCalculator can be used to convert file data from one geological orientation convention and format to another. As well as a numerical results window, calculation results can be displayed in stereographic projection. Oriented drillhole calculations include not only the commonly used oriented core calculations, but also calculations for partially oriented core, a method devised by the author for using known local structural features in unoriented core as a proxy for the orientation mark in oriented core. Thus oriented core marks can be extrapolated to unoriented sections of the same core, or old unoriented core can be processed as if it were oriented.