Mode is a fundamental concept in Western music theory, as well as many aspects of music perception. It is foundational to tasks ranging from identifying chords, detecting cadences, and assessing form, as well as the encoding/decoding of musical emotion and expression. Here we expand the categorical notion of mode as major or minor to a continuum, an approach we refer to as relative mode. We formulate and evaluate a computational model that calculates this property from either symbolic or audio representations of music. Building on common key-finding techniques created to identify mode in a categorical manner, here we use them to infer the relative mode based on the difference between the potential key candidate strengths in major and minor keys. The model evaluation is based on a corpus consisting of excerpts from Preludes by Bach, Chopin, and Shostakovich previously assessed by expert music analysts. Our results suggest that the model is able to predict the relative mode to a degree that closely matches evaluations by experts using both audio and notated scores. A pragmatic set of parameters for the model is identified and the shortcomings and the applicability of the model to other eras and genres are discussed