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A liquid crystal variable retarder (LCVR) enables fast, automated control of retardance that can be used as a variable waveplate in polarimetric instruments. However, precise control of the polarization state requires calibration of the LCVR. A manufacturer calibration curve is typically supplied for a single specific wavelength and temperature, but for applications under different conditions, additional calibration is needed. Calibration is typically performed with crossed polarizers to generate an intensity curve that is converted to retardance, but this method is prone to noise when retardance is close to zero. Here, we demonstrate a simple common-path Sagnac interferometer to measure retardance and provide open source software for automated generation of calibration curves for retardance as a function of wavelength and voltage. We also provide a curve fitting method and closed-form functional representation that outputs the voltage needed to achieve a desired retardance given a specified wavelength.