TDFCalc is an application which implements the linear-quadratic (LQ) method for comparing the biologically effective dose (BED) of external beam radiation therapy (EBRT) fractionation regimens as well as low dose rate (LDR) and permanent implant brachytherapy treatments. It calculates BED for a given alpha/beta ratio as a function of the dose per fraction and number of fractions for EBRT, and initial dose rate for brachytherapy. For a predetermined BED, the app solves the quadratic equation to calculate dose per fraction as a function of the number of fractions desired. This enables quick comparison of modified fractionation regimens (e.g. hypofractionation or compensating for missing treatment fractions) vs a conventional and/or originally intended regimen.