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We present a software tool for planning two-phase case-control studies assuming categorical covariates. Two-phase designs, in which validation or complete information data are sampled stratified both on a dichotomous outcome and a covariate from a first-phase study with incomplete data, result in efficient estimates compared to standard designs. Efficiency and power depend on sample size, sampling fractions within each stratum x outcome cell, distributional assumptions and the regression model. Our software, called Two-Phase Planning Tool (or 2P Planning Tool), offers a graphical user interface (GUI) to organize and input the relevant anticipated entities and calculates a normed, expected two-phase case-control study. The 2P Planning Tool is especially helpful in selecting a stratification. The data are output into an Excel-sheet, which in turn can be read into a standard statistics package to perform "experimental" power calculations. Its use is illustrated by an example from epidemiology. Software for analyzing logistic two-phase studies is also provided.