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The frequency domain reflectometry (FDR) meter is a convenient sensor used to measure the water content of soils and substrates. An adequate calibration equation evaluated by statistical techniques is important to ensure the performance of the FDR meter. In this study, the calibration equation was used to express the relationship between the volumetric water contents of substrates measured with the gravimetric method then calculated with bulk density and the values read from the FDR meter for three kinds of substrates. Two types of FDR probes were applied to illustrate the improvement with calibration equations for the volumetric water contents of three kinds of substrates. Seven calibration equations were selected to evaluate the fitting agreement. No unique FDR calibration equation could be proposed to fit all FDR probes and substrates. Each FDR probe had a specific calibration equation for the different substrates. The accuracy of an FDR probe could be improved significantly with the selection of an adequate calibration equation.