Moisture content measurements using time domain reflectometry (TDR) or water content reflectometry (WCR) are basic in many research areas. The goals of this study were to establish a relation between apparent dielectric number and WCR sensor output, to compare TDR and WCR calibration equations for two coarse road construction materials, and to investigate the influence of sampling volume for horizontally installed sensors. Measurements were performed in fluids of known dielectric number and in two incrementally saturated coarse materials. The effect of sampling volume was evaluated using an electrostatic finite-element model. A two-parameter equation was determined relating the apparent dielectric number to Campbell CS616 (Campbell Scientific, Logan, UT) WCR output (r2 > 0.99). A simple calibration can adapt the equation to individual CS616, or similar, sensors. For the finer, coarse material, a three-phase mixing model proved best, while for the coarser material no equation adequately described the measurements. Numerical simulations indicated that limited capillary rise, creating a rapid transition from wet to dry close to saturation, was the explanation, warranting caution when interpreting measurements in nearly saturated coarse materials.