Time domain reflectometry (TDR) is widely used to measure and monitor soil water. The commonly used calibration curve is the third-degree universal polynomial of Topp et al. <1980>. The most common refinement is calibration to a specific soil but still using four parameters (coefficients) from fitting a third-degree polynomial. Here we demonstrate that a three-parameter expression, &thgr;v=aKa&agr;+b, fits as close as or more closely than the four-parameter polynomial (where the three parameters a, b, and &agr; are determined by fitting water content &thgr;u to the dielectric coefficient Ka). This form is consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution this expression is further simplified to two parameters by taking &agr;=0.5. When &agr; is 0.5, its calibration is equivalent to the linear calibration between &thgr;u and the travel time along the waveguide. In addition, the simple three-parameter expression can be easily inverted without losing accuracy with regard to the original calibration. The TDR calibration expressed in a three-parameter form not only achieves a good fit but also conveys a physical connotation.¿ 1997 American Geophysical Union