Knowledge of soil moisture is needed to understand crop water use, hydrology, and microclimate. A reliable, rapid technique is needed, and recently an impedance soil moisture probe (Theta Probe) has been accepted by the scientific community. The purposes of this study were to calibrate the probe for soils of Central Iowa through field sampling, to determine the number of samples needed for calibration, and to determine the effect of temperature on calibration. Laboratory calibration was conducted on Des Moines lobe soils across a range of water contents and temperatures. Including a temperature term increased the R2 from 0.85 to 0.87. Field calibration was based on Theta Probe measurements on similar soils combined with gravimetric sampling and soil temperature determination. Although some scatter existed, the field calibration was adequate for Iowa soils (R2 = 0.77). Inclusion of temperature did not significantly improve the calibration for the field data. To determine the appropriate number of samples needed for the field calibration, regression equations were determined from sample numbers ranging from 2 to 89, and the standard error was determined for each. Based on the standard error analysis, 20 samples was an adequate number, with no further improvement for additional data points.