Gravimetric water content is the weight of soil water per unit weight of dry soil. Volumetric water content is the volume of soil water per unit of total volume. Volumetric and gravimetric water content are related by the bulk density of the soil. Gravimetric water content is the most easily measured, requiring only a glass jar with a lid, an accurate scale, and an oven. Indeed, while GWC may be more easily obtained and more intuitive, the output of most soil water content sensors (including ECHO probes) is correlated with VWC. One common calibration error is to relate probe output with GWC.