What is the CT number for water?

The CT number for water is universally standardized to 0 Hounsfield Units (HU). This scale, developed by Sir Godfrey Hounsfield, assigns values based on the linear attenuation coefficients of the materials being imaged in comparison to water. Since water has a defined baseline attenuation, it serves as the reference point in the Hounsfield scale. Consequently, all substances are measured relative to water, where air is assigned a negative value (commonly around -1000 HU), and various tissues or materials have positive values depending on their density and atomic composition. Thus, the CT number for water being 0 HU ensures consistency across imaging techniques and provides a stable basis from which to interpret densities of other materials.