The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement in one in which the radiodensity of distilled water at standard pressure and temperature (STP) is defined as zero Hounsfield units (HU), while the radiodensity of air at STP is defined as -1000 HU.
For a material X with linear attenuation coefficient μX, the corresponding HU value is therefore given by where is the linear attenuation coefficient of water.
Thus, a change of one Hounsfield unit (HU) represents a change of 0.1% of the attenuation coefficient of water since the attenuation coefficient of air is nearly zero.
It is the definition for CT scanners that are calibrated with reference to water.
Tissue CT number (HU)

Bone	1000
Liver	40 - 60
White matter	~20-30 HU
Grey matter	~37-45 HU
Blood	40
Muscle	10 - 40
Kidney	30
Cerebrospinal fluid	15
Water	0
Fat	-50 - -100
Air	-1000