The absorbance of a material, denoted , is given by
where
*  is the radiant flux  by that material,
*  is the radiant flux  by that material, and
*  is the transmittance of that material.
Absorbance is a dimensionless quantity. Nevertheless, the absorbance unit or AU is commonly used in ultraviolet–visible spectroscopy and its high-performance liquid chromatography applications, often in derived units such as the milli-absorbance unit (mAU) or milli-absorbance unit-minutes (mAU×min), a unit of absorbance integrated over time.
Absorbance is related to optical depth by
where  is the optical depth.