The rontgen or roentgen (symbol R) is a unit of measurement for ionizing radiation (such as X-ray and gamma rays), and is named after the German physicist Wilhelm Rontgen. Adopted in 1928, 1 R is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm of dry air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08x109 ion pairs.Until 2006, the rontgen was accepted for use with the SI system but in this case its value is expressed in terms of the SI units charge divided by unit mass (C/kg) rather than as in the original definition. Although its use was allowable under the SI system, it is not itself an SI unit and continued use is "strongly discouraged" by the National Institute of Standards and Technology.

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