Roentgen (unit)

The roentgen (symbol R) or röntgen (in German) is a legacy unit to measure radiation exposure. It was defined as the quantity of x-rays that produce 2.580 × 10 coulombs of charge collected per unit mass (kilograms) of air at standard temperature and pressure (STP): 1 R = 0.000258 coulombs per kilogram (Ckg) of air.


As for all other eponymous units when the name is written out in full it is not capitalized, but its symbol is capitalized.

History and etymology

Named after Wilhelm Conrad Roentgen for his discovery of x-rays in 1895.

In July 1974, the International Commission on Radiation Units and Measurements (ICRU) recommended that the roentgen no longer be used as the unit of radiation exposure, this was ratified by the International Committee of Weights and Measures (CIPM) at the 15th General Conference of Weights and Measures (CPGM) in May-June 1975. No specific name was adopted for a replacement SI unit for radiation exposure and it was recommended that the roentgen be replaced with coulombs per kilogram (Ckg) .

It was advised that the roentgen be phased out over at least ten years , i.e. not before 1985.