Gray (SI unit)

Last revised by Daniel J Bell on 26 Nov 2021

The gray (symbol Gy) is the SI unit of absorbed dose and is defined as the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter, i.e. one gray = 1 J/kg 2.


One gray is a large unit and is usually used with a prefix, e.g. milligray (mGy), microgray (μGy).

As per all other eponymous SI units when the unit is written out in full it is not capitalized, but when shortened to its symbol it is capitalized.

History and etymology

In July 1974, the International Commission on Radiation Units and Measurements (ICRU) recommended that a specific name, the gray, be adopted for the SI unit 'joule per kilogram' when used for absorbed radiation dose. This recommendation was expedited and passed by the International Committee of Weights and Measures (CIPM) at the 15th General Conference of Weights and Measures (CPGM) in May-June 1975 2.

The unit is named for the British physicist Louis Harold Gray (1905-1965) 1, who made a major contribution to the field of radiation dosimetry with the Bragg-Gray principle 2.

ADVERTISEMENT: Supporters see fewer/no ads