The sievert (symbol: Sv) is the International System of Units (SI) SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dosage measurement and research into the biological effects of radiation.

The unit gray measures the absorbed dose of radiation (D), absorbed by any material. The unit sievert measures the equivalent dose of radiation (H), having the same damaging effect as an equal dose of gamma rays.

Both the gray, with symbol Gy and the sievert, with symbol Sv are SI derived units, defined as a unit of energy (joule) per unit of mass (kilogram):

1 Gy = 1 Sv = 1 J / kg