What is radiation measured in?

The story behind the answer

Gray (Gy) is the SI unit of absorbed dose of ionizing radiation—how much energy the tissue actually absorbs. One gray equals one joule of radiation energy deposited per kilogram of matter. This is different from measuring how radioactive a source is (activity, in becquerels) or how risky the exposure is biologically (effective dose, in sieverts).

Named after radiobiologist Louis Harold Gray, the unit helps quantify the physical impact of radiation on materials and the human body (1 Gy = 100 rad in older units). Remember: Gray = energy absorbed; Sievert = biological effect. By contrast, decibel measures sound intensity, watt measures power, and kelvin measures temperature—so they don’t describe radiation dose.