For other uses, see Curie (disambiguation).

The curie (symbol Ci) is a former unit of radioactivity, defined as 3.7×10¹⁰ decays per second. This is roughly the activity of 1 gram of the radium isotope ²²⁶Ra, a substance studied by the pioneers of radiology, Marie Curie and Pierre Curie (the unit is named for them). The curie has been replaced by an SI derived unit, the becquerel (Bq), which equates to one decay per second. Therefore 1 curie is 37 gigabecquerels (GBq) exactly and 1 Bq = 2.7027×10^-11 Ci.