The curie or Ci is the non-SI unit for radioactive decay measuring the radioactivity of a substance. However, today the official SI unit for radioactive decay is the Becquerel. The curie specifies the amount of ionizing radiation energy that is emitted from an unstable isotope as it decays.[1] Named after Marie Curie, a chemist and the first female to win a Nobel prize after coining the term radioactivity,[2] the Ci also measures the amount of disintegrations per second coming from a decaying element such as Uranium. Originally, the curie was a comparison of the activity of a sample to the activity of one gram of radium, which was measured at 37 billion disintegrations per second.[3] For example, a radioactive sample undergoing 74 billion disintegrations per second has an activity of 2 curies.
The ability to measure the activity of a substance is practical and has many applications. For example, measuring the activity of a substance can help determine the half-life of a sample. One useful application involving half-life is radioactive dating, which is used to determine the age of rocks and other materials, which is how the age of the Earth is known.[4] The curie is a fairly large unit of measurement, thus it is commonly prefixed to represent a wide range of different activity level, for example:

  • Picocuries (pCi): are 1 million millionth of a curie (1 x 10-12 Ci). Picocuries are used in measuring the typically small amount of radioactivity in air and water.
  • Megacuries (MCi) are 1 million curies (1 x 106 Ci). Megacuries are used in measuring the very large amount of radioactivity released from nuclear weapons.

Radiation-related quantities

The following table shows radiation quantities in SI and non-SI units.[5][6]

Quantity Name Symbol Unit System
Exposure Roentgen R 2.58x10-6 C/kg Non-SI
Absorbed Dose Rad
10-2 Gy
Activity Curie
3.7x1010 Bq
Dose Equivalent Roentgen Equivalent Man
10-2 Sv

Curie Unit Converter


  1. U.S.NRC. (06, 21, 2016). Curie (Ci) [Online]. Available:
  2. UNEP. “Radiation Effects and Sources,” United Nations Environment Programme, Austria. Sci. Rep. 978-92-807-3517-8. 2016.
  3. E.E. Anderson. “Units of Radiation and Radioactivity.” Public Health Reports, Oak Ridge National Laboratory, Oak Ridge, Tenn. Vol. 67, No. 3, March 1952
  4. The TalkOrigins Archive, "The Age of the Earth" [Online], Available:
  5. NIST. (2016, February 10). Chapter 5. Guide to the SI, Units Outside the SI [Online]. Available:
  6. NIST. (2016, February 19). Chapter 4. Guide to the SI, The Two Classes of SI Units and the SI Prefixes [Online]. Available:

Authors and Editors

Celeste Pomerantz, Kailyn Stenhouse, Jason Donev