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 ([math]1 \times 10^{-12}[/math] Ci). Picocuries are used in measuring the typically small amount of radioactivity in air and water.
  • Megacuries (MCi) are 1 million curies ([math]1 \times 10^6[/math] 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

For Further Reading


  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

Allison Campbell, Celeste Pomerantz, Ashley Sheardown, Kailyn Stenhouse, Jason Donev
Last updated: January 4, 2019
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