The Roentgen Equivalent Man or REM is an older, non-SI unit of dose equivalent.[1] Depending on the effect under study, REM can also be referred to as roentgen equivalent mammal.[2] Measuring the quantity of radiation damage on human tissue by x-rays, gamma or any other type of ionizing radiation such as alpha and beta radiation.[1] However, REM does not measure the amount of energy absorbed or the amount of ionization accumulated.[3] REM was first proposed in 1945 before Sievert was an established SI unit. One hundred rem can be defined as one Sievert or 1 rem = 1 cSv = 10-2 Sv.[4]

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 rad 10-2 Gy Non-SI
Gray Gy Jkg-1 SI
Activity Curie Ci 3.7x1010 Bq Non-SI
Becquerel Bq s-1 SI
Dose Equivalent Roentgen Equivalent Man rem 10-2 Sv Non-SI
Sievert Sv Jkg-1 SI

Rem Unit Converter

For Further Reading


  1. 1.0 1.1 U.S.NRC. (2016, June 21).REM (Roentgen equivalent man) [Online]. Available:
  2. S.T. Cantril et al., “The Tolerance Dose,” United States Atomic Energy Commission, Argonne National Laboratory, Technical Information Division, Oak Ridge, Tenn. MDDC-1100. 1945.
  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. Available:
  4. U.S.NRC. (2015, December 02). Units od radiation dose [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, Jason Donev
Last updated: June 25, 2018
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