Guidance for Radiation Accident Management

Activity: How much is present?

Radioactive material is usually not described in terms of its mass or volume. Instead, the amount of radioactive material present is communicated in terms of how quickly the material decays, or its activity. In the International System of units (SI), the unit of activity is the becquerel (Bq). A becquerel is one decay per second (dps).

The curie (Ci) is the traditional unit of radioactivity and is the unit most commonly used in the United States. One curie is 37 billion Bq. Since the Bq represents such a small amount, you are likely to see a prefix used with Bq, as shown below:

  • 1 MBq (27 microcuries)
  • 1 GBq (27 millicuries)
  • 37 GBq (1 curie)
  • 1 TBq (27 curies)

A large amount of material can have a very small amount of radioactivity; a very small amount of material can have a lot of radioactivity.

For example, uranium-238 has 0.00015 curies of radioactivity per pound (0.15 millicuries), while cobalt-60 has nearly 518,000 curies per pound.

In the International System of units (SI), the becquerel (Bq) is the unit of radioactivity. One Bq is 1 disintegration per second (dps).

SI units and prefixes

The International System of Units has been given official status and recommended for universal use by the General Conference on Weights and Measures.

Radiation Measurements

  Radioactivity Absorbed Dose Dose Equivalent Exposure
Common Units curie (Ci) rad rem roentgen (R)
SI Units becquerel (Bq) gray (Gy) sievert (Sv) coulomb/kilogram (C/kg)

Following is a list of prefixes and their meanings that are often used in conjunction with SI units:

Multiple Prefix Symbol
1012 tera T
109 giga G
106 mega M
103 kilo k
10-2 centi c
10-3 milli m
10-6 micro µ
10-9 nano n


Conversions   Conversion Equivalence
1 curie = 3.7 x 1010
disintegrations per second
1 becquerel =
1 disintegration per second
1 millicurie (mCi) = 37 megabecquerels (MBq)
1 rad = 0.01 gray (Gy)
1 rem = 0.01 sievert (Sv)
1 roentgen (R) = 0.000258 coulomb/kilogram (C/kg)
1 megabecquerel (MBq) = 0.027 millicuries (mCi)
1 gray (Gy) = 100 rad
1 sievert (Sv) = 100 rem
1 coulomb/kilogram (C/kg) = 3,880 roentgens

Conversion Factors

To convert from To Multiply
Curies (Ci) becquerels (Bq) 3.7 x 1010
millicuries (mCi) megabecquerels (MBq) 37
microcuries (µCi) megabecquerels (MBq) 0.037
millirads (mrad) milligrays (mGy) 0.01
millirems (mrem) microsieverts (µSv) 10
milliroentgens (mR) microcoulombs/kilogram (µC/kg) 0.258
becquerels (Bq) curies (Ci) 2.7 x 10-11
megabecquerels (MBq) millicuries (mCi) 0.027
megabecquerels (MBq) microcuries (µCi) 27
milligrays (mGy) millirads (mrad) 100
microsieverts (µSv) millrems (mrem) 0.1
microcoulombs/kilogram (µC/kg) milliroentgens (mR) 3.88