|Unit system||SI derived unit|
|Named after||Henri Becquerel|
|SI base unit||s-1|
The becquerel (; symbol: Bq) is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. For applications relating to human health this is a small quantity, and SI multiples of the unit are commonly used.
1 Bq = 1 s-1
A special name was introduced for the reciprocal second (s-1) to represent radioactivity to avoid potentially dangerous mistakes with prefixes. For example, 1 µs-1 would mean 106 disintegrations per second: 1·(10-6 s)-1 = 106 s-1, whereas 1 µBq would mean 1 disintegration per 1 million seconds. Other names considered were hertz (Hz), a special name already in use for the reciprocal second, and Fourier (Fr). The hertz is now only used for periodic phenomena. Whereas 1 Hz is 1 cycle per second, 1 Bq is 1 aperiodic radioactivity event per second.
The gray (Gy) and the becquerel (Bq) were introduced in 1975. Between 1953 and 1975, absorbed dose was often measured in rads. Decay activity was measured in curies before 1946 and often in rutherfords between 1946 and 1975.
As with every International System of Units (SI) unit named for a person, the first letter of its symbol is uppercase (Bq). However, when an SI unit is spelled out in English, it should always begin with a lowercase letter (becquerel)--except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.
Like any SI unit, Bq can be prefixed; commonly used multiples are kBq (kilobecquerel, 103 Bq), MBq (megabecquerel, 106 Bq, equivalent to 1 rutherford), GBq (gigabecquerel, 109 Bq), TBq (terabecquerel, 1012 Bq), and PBq (petabecquerel, 1015 Bq). Large prefixes are common for practical uses of the unit.
With = , the Avogadro constant.
Since is the number of moles (), the amount of radioactivity can be calculated by:
For instance, on average each gram of potassium contains 0.000117 gram of 40K (all other naturally occurring isotopes are stable) that has a of = , and has an atomic mass of 39.964 g/mol, so the amount of radioactivity associated with a gram of potassium is 30 Bq.
For practical applications, 1 Bq is a small unit. For example, the roughly 0.0169 g of potassium-40 present in a typical human body produces approximately 4,400 disintegrations per second or 4.4 kBq of activity.
The global inventory of carbon-14 is estimated to be (8.5 EBq, 8.5 exabecquerel). The nuclear explosion in Hiroshima (an explosion of 16 kt or 67 TJ) is estimated to have injected (8 YBq, 8 yottabecquerel) of radioactive fission products into the atmosphere.
These examples are useful for comparing the amount of activity of these radioactive materials but should not be confused with the amount of exposure to ionizing radiation that these materials represent. The level of exposure and thus the absorbed dose received are what should be considered when assessing the effects of ionizing radiation on humans.
The following table shows radiation quantities in SI and non-SI units. WR (formerly 'Q' factor) is a factor that scales the biological effect for different types of radiation, relative to x-rays. (e.g. 1 for beta radiation, 20 for alpha radiation, and a complicated function of energy for neutrons) In general conversion between rates of emission, the density of radiation, the fraction absorbed, and the biological effects, requires knowledge of the geometry between source and target, the energy and the type of the radiation emitted, among other factors. 
|Activity (A)||becquerel||Bq||s-1||1974||SI unit|
|curie||Ci||3.7 × 1010 s-1||1953||3.7×1010 Bq|
|rutherford||Rd||106 s-1||1946||1,000,000 Bq|
|Exposure (X)||coulomb per kilogram||C/kg||C?kg-1 of air||1974||SI unit|
|röntgen||R||esu / 0.001293 g of air||1928||2.58 × 10-4 C/kg|
|Absorbed dose (D)||gray||Gy||J?kg-1||1974||SI unit|
|erg per gram||erg/g||erg?g-1||1950||1.0 × 10-4 Gy|
|rad||rad||100 erg?g-1||1953||0.010 Gy|
|Equivalent dose (H)||sievert||Sv||J?kg-1 × WR||1977||SI unit|
|röntgen equivalent man||rem||100 erg?g-1 x WR||1971||0.010 Sv|
|Effective dose (E)||sievert||Sv||J?kg-1 × WR × WT||1977||SI unit|
|röntgen equivalent man||rem||100 erg?g-1 × WR × WT||1971||0.010 Sv|
(d) The hertz is used only for periodic phenomena, and the becquerel is used only for stochastic processes in activity referred to a radionuclide.