Key facts
- 1 curie (Ci) = 3.7 × 10<sup>10</sup> Bq exactly, by definition.
- 1 gray (Gy) = 100 rad, and 1 sievert (Sv) = 100 rem.
- Bq measures activity, Gy absorbed dose and Sv weighted dose: they measure different things and cannot be converted into one another without further information.
Three families of units for three different quantities
| Quantity | SI unit | Legacy unit | Conversion |
|---|---|---|---|
| Activity (decays per second) | becquerel (Bq) | curie (Ci) | 1 Ci = 3.7 × 1010 Bq |
| Absorbed dose (energy per mass) | gray (Gy) | rad | 1 Gy = 100 rad |
| Equivalent and effective dose | sievert (Sv) | rem | 1 Sv = 100 rem |
The becquerel is one decay per second, the gray is one joule absorbed per kilogram, and the sievert is also one joule per kilogram but multiplied by weighting factors for the radiation type and, for effective dose, the tissues irradiated. The concepts are explained further on the dose and risk page.
The curie and the becquerel
The curie was originally defined by reference to the activity of one gram of radium-226, a value now fixed by definition at 3.7 × 1010 decays per second (about 3.66 × 1010 Bq is the measured activity of a gram of Ra-226, a small difference from the defined curie). Because the becquerel is very small, you will see prefixes constantly: 1 kBq = 103 Bq, 1 MBq = 106 Bq, 1 GBq = 109 Bq. Common sub-multiples of the curie are:
- 1 millicurie (mCi) = 37 MBq
- 1 microcurie (µCi) = 37 kBq
- 1 picocurie (pCi) = 0.037 Bq
The picocurie is still widely used in the US for environmental limits. Concentrations are given as activity per volume: 1 pCi/L = 37 Bq/m3. This matters for radon, where the US action level of 4 pCi/L equals 148 Bq/m3.
Absorbed dose: gray and rad
One gray is 100 rad, so 1 rad = 0.01 Gy = 10 mGy. For gamma and X-rays, an absorbed dose of 1 mGy in tissue is numerically about 1 mSv of equivalent dose because the radiation weighting factor is 1. For alpha particles the weighting factor is 20, so 1 mGy of alpha absorbed dose corresponds to 20 mSv of equivalent dose. That is why Gy and Sv must not be used as synonyms.
Dose equivalents: sievert and rem
One sievert is 100 rem, so 1 rem = 10 mSv and 1 mrem = 10 µSv. Natural background of around 2.4 to 3 mSv per year (see dose and risk basics) is therefore about 240 to 300 mrem per year, and the ICRP public limit of 1 mSv per year is 100 mrem per year. The older roentgen (R) measures exposure in air for X- and gamma rays; it is close to 0.01 Sv for soft tissue but is not an exact synonym, and it should not be converted without stating the assumptions.
Worked examples, including uranium
- Bq to Ci: 5 MBq = 5 × 106 / 3.7 × 1010 = 1.35 × 10-4 Ci = 135 µCi.
- Ci to Bq: 2 µCi = 2 × 37 kBq = 74 kBq.
- rem to mSv: 250 mrem = 2.5 mSv.
- Uranium by mass: U-238 has a specific activity of about 12.4 kBq/g (see radioactive properties of uranium), so 1 µCi (37 kBq) corresponds to about 3 g of U-238. Natural uranium, at roughly 25 kBq/g, is about 1.5 g per µCi.
- Dose rate: a survey meter reading 0.12 µSv/h equals 12 µrem/h.
Choosing units in practice: who uses what
Most national regulators and international bodies now use SI units, and so does ICRP. Legacy units persist in several places: US regulations and many US instruments and documents still show curies, pCi/L, rad and rem, and older scientific literature uses them throughout. A good habit when reading any document is to look at the unit before the number, to write the unit with every value, and to keep the original value and unit when quoting. When a figure is converted, state both. For example, a radon concentration of 148 Bq/m3 (4 pCi/L) is clearer than either alone.
Instrument readouts add another layer. Survey meters often read in counts per second, which is not a dose unit at all, or in µSv/h calibrated for a particular gamma energy. A count rate cannot be converted into a dose rate without the instrument's calibration for the radiation being measured.
Common pitfalls when converting
Conversion works inside each family, and only there. A becquerel cannot be turned into a sievert without knowing the nuclide, the chemical form, the route of intake and the person's age; ICRP dose coefficients are used for that. A gray of absorbed dose cannot become a sievert unless radiation type and organ are known. Check prefixes (milli versus micro is a common error of a factor of 1000), and check whether a source writes mSv per hour or per year. For soil and building materials, see estimating dose from soil and building materials.
Prefixes and orders of magnitude
The prefixes below cover almost all values encountered with uranium and environmental radiation.
| Prefix | Symbol | Factor |
|---|---|---|
| giga | G | 109 |
| mega | M | 106 |
| kilo | k | 103 |
| milli | m | 10-3 |
| micro | µ | 10-6 |
| nano | n | 10-9 |
| pico | p | 10-12 |
The ground dose rates discussed on the soil and building materials page are given in nGy/h, while workplace and medical doses are usually in µSv or mSv. To compare them, convert to the same unit and remember that nGy/h is absorbed dose in air, not effective dose.
Frequently asked questions
How many becquerels are in a curie?
By definition, 1 curie equals 3.7 × 10^10 becquerels, or 37 gigabecquerels.
How many millisieverts are in a rem?
One rem equals 10 millisieverts, so 100 rem make 1 sievert.
Is a gray the same as a sievert?
No. The gray measures absorbed energy, the sievert is a weighted quantity for biological effect. For gamma and X-rays they are numerically equal for whole-body exposure, but for alpha particles they differ by a factor of 20.
Can I convert becquerels to sieverts?
Not directly. You need the radionuclide, its form, the way it entered the body and the age of the person, and then use published dose coefficients.
What is 1 pCi/L in becquerels per cubic metre?
37 Bq/m3. The US radon action level of 4 pCi/L is therefore 148 Bq/m3.
Sources and further reading
- Bureau International des Poids et Mesures — The International System of Units (SI Brochure) (2019)
- International Commission on Radiological Protection — ICRP Publication 103: The 2007 Recommendations of the ICRP (2007)
- International Commission on Radiological Protection — Dose limits (ICRPaedia) (2024)
- United Nations Scientific Committee on the Effects of Atomic Radiation — UNSCEAR 2008 Report, Volume I: Sources of Ionizing Radiation (2010)
- US Environmental Protection Agency — Radionuclide basics: uranium (2024)
- Agency for Toxic Substances and Disease Registry — Toxicological Profile for Uranium (2013)
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