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Units of Measure

Phil B. Fontanarosa

and Stacy Christiansen

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Subscriber: null; date: 18 June 2018


UPDATE: We will discontinue using quotation marks to identify parts of an article, but retain the capitalization; eg, This is discussed in the Methods section (not the “Methods” section). This change was made February 14, 2013.

Measurements of ionizing radiation and radioactivity should be reported by means of SI units. The SI units for radiation are established by international agreement.1 The unit for activity of a radionuclide is the becquerel; the absorbed dose of radiation (absorbed per unit weight of tissue) is the gray (Gy); and the dose equivalent used to indicate the detrimental effects of an absorbed radiation dose on biological tissue is the sievert (Sv).

A 1-Gy dose is equivalent to 1 joule (J) of radiation energy absorbed per kilogram of organ or tissue weight. Rad is the older, non-SI term and is still in use as a unit of absorbed dose (100 rad = 1 Gy). However, equal doses of all types of ionizing radiation are not equally harmful. Alpha particles produce greater harm than beta particles, γ rays, and x-rays for a given absorbed dose. To account for this difference, radiation dose is expressed as equivalent dose in sieverts (Sv).9

SI units for radiation and factors to convert values from SI units to conventional units are shown below.


SI Unit (Symbol)

Conversion Factors

Non-SI Unit


becquerel (Bq)

1 Bq = 2.7 × 10−11

Ci (approx)

1 Ci = 3.7 × 1010 Bq

1 Bq = 27 picocurie (pCi)


Absorbed dose

gray (Gy)

1 Gy = 100 rad

1 rad = 0.01 Gya


“Dose” equivalent

sievert (Sv)

1 Sv = 100 rem

1 rem = 0.01 Sv


a Although 1 rad = 1 cGy, the centi- prefix is generally not preferred in SI. Therefore, despite the appeal of one-to-one conversion, rad should be converted to gray, not centigray.

Although SI units are preferred, authors of some articles, such as those reporting studies involving nuclear medicine or radiation oncology, may prefer to report results in both SI units and non-SI units. As with units for laboratory results, conversion factors to convert radiation units from SI units to conventional units should be provided in the article, either in the text, in footnotes to tables or figures, or in the Methods section.

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