Absorbed Radiation Dose Conversion Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
1 gray (Gy) equals exactly 100 rad, by definition. A dose of 5 Gy therefore equals 500 rad.
Explanation
Absorbed dose measures the amount of ionizing radiation energy deposited per unit mass of irradiated matter (living tissue, material): the gray (Gy), the SI unit, corresponds exactly to 1 joule of energy deposited per kilogram of matter. The rad ("radiation absorbed dose"), an older CGS unit, corresponds to 0.01 joule per kilogram, which gives the exact relationship 1 Gy = 100 rad. The rad is still used in the United States in some radiation-protection and medical-dosimetry contexts, alongside the gray, now standard in most other countries — an absorbed dose that remains a form of energy referred to a mass, and that must not be confused with the activity of a radioactive source itself, covered by our radioactivity conversion calculator. **Important point of vigilance**: this calculator converts only the ABSORBED dose (the energy physically deposited), not to be confused with the EQUIVALENT dose (expressed in sievert or rem), a distinct biological quantity that weights the absorbed dose by a factor depending on the type of radiation (alpha particles, for example, cause more biological damage than X-rays for the same absorbed dose). For X or gamma radiation, the weighting factor is 1, which makes the two quantities numerically equal in that specific case (1 Gy ≈ 1 Sv), but this is true only for that type of radiation — this calculator deliberately does not handle that conversion between different quantities, so as not to mislead for radiation types where the numerical equality does not hold.
Example: 5 Gy to rad
Inputs
Value: 5. Starting unit: Gy. Target unit: rad.
Calculation
5 Gy × 100 (factor from gray to rad) = 500 rad.
Result
An absorbed dose of 5 Gy equals 500 rad.
Frequently asked questions
Are the gray and the sievert the same thing?
No, they are two physically distinct quantities: the gray measures the radiation energy actually deposited per kilogram of matter (absorbed dose), while the sievert measures the biological effect of that dose, weighting it by the type of radiation (weighting factor of 1 for X and gamma rays, but markedly higher for alpha particles, for example). For X or gamma radiation only, the two numerical values coincide, but this is not a unit conversion valid for all radiation types.
Why is the rad still used when the gray is the SI unit?
Out of regulatory and historical habit, particularly in the United States, where some radiation-protection texts and older medical dosimetry equipment continue to display values in rad rather than in gray, the unit now standard in most other countries and in recent international scientific literature.
Where does the factor 100 between gray and rad come from?
The gray is defined as 1 joule of energy deposited per kilogram of matter, while the rad (an older CGS unit) is defined as 100 ergs per gram, that is exactly 0.01 joule per kilogram once converted to SI units. The ratio between the two definitions gives the factor 100 directly.