Radiation Dose Converter (Gy, Sv, rad, rem)

Two families of units in one place. Absorbed dose — how much energy the material took up per unit mass — is measured in gray, with the older rad at exactly one hundredth of it, so 1 Gy is 100 rad. Equivalent dose — absorbed dose scaled by how damaging that particular radiation is to tissue — is measured in sievert, with the rem again at exactly one hundredth, so 1 Sv is 100 rem. Every conversion inside a family is exact, because the rad, the rem and the metric prefixes are all defined rather than measured.

What this tool deliberately will not do is convert a gray straight into a sievert. Both are joules per kilogram, they have the same dimensions, and a calculator that only looks at dimensions will happily treat them as the same unit. They are not: the sievert carries a radiation weighting factor on top of the energy. For photons, X-rays, gamma rays and electrons that factor is 1, which is exactly why the two units look interchangeable in medical imaging — 1 mGy from a chest X-ray really is 1 mSv. For protons it is 2, for alpha particles and heavy ions it is 20, and for neutrons it runs from about 2.5 to 20 depending on their energy. So the same 1 mGy of alpha exposure is 20 mSv, twenty times the risk for the same deposited energy.

That is why the cross-quantity card asks for the factor instead of assuming one. Enter the w_R for the radiation you are dealing with and it applies Sv = Gy × w_R in one direction and Gy = Sv ÷ w_R in the other; leave it at 1 and you are stating that you are dealing with photons or electrons.

For a sense of scale, the sievert is an enormous unit for everyday purposes: natural background is on the order of a few millisieverts a year, a chest radiograph is around a tenth of a millisievert, and the microsievert is the unit that survey meters actually read in. Values below 0.0001 are shown in exponential form and everything else to eight significant digits.