Rad and Gray are absorbed dose units. When we look at radiation being absorbed in tissue, the absorption
varies with the energy of the radiation. With a higher energy deposition in tissue, there are more rads or more Grays deposited than a lower energy deposition at the same rate (particles or photons per second).
Now, what is a rem and what is a a Sievert? The term rem came from an acronym that means Roentgen Equivalent Man, in another words the equivalent biological damage done to human tissue. Some radiation emissions, when depositing the same energy as other radiation emissions,
do more biological damage to the human organism than others. How does one convert?
To go from rad to rem or from Gray to Sievert, you need a multiplication factor that represents the effective biological damage. Most training texts call this a quality factor (QF) or a radiation weighting factor. Some training texts call it a biological damage conversion factor but what it truly represents is the the ratio of biological damage done by radiation types to the biological damage done by gamma radiation.
For gamma, x-ray and beta radiation, this factor is 1. For alpha, it is 20. For neutrons it is between 3 and 10, and is generally conservatively taken as 10. What this implies is that a rad or Gray of alpha energy absorbed by soft human tissue does 20 times more damage than a rad or Gray of gamma, x-ray or beta energy absorbed.
Since for gamma, x-ray and beta, the multiplication factor is 1, one rad equals one rem and one Gray equals one Sievert. There is an excellent

video animation on this subject by Ionactive Consulting of the United Kingdom on their website.

View the video if you want a more detailed explanation these concepts.

Please also read FAQ 1 above. A Roentgen (R) is a unit of exposure from the old US customary system. It is directly related to how many ion pairs are generated when
gamma or x-ray radiation passes through air. The words gamma and x-ray in the previous sentence are key. A Roentgen (R) only applies to electromagnetic energies. It should not be used for alpha, beta or neutron doses.
There actually is a conversion factor, for gammas, from the old Roentegen (R) to rad. Once a gamma ray is absorbed, its units then become a rad. Depending on what textbook that you read, one R equals 0.877 rad or 0.87 rad in air. We chose 0.877
One should note that the rule of thumb 6CEN formula is in Roentgens (1 R/hr = 6CEN). That formula is derived from fluence-rate formulas The SI system has no equivalent for Roentgen. Sieverts are related to rem. Considering FAQ 1 above, we know that 1 rad equals 1 rem and 1 Gray equals 1 Sievert, for gamma rays. Since 1 Roentgen equals
0.877 rad, then 1 Roentgen equals 0.877 rem and 1 Roentgen = .00877 Sieverts. That is why the gamma calculator has the 0.877 conversion factor built in. Look at it for yourself on the Rad Pro Calculator gamma tab/page. If you input 10 microcuries Cs-137 at a distance of 50 cm, you get
12.88 microR per hour. Change the dose rate units to microrem per hour and you get 11.30 microrem per hour, 87.7% less than microR per hour. Change to microSievert per hour and you get 0.113 microSievert per hour, related by a factor of 100 to microrem per hour, not microR per hour.
There is more on this at the

US Health Physics Society website. Also read

this. Note that the 0.87 factor is higher for soft tissue (0.92 to 0.96). Rad Pro Calculator only considers air exposure.

This is not so much a FAQ but a search term that we see every day when viewing website statistics. Many people are searching for How do I convert activity to dose or dose-rate? We think that conversion is the wrong term. Conversion usually means, what number do I multiply Ci and Bq by to obtain R, rad, rem and Gy or Sv?
What you should be asking is how do a I calculate dose-rate or dose for a given activity of an isotope? It is indeed a complicated calculation, not a simple conversion.
There is no number that you multiply Ci and Bq by to get R or rad and Sv or Gy.
Different isotopes emit different energy gammas and some emit more than one gamma. At the same rate of emission or decay, an isotope that emits more and higher energy gammas will give a higher rem/hr or Sv/hr. Ci and Bq are based on dpm/sec (dps). Consider the same emission rate (dps) for Cs-137 and Co-60. Co-60 will give you a higher dose-rate and dose because it emits two gammas above 1000 keV where Cs-137 emits only 1 gamma at 661.8 keV.
Obviously Co-60 is emitting more electromagnetic energy per decay than Cs-137 which will give one a higher dose-rate in R/hr or Sv/hr. Also, the dose and dose-rate varies with the distance from the source. You cannot convert from Ci and Bq to R, rem or rad and Sv or Gy. You must do a complicated calculation on an isotope by isotope and a distance by distance basis. The formula can be found

here. If you can eliminate the word "convert" from your question and replace it with "calculate", Rad Pro Calculator can give you your answer,

here. Rad Pro Calculator also calculates dose-rate in rad/hr and Gy/hr from beta emitter activity

here.

You have turned on the Excel feature and started a spreadsheet. Since the units are written into the Excel column headers, the units must be frozen to eliminate the possibility of mistakenly having mixed units in your columns. One would not want mSv/hr numbers in a column labeled as uSv/hr. Close the program and restart it to turn on the units selections. Do not push the Excel buttons unless you intend to create a spreadsheet.

This is my most frequently asked question.

NIST gives us two coefficients, attenuation and absorbed. Using the attenuation coefficient overestimates the shielding effectiveness (underestimates the dose-rate) and using the absorbed coefficient underestimates the shielding effectiveness (overestimates the dose-rate).
The number that is closer to life is using the attenuation coefficient multiplied by a correction factor that accounts for Compton scatter and pair production gammas, also known as a buildup factor. Rad Pro Calculator allows all methods of gamma shielding calculation. You get to choose which method is appropriate for you.
One would normally use the more accurate buildup factor with attenuation coefficient. If you wish to have ultimate conservatism, then use the absorbtion coefficient with no buildup. The attenuation coefficient with no buildup may be useful for educational exercises.
I wrote a white paper that explains this in detail.

Click here to read.

Some selections are dependent on other selections. Rad Pro Calculator knows this and will not allow you to make an incorrect selection. For instance, buildup factor for gamma shielding must by definition be used with the attenuation coefficient. Using buildup with the energy absorption coefficient makes no mathematical sense and would produce nonsense numbers. Therefore, if you select "Use Buildup Factor" in the gamma shielding box, the energy absorption coefficient option is disallowed. These user helpers are programmed throughout the software and website. Seeing grayed out and unavailable options is normal.