For those needing portability, Rad Pro for Desktop works with Windows 8.1/10 tablets. Will not work with Surface tablets running Windows RT.
 
                =Select Activity UnitsCalculated GramsSelect Isotope

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Uranium and Plutonium Grams Calculator
Select Isotope Group
Select Number of Isotopes
Enter pCi
Plutonium SA Uranium SA
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Utilizing the specific activities of the uranium and plutonium isotopes, calculate the grams of PU and U from known activities or calculate the activities from know masses.  If you select multiple isotopes, the total uranium and total plutonium are also calculated for you.