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

Select Calculation Direction
Uranium and Plutonium Grams Calculator
Select Isotope Group
Select Number of Isotopes
Enter pCi
Plutonium SA Uranium SA
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.