What is the average molecular weight of a siRNA, and how do I convert uM to ug values?

The Molecular Weight (MW) of a 21 nucleotide double-stranded siRNA molecule is approximately 13-15 ug/nmol. The exact MW depends on the sequence of the siRNA. 20 uM of double-stranded 21 nt siRNA is equivalent to approximately 0.25 ug/ul.