The formula to calculate the molecular weight of DNA oligonucleotides is:

MW (g/mol)= (nA x 249.08619) + (nG x 265.0811) + (nC x 225.07496) + (nT x 240.07462)

To calculate ε (epsilon, the extinction coefficient) of an oligo, the formula is:

ε (in l/mmol) = amount of As x 15.4 + amount of Cs x 7.3 + amount of Gs x 11.4 + amount of Ts x 8.8

ε is needed to correctly calculate the concentration of an oligo based on the OD at 260nm. The formula is:

c (in mmol/l) = OD(260nm) / ε