IDT offers a free online calculator, the OligoAnalyzer tool, that will determine the Tm of your oligonucleotide sequence, for reaction conditions you specify. Access it at http://www.idtdna.com/analyzer/Applications/OligoAnalyzer
. Be sure to include the Mg2+
and dNTP concentrations you plan to use in your reaction, as this will affect the Tm for your oligo.