The invention relates to methods and systems for predicting or estimating the melting temperature of duplex nucleic acids, in the presence of divalent cations, particularly duplexes of oligonucleotides which may be used as, for example, but not limited to primers or probes in PCR and/or hybridization assays. The methods and algorithms use novel formulas, having terms and coefficients that are functions of the particular nucleotide sequence, to estimate the effect of divalent cation salt conditions on the melting temperature.