How Tm is calculated
The melting temperature (Tm) is the temperature at which half of the primer is bound to its complement. This calculator uses the nearest-neighbour thermodynamic model (SantaLucia, 1998), which sums the enthalpy and entropy of each adjacent base pair, and corrects for monovalent and Mg²⁺ ions following Owczarzy et al. (2004, 2008, simplified).
Simple rules such as the Wallace rule (2 °C per A/T, 4 °C per G/C) are only adequate for very short oligos and can be off by several degrees.
Good primer properties
Typical PCR primers are 18–25 nucleotides long with 40–60% GC content, a Tm of 55–65 °C, and forward and reverse primers within about 2–3 °C of each other. One or two G or C bases at the 3′ end (a GC clamp) help priming, but more than three can promote mispriming.
Reaction conditions matter
Tm depends on salt, magnesium and primer concentration. Use the values of your actual PCR buffer; the defaults (50 mM Na⁺, 1.5 mM Mg²⁺, 0.2 mM dNTPs, 250 nM primer) are typical for Taq polymerase reactions.