Formula to Calculate the Annealing Temperature of Oligonucleotides for PCR

The thumbrule for calculating the annealing temperature for a PCR primer is

Tm (°C) = 81.5 + 0.41(%GC) - (675/N)

where %GC is the percentage of G and C nucleotides in the oligo and N is the length of the oligo given in nucleotides.

Example calculation:
Tm = 81.5 + 0.41(33.3) - (675/21)
Tm = 81.5 + 13.7 - 32.1 (rounded to one decimal)
Tm = 63.1°C

When choosing the temperature for the PCR annealing step a good starting point is Tm - 5°C, for the above example that is 58°C.