Tm is the temperature at which one half of a DNA molecule is denatured.
The bond between the AT bases is weaker (because there are two hydrogen bonds) than the bond between GC bases (because there are three hydrogen bonds).
A Tm calculation must take into account:
the number of matching AT bases (= 2°C for each base pair)
the number of matching GC bases (= 4°C for each base pair)
Tm= 2(AT) + 4(GC)
AT = 2°C
GC = 4°C
This formula is restricted to small-sized DNA molecules (between 17 and 25 base pairs).