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Primer Tm calculation. - (Mar/23/2012 )

Why SIGMA calculator (above) different from the formula below?

Tm= 2(A+T)+4(G+C)

Any tips will be appreciated.Thanks!


the formula you give is very general, rough "rule of thumb" and will give you a "ballpark" figure.

the calculator uses more precise values for the various bases and may be taking other factors into consideration (i can't list those factors because i don't use the sigma calculator, this statement is for the calculators in general).