DNA Melting Temperature (Tm) Calculator

What Is Melting Temperature (Tm)? The melting temperature of a DNA molecule is the temperature at which 50% of the DNA strands are in double-helix form and 50% are single-stranded (denatured). It is one of the most important parameters in molecular biology, especially for designing PCR (Polymerase Chain Reaction) primers.

Why Tm Matters In PCR, primers must bind specifically to their target DNA. If the annealing temperature is too high, primers will not bind. If too low, primers may bind nonspecifically to wrong sequences. The optimal annealing temperature is typically 3-5 degrees Celsius below the Tm of the primers.

The Basic Formula (Wallace Rule) For oligonucleotides of 20 bases or fewer: Tm = 2(A+T) + 4(G+C) degrees Celsius. Where A, T, G, C are the counts of each nucleotide. This formula accounts for the fact that G-C base pairs have 3 hydrogen bonds (stronger) while A-T base pairs have only 2.

The Salt-Adjusted Formula Past 20 bases: Tm = 81.5 + 16.6 * log10([Na+]) + 41 * (GC fraction) - 675/N. Where [Na+] is the sodium ion concentration (typically 0.05 M for standard PCR), GC fraction is the proportion of G and C bases, and N is the sequence length. This accounts for ionic strength effects on DNA stability.

Which formula, and why the cutoff sits at 20 The two formulas disagree, and on an ordinary primer they can disagree by more than 10 degrees, so the cutoff is not a detail. Take a 19-base primer at 42% GC: Wallace gives 54.0°C and the salt-adjusted formula gives 41.6°C. Anneal at Wallace minus 5 and you run at 49°C, which is a normal PCR. Anneal off the salt-adjusted figure and you run at 36.6°C, cold enough that primers start binding sequences they only partly match, and you get a smear of non-specific bands instead of one product.

Bench protocols use Wallace across the 18 to 25 base range that primers actually live in, so this calculator does too. The salt-adjusted formula was derived for long duplexes, where the 675/N term stops dominating and ionic strength genuinely drives the answer, and it takes over past 20 bases. Either way, both numbers are shown below the result and the gap between them is reported, because for anything critical a nearest-neighbour calculation beats both.

GC Content and Stability DNA sequences with higher GC content have higher melting temperatures because G-C pairs form three hydrogen bonds versus two for A-T pairs. The GC content of a primer is ideally between 40-60% for optimal PCR performance.

Practical PCR Guidelines Primers should typically be 18-25 bases long. The Tm of forward and reverse primers should be within 5 degrees of each other. Primer GC content between 40-60% is ideal.

The GC clamp is the rule people most often get backwards, so it is worth stating in both directions. The last 5 bases at the 3’ end want one or two G or C bases, and no more. Three or more give that end enough grip to prime off a partial match, which is where nonspecific product comes from. None at all is the opposite problem: an all-AT tail breathes open and the polymerase struggles to get a purchase. This calculator checks the 3’ end for you and says which way it is off.