Primer Tm Calculator
Calculate the melting temperature (Tm) of a DNA primer.
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What it does
The primer Tm calculator estimates the melting temperature of a DNA oligonucleotide by two independent rules, and also reports its GC content, length and molecular weight. Tm is the temperature at which half of the duplex has dissociated into single strands, and it is the number that sets the annealing step of a PCR programme or the wash stringency of a hybridization. Only A, C, G and T are counted, so modified or ambiguous positions are excluded from the estimate.
How it works
Two formulas are reported. The Wallace rule, Tm = 2 x (A + T) + 4 x (G + C), assigns 2 degrees to each A or T and 4 degrees to each G or C; it is quick and works reasonably for short primers of roughly 14 to 20 nucleotides at standard salt. The salt-adjusted estimate, Tm = 64.9 + 41 x (G + C - 16.4) / N where N is the primer length, scales with GC fraction instead of absolute counts and behaves better as primers get longer. Neither formula considers sequence context, primer concentration or magnesium, so for design-critical work a nearest-neighbour thermodynamic model, which sums stacking enthalpies and entropies for each adjacent base pair, is more accurate. The molecular weight is computed from the mass of each nucleotide residue in the single strand, and is what you need to convert a synthesis yield in micrograms into nanomoles.
Worked example
For the 20-mer primer 5'-ATGCGGATCCATGCTAAGGT-3' the counts are A = 5, T = 5, G = 6 and C = 4, so G + C = 10 and the GC content is 50%. The Wallace rule gives Tm = 2 x 10 + 4 x 10 = 60.0 degrees Celsius, while the salt-adjusted formula gives 64.9 + 41 x (10 - 16.4) / 20 = 51.8 degrees Celsius, and the molecular weight is about 6157 daltons. The eight-degree gap is a useful reminder that these are estimates: for a 20-mer the salt-adjusted value is generally the more trustworthy of the two, and a gradient PCR around it will settle the question empirically.
When to use it
When writing a PCR programme, calculate the Tm of both primers and set the annealing temperature a few degrees below the lower one. When designing a primer pair, adjust the length or position of the primers until their Tm values agree within about 5 degrees, since a mismatched pair lets one primer dominate and generates non-specific product. When designing a probe or a nested primer that must anneal while another does not, compare Tm values to confirm the intended order of binding as the reaction cools.
FAQ
- What is a good Tm for PCR primers?
- Most primers are designed for a Tm of about 55 to 65 degrees Celsius, which allows an annealing step comfortably above the temperature at which non-specific products form and within the working range of standard polymerases. Just as important, the two primers of a pair should be within roughly 5 degrees of each other so that both anneal efficiently in the same cycle.
- Why do the Wallace and salt-adjusted Tm values differ?
- They are different approximations fitted to different situations. The Wallace rule adds a fixed contribution per base and so grows without limit as the primer lengthens, which makes it overestimate Tm for oligos longer than about 20 nucleotides, while the salt-adjusted formula uses GC fraction and a length term and stays more realistic. Treat the two values as a range and verify empirically if the difference matters.
- Which Tm formula should I use for a 25-mer primer?
- Prefer the salt-adjusted value over the Wallace rule, which is calibrated for shorter oligos and will read too high at 25 nucleotides. For a primer you intend to rely on, a nearest-neighbour calculation that accounts for sequence context, salt and primer concentration is better still, and gradient PCR remains the definitive test.
- Does Tm depend on salt and primer concentration?
- Yes. Higher monovalent salt and magnesium screen the negative charge of the phosphate backbone and stabilize the duplex, so Tm rises with ionic strength; that dependence is exactly what the salt-adjusted formula tries to capture. Tm also shifts with oligo concentration because duplex formation is bimolecular, which is why measured annealing behaviour can differ between reactions using different primer amounts.