Melting Temperature Calculator

The melting temperature, abbreviated Tm, is the temperature at which exactly half of a double-stranded DNA duplex has separated, or "melted," into two single strands. At this point the population of molecules sits in equilibrium: 50% remain paired as a double helix and 50% have dissociated. This melting temperature calculator estimates Tm for a short DNA oligonucleotide such as a PCR primer, which is one of the single most important numbers in molecular biology because it dictates how hot or cold you run the annealing step of a polymerase chain reaction.

Base pairing in DNA is held together by hydrogen bonds and base-stacking interactions. Guanine and cytosine (G-C) pairs share three hydrogen bonds, while adenine and thymine (A-T) pairs share only two. Because G-C pairs are more stable, a primer rich in G and C melts at a higher temperature than an A-T rich primer of identical length. That is why GC content appears directly in nearly every Tm equation, and why this calculator reports the percentage of G plus C alongside every result.

Salt concentration matters too. Positively charged sodium (Na+) and magnesium (Mg2+) ions shield the negatively charged phosphate backbone, allowing the two strands to stay together at higher temperatures. Raising the salt concentration raises the Tm; lowering it drops the Tm. A reliable Tm calculator therefore takes sequence, length, GC content, and ionic strength into account, and this tool lets you enter primer concentration, Na+, and Mg2+ so the salt-adjusted estimate reflects your real reaction buffer.

This melting temperature calculator reports Tm from several published methods so you can compare them. The headline "Recommended Tm" comes from the salt-adjusted formula, which is appropriate for typical primers in the 14 to 70 base-pair range. The other methods are shown for reference and for very short or very long oligos.

The Basic (Wallace) rule is the classic quick estimate for short oligonucleotides under about 14 bp: each A-T pair contributes 2 degrees and each G-C pair contributes 4 degrees Celsius. It ignores salt entirely, so it overestimates Tm for longer primers but is handy for a fast mental check.

The nearest-neighbor (NN) result uses a thermodynamic relationship built from average enthalpy (ΔH) and entropy (ΔS) values per base pair plus the primer concentration term. It is the basis of the most accurate published Tm models. Note that the version in this calculator uses single averaged ΔH and ΔS values rather than a full 16-doublet table, so it is a simplified approximation and tends to read high; treat the salt-adjusted number as your working value.

In all of these, L is the primer length in bases, GC% is the percentage of guanine plus cytosine, and [Na+] is the sodium concentration in millimolar (converted to molar inside the log10 term by dividing by 1000).

Using this Tm calculator takes only a few seconds. Start by pasting your DNA primer sequence into the sequence box, written 5' to 3'. The tool automatically uppercases the text and strips anything that is not an A, T, G, or C, so spaces, line breaks, and accidental characters are ignored. A valid sequence must contain at least four bases for a result to appear.

Next, set the three reaction parameters. Primer (nM) is the oligonucleotide concentration in nanomolar, which feeds the nearest-neighbor thermodynamic term. Na+ (mM) is the monovalent salt concentration; 50 mM is a common default for many buffers. Mg2+ (mM) records the magnesium concentration of your reaction, which is reported in the results for documentation. Three preset buttons load a balanced sequence, an all-GC sequence, and an all-AT sequence so you can instantly see how composition shifts the Tm.

The results panel shows the "Recommended Tm" from the salt-adjusted method in large type, followed by Tm from every method side by side, a suggested annealing temperature of Tm minus 5 degrees, a standard 72 degree extension temperature for Taq polymerase, and a primer quality assessment that flags length, GC content, and Tm against ideal design ranges. Use the recommended annealing temperature as a starting point and optimize empirically with a gradient PCR if your product is weak or nonspecific.

Three variables dominate the melting behavior of any primer, and each one appears explicitly in the salt-adjusted equation. Understanding them helps you design primers that actually work the first time rather than guessing at annealing temperatures.

Length (L). Longer primers have more base pairs and more cumulative hydrogen bonding, so they melt at higher temperatures. In the salt-adjusted formula the term −675/L shrinks as L grows, which raises the Tm. A 12-base oligo and a 30-base oligo of the same GC content can differ by more than ten degrees. Most PCR primers land in the 18 to 25 base sweet spot, which the quality assessment flags as optimal.

GC content. Because G-C pairs carry three hydrogen bonds versus two for A-T, a higher GC percentage means a more thermally stable duplex. The 0.41·GC% term captures this: every additional percentage point of GC adds roughly four-tenths of a degree to the Tm. The calculator also warns when GC content falls outside the 40 to 60 percent window, which is the range that best balances stability and specificity.

Salt (Na+). Sodium and other monovalent cations neutralize the phosphate backbone's negative charge, reducing electrostatic repulsion between the strands and stabilizing the duplex. The 16.6·log10([Na+]/1000) term is negative at low salt and rises toward zero as salt increases, so doubling the salt concentration meaningfully raises the predicted Tm. Magnesium ions (Mg2+) have an even stronger per-ion effect in real reactions, which is why this melting temperature calculator records your Mg2+ value even though the salt-adjusted equation is driven primarily by Na+.

A great melting temperature calculator is only useful if you act on the numbers. For standard PCR, aim for primers with a Tm between 55 and 65 degrees Celsius, and keep the forward and reverse primer Tm values within about 5 degrees of each other so they anneal efficiently in the same cycle. Matching the pair is often more important than hitting an exact absolute value.

Set your initial annealing temperature about 5 degrees below the lower primer Tm; this tool suggests exactly that with its annealing temperature output. If you see primer-dimers or nonspecific bands, raise the annealing temperature to increase stringency. If the target band is faint or absent, lower it a few degrees. A gradient thermocycler lets you test a range in a single run and pick the cleanest result.

Beyond Tm, good primers avoid long runs of a single base, strong self-complementarity that forms hairpins, and 3' ends that are complementary to each other. Keeping GC content in the 40 to 60 percent band and length in the 18 to 25 base range, both of which the quality panel checks, gives you a primer that is specific and amplifies cleanly. Use the GC clamp principle by ending the primer with one or two G or C bases to anchor the 3' end during extension.

No Tm formula is perfect, and the methods in this calculator each have a domain where they shine and ranges where they drift. The Wallace rule is fast but ignores salt and length corrections, so it inflates Tm for primers longer than about 14 bases. The salt-adjusted and GC-adjusted equations are solid empirical fits for the 14 to 70 base range and are what most everyday primer design relies on.

The nearest-neighbor output in this tool uses single averaged enthalpy and entropy constants rather than the full SantaLucia 16-dinucleotide parameter set, so it is an approximation that systematically reads several degrees high; use it for relative comparison, not as an absolute target. Likewise, the calculator emphasizes monovalent Na+ in the salt term, while real PCR is strongly influenced by Mg2+, dNTPs, and DMSO that are not fully modeled here.

Treat every computed Tm as a well-informed starting estimate rather than a guaranteed value. The most reliable workflow is to use this melting temperature calculator to get into the right neighborhood, then confirm the optimal annealing temperature empirically with a gradient PCR. For demanding applications such as qPCR probe design or allele-specific amplification, cross-check with a full nearest-neighbor thermodynamic tool that uses experimentally measured parameters.