DNA Melting Temperature Calculator

The DNA melting temperature, written as Tm, is the temperature at which exactly half of a double-stranded DNA population has separated into single strands. At the Tm, 50% of the duplex molecules are still paired and 50% have "melted" apart, so it is the midpoint of the helix-to-coil transition rather than the point of complete denaturation. This DNA melting temperature calculator estimates the Tm of a short oligonucleotide directly from its sequence, oligo concentration, and salt concentration.

Tm is one of the most important numbers in molecular biology. It controls how PCR primers anneal, how hybridization probes bind their targets, and how stable a primer-template duplex will be at a given reaction temperature. A primer with a Tm that is too low will not bind reliably, while one that is too high can encourage non-specific priming. Because Tm depends on base composition, length, and the surrounding ionic environment, a quick, reliable calculator is far more practical than measuring melting curves on a spectrophotometer for every new oligo.

Three things raise Tm: a higher proportion of guanine-cytosine (GC) pairs, a longer sequence, and a higher salt concentration. GC pairs form three hydrogen bonds versus two for adenine-thymine (AT) pairs and stack more strongly, so GC-rich sequences resist melting. Longer duplexes have more stacking interactions holding the strands together, and positively charged counter-ions such as Na+ shield the negatively charged phosphate backbone, stabilising the double helix.

The calculator first cleans your input, converting it to uppercase and stripping any character that is not A, T, G, or C. It then counts each base, derives the length, and computes the GC content as a percentage. Based on the calculation method you select, it applies one of three formulas: the basic Wallace rule, a salt-adjusted equation, or a nearest-neighbor thermodynamic model. The result panel reports the Tm in degrees Celsius alongside the length, GC content, and individual A/T and G/C counts.

The Basic (Wallace rule) method is the simplest. It assigns 2°C to every A or T and 4°C to every G or C and sums them. This reflects the extra hydrogen bond in GC pairs and works reasonably for very short oligonucleotides under about 14 bases, but it ignores salt and concentration entirely.

The Salt-Adjusted method scales Tm with GC content and length while correcting for the monovalent salt concentration through a logarithmic term. It is a good general-purpose estimate for primers and probes in the 14-50 base range.

The Nearest-Neighbor method is the most accurate. Instead of treating each base independently, it sums published enthalpy (dH) and entropy (dS) values for every overlapping dinucleotide step in the sequence, then converts those thermodynamic totals into a Tm using the oligo concentration and a final salt correction. Stacking energy between adjacent bases is the dominant factor in duplex stability, which is why this approach outperforms the simpler rules.

For short oligonucleotides the calculator offers the classic Wallace rule, which estimates Tm purely from base counts. It is fast and surprisingly useful for primers up to roughly 14 bases, but it overestimates Tm for longer sequences and never accounts for buffer conditions.

The salt-adjusted equation blends GC content, length, and ionic strength. Notice that increasing the salt concentration makes the logarithmic term less negative and therefore raises Tm, exactly as observed experimentally. Lengthening the sequence shrinks the 675/length penalty, again raising Tm. The table below summarises the strengths of each method.

The salt-adjusted formula used here is Tm = 81.5 + 16.6 x log10(salt/1000) + 0.41 x %GC - 675/length, where the salt term converts your millimolar input to molar before taking the logarithm. Because it includes both GC content and an explicit ionic correction, it tracks real primer behaviour better than the Wallace rule while remaining easy to interpret.

In PCR, the annealing temperature is normally set a few degrees below the primer Tm. If your forward and reverse primers have very different melting temperatures, one may bind well while the other binds poorly, producing weak or biased amplification. Most protocols therefore aim to keep the two primer Tm values within about 2-5°C of each other and the working annealing temperature roughly 3-5°C below the lower Tm.

Beyond PCR, Tm guides the design of hybridization probes, molecular beacons, microarray oligos, and antisense reagents. For a probe to discriminate a single-base mismatch, you typically want it to operate near its Tm, where binding is most sensitive to small destabilisations. For stringent washes in Southern or Northern blotting, raising the temperature toward Tm strips away weakly bound, non-specific hybrids.

Because Tm shifts with buffer composition, always calculate it under conditions close to your real reaction. A primer designed at 50 mM salt will melt at a different temperature in a low-salt buffer. This DNA melting temperature calculator lets you change the oligo concentration and salt concentration so the estimate reflects your actual experiment rather than a generic default. Pair it with a GC content check and a quick secondary-structure scan for the most reliable primer design.

Several variables push Tm up or down, and understanding them helps you interpret the calculator's output and troubleshoot failed reactions.

• GC content: GC pairs share three hydrogen bonds and stack tightly, so a higher GC percentage raises Tm. AT-rich regions melt first.
• Sequence length: Longer duplexes have more base-stacking interactions, so Tm climbs with length, though the gain per base diminishes as sequences get long.
• Salt (ionic strength): Monovalent cations such as Na+ and K+ neutralise the phosphate backbone's negative charge, stabilising the duplex and increasing Tm. Divalent Mg2+ in PCR buffers has an even stronger stabilising effect.
• Oligo concentration: In the nearest-neighbor model, higher strand concentration drives annealing and modestly raises Tm, which is why concentration appears explicitly in that formula.
• Mismatches and secondary structure: A single mismatch can lower effective Tm by several degrees, and self-complementary regions can form hairpins that compete with target binding.
• Destabilising agents: Formamide, DMSO, and urea lower Tm and are sometimes added to GC-rich templates to ease denaturation.

Keeping these factors in mind, you can read a low calculated Tm as a sign to lengthen the primer or increase GC content, and an excessively high Tm as a cue to shorten it or accept a higher annealing temperature.