Gera Tools

Primer Tm (Melting Temperature) Calculator

Estimate primer melting temperature by nearest-neighbour or basic method

Calculate PCR primer melting temperature from its sequence using the basic 2(A+T)+4(G+C) rule, the salt-adjusted Marmur formula, and SantaLucia nearest-neighbour thermodynamics with salt and primer concentration. For molecular biology and assay design. It runs free in your browser on Gera Tools, with nothing uploaded.

Last updated Source: Gera Tools

What is primer melting temperature?

The melting temperature, or Tm, is the temperature at which half of the primer-template duplexes have separated into single strands. It guides the annealing temperature of a PCR, since primers must bind their target but not stick non-specifically.

A primer’s melting temperature sets the annealing temperature of your PCR and strongly affects specificity. This calculator reports the nearest-neighbour Tm — the most accurate method — alongside the classic basic and salt-adjusted estimates, straight from the primer sequence.

How it works

Three methods are computed in parallel so you can see how much they differ for your specific oligo.

Basic Wallace rule — a quick count of bases, calibrated for short sequences under about 14 bases:

basic Tm = 2 × (A+T) + 4 × (G+C)

Salt-adjusted Marmur formula — adds length and a GC term but still no sequence-context information:

salt-adj Tm = 64.9 + 41 × (G+C − 16.4) / N

Nearest-neighbour thermodynamics (SantaLucia 1998 parameters) — sums experimentally measured enthalpy (ΔH) and entropy (ΔS) for every adjacent base-pair step, adds initiation terms, then solves for temperature with salt and primer-concentration corrections:

Tm(K)  = ΔH × 1000 / ( ΔS + R × ln(Ct/4) )
Tm(°C) = Tm(K) − 273.15 + 16.6 × log10([Na+])

where R is the gas constant (1.987 cal/mol·K), Ct the total primer concentration in molar, and [Na+] the monovalent cation concentration in molar.

Understanding the output

For the M13 forward primer GTAAAACGACGGCCAGT at 50 mM NaCl and 250 nM primer, the nearest-neighbour method gives a Tm in the low-to-mid 50s °C. The basic rule tends to over-estimate this sequence by several degrees because it ignores that A-T steps stack more weakly than G-C steps.

The three methods diverge most for:

  • GC-rich primers — the nearest-neighbour result is noticeably more accurate because consecutive G-C steps have unusually high stacking enthalpy.
  • Short primers under 15 bases — the basic rule is surprisingly reasonable here; nearest-neighbour gains the most over it for longer oligos.
  • Low-salt conditions — the salt correction matters more when the cation concentration diverges from the 50 mM default assumed by the Wallace rule.

Choosing the annealing temperature

A standard starting point is Tm − 3 °C to Tm − 5 °C, where Tm is the lower of your two primers’ nearest-neighbour values. For example, if the forward primer gives 60 °C and the reverse gives 58 °C, try 53–55 °C first.

  • If you see non-specific bands, raise the annealing temperature 1–2 °C at a time.
  • If the product is faint or absent, lower it and verify that both primers actually bind your target sequence (check for SNPs or sequence errors).
  • Gradient PCR across a range of annealing temperatures is the fastest way to find the empirical optimum without iterating blind.

Primer design tips for Tm

  • Keep both primers within 5 °C of each other. A large Tm mismatch means one primer dominates anneal efficiency, reducing yield and increasing non-specific products.
  • Aim for a GC content of 40–60% for stable, specific binding.
  • Avoid runs of the same base (especially poly-G) and intra-primer complementarity that could form hairpin structures — these reduce the effective Tm in ways the calculator cannot fully predict.
  • Self-complementary or 3’-complementary primer pairs can form primer-dimers; check both sequences for complementarity before ordering.