primer tm calculator

What is primer Tm?

Primer melting temperature (Tm) is the estimated temperature at which half of a primer is bound to its complementary DNA target and half is unbound. In PCR, Tm helps you choose an annealing temperature that promotes specific binding while minimizing off-target amplification.

How this primer Tm calculator works

This calculator uses two common methods:

• Wallace Rule: Tm = 2°C × (A + T) + 4°C × (G + C)
• Salt-adjusted formula: Tm = 81.5 + 16.6 × log₁₀[Na⁺] + 0.41 × (%GC) − (600 / N)

In auto mode, short primers generally use Wallace-style estimation, while typical PCR-length primers use the salt-adjusted model. This gives a practical estimate for quick primer screening.

Inputs included in the calculation

• Sequence length (N): total primer bases
• Base composition: counts of A, T, G, and C
• GC content: higher GC usually increases Tm
• Salt concentration: ionic strength stabilizes duplex formation

How to use the calculator

1. Paste or type a primer sequence using only A/T/C/G.
2. Set monovalent salt concentration (50 mM is a common default).
3. Select Auto mode or force a specific formula.
4. Click Calculate Tm and review the suggested annealing range.

Interpreting your result

A practical PCR annealing temperature is often set about 3–5°C below primer Tm. If you are using a primer pair, set conditions based on the lower Tm primer and then optimize experimentally.

• Too low annealing temperature: more non-specific bands
• Too high annealing temperature: weak or no amplification
• Balanced condition: strong, specific product

PCR primer design tips for better Tm behavior

• Aim for primer length around 18–25 nucleotides for most assays.
• Target GC content in the ~40–60% range.
• Avoid long homopolymer runs (e.g., AAAA or GGGGG).
• Limit strong 3' complementarity between primer pairs to reduce primer-dimers.
• Use similar Tm values for forward and reverse primers (within ~2–3°C).

Important limitations

This tool is meant for rapid estimation, not full thermodynamic modeling. Exact Tm can vary with Mg²⁺, dNTP concentration, additives (DMSO, betaine), template complexity, and nearest-neighbor effects. For critical assays (qPCR, multiplex PCR, diagnostic work), confirm with advanced software and lab optimization.