DNA Melting Temperature (Tm) Calculator

The DNA Melting Temperature (Tm) Calculator calculates the temperature at which a DNA primer separates from its complementary strand — a critical value for designing PCR experiments and optimizing annealing conditions. Enter your DNA sequence (5' to 3'), primer concentration, salt concentration, and optional Mg²⁺ concentration, then select a calculation method (Wallace Rule or Nearest Neighbor) to get your Melting Temperature (Tm). Secondary outputs include GC content, sequence length, molecular weight, and a suggested annealing temperature for PCR setup.

Enter the DNA primer sequence in 5' to 3' direction

μM

Concentration of primer in micromolar (μM)

mM

Monovalent cation concentration (Na+, K+) in millimolar

mM

Magnesium ion concentration in millimolar

Choose calculation method for Tm determination

Results

Melting Temperature (Tm)

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GC Content

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Sequence Length

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Molecular Weight

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Suggested Annealing Temperature

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Frequently Asked Questions

What is DNA melting temperature (Tm)?

Tm is the temperature at which 50% of DNA strands dissociate from their complementary strands. It's crucial for designing PCR primers and optimizing reaction conditions.

Which calculation method should I use - Basic or Nearest Neighbor?

Nearest Neighbor is more accurate as it considers base pair interactions and thermodynamic parameters. Basic method uses simple rules and is less precise but faster for rough estimates.

How does salt concentration affect Tm?

Higher salt concentrations increase Tm by stabilizing DNA duplexes. Typical PCR reactions use 50-100 mM monovalent salt concentration for optimal primer annealing.

What annealing temperature should I use for PCR?

Generally, use 3-5°C below the calculated Tm. Our calculator suggests an annealing temperature that works well for most PCR applications.

Why is primer concentration important for Tm calculation?

Primer concentration affects the equilibrium between bound and unbound DNA strands. Higher concentrations increase effective Tm, impacting PCR efficiency and specificity.

How does GC content affect melting temperature?

Higher GC content increases Tm because G-C base pairs form three hydrogen bonds compared to two for A-T pairs, making them more thermally stable.

Can I use this calculator for RNA sequences?

This calculator is optimized for DNA sequences. RNA has different thermodynamic properties and would require specific RNA Tm calculation parameters.

What if my primer contains mismatches or modifications?

This calculator assumes perfect complementarity. For primers with mismatches or chemical modifications, consider only the perfectly matched portion for accurate Tm calculation.