Calculate the melting temperature (Tm) of your primers for PCR. Uses the nearest-neighbor thermodynamic method for accurate Tm estimation.
Primer Sequence
Reaction Conditions
Polymerase Type
Results
Method: —
ΔH: — kcal/mol
ΔS: — cal/(mol·K)
Primer Length: —
The NEB Tm Calculator helps molecular biologists determine the melting temperature of oligonucleotide primers for PCR reactions. Knowing the correct Tm is essential for designing efficient PCR experiments - it determines the optimal annealing temperature to use during thermal cycling.
Our NEB Tm Calculator uses two methods depending on primer length:
The annealing temperature (Ta) is calculated as Tm − 5°C, which is the standard recommendation for most polymerases. For Q5 and Phusion polymerases, adjustments are automatically applied.
Get the right annealing temperature to avoid non-specific binding, primer dimers, and failed amplifications in your PCR experiments.
Different polymerases have different optimal temperature ranges. Our NEB Tm Calculator adjusts for Taq, Q5, and Phusion enzymes automatically.
Adjust primer concentration, Na⁺ and Mg²⁺ concentrations to match your exact reaction setup for the most accurate Tm calculation.
Calculate Tm in real-time as you type your primer sequence. No waiting, no page reloads - just instant, accurate melting temperatures.
Tm is the temperature at which 50% of the primer-template DNA duplex is dissociated into single strands. It's a critical parameter for PCR because the annealing step temperature should be set close to (typically 5°C below) the Tm for optimal primer binding.
The nearest-neighbor method calculates Tm by considering the thermodynamic stability of each pair of adjacent bases (dinucleotides) in the primer sequence. It accounts for stacking interactions between neighboring base pairs, making it more accurate than simpler methods that only count GC content. NEB uses this method in their official Tm calculator.
Higher salt (Na⁺) concentrations stabilize DNA duplexes by neutralizing the negative charges on the phosphate backbone, which increases Tm. The nearest-neighbor Tm formula includes a salt correction term: Tm increases by approximately 7.21°C per log₁₀ increase in Na⁺ concentration. Mg²⁺ ions also stabilize duplexes and are accounted for in the calculation.
Most PCR primers are 18–25 nucleotides long. Shorter primers (<15 bp) may bind non-specifically, while very long primers (>30 bp) increase the risk of secondary structure formation. The optimal Tm range for most primers is 55–65°C.
Different polymerases have different buffer compositions and optimal temperature ranges. Q5 and Phusion (high-fidelity polymerases) work best with higher annealing temperatures compared to standard Taq polymerase. NEB recommends using their Tm calculator with the specific polymerase you're using for best results.
The ideal GC content for PCR primers is 40–60%. Primers with very low GC content will have low Tm values, while primers with very high GC content may form stable secondary structures. A balanced GC content ensures reliable primer binding and amplification.