Primer Melting Temperature (Tm) Calculator
What is this tool?
This free online primer melting temperature (Tm) calculator uses the SantaLucia nearest-neighbour thermodynamic model to compute Tm, GC content, delta-G, and molecular weight for DNA primer sequences. It is designed for molecular biology researchers and PCR assay developers who need accurate thermodynamic primer validation.
Calculate melting temperature, GC content, and free energy for PCR primers using SantaLucia nearest-neighbour thermodynamics. Supports monovalent and divalent salt concentration corrections.
Last updated: July 2026 · Reviewed by VigyanLLM Research Team
VigyanLLM's primer Tm calculator uses the SantaLucia 1998 nearest-neighbor model with salt correction to calculate melting temperature, GC content, and ΔG values. Accurate Tm prediction is essential for successful PCR annealing temperature optimization.
| Parameter | Formula / Method |
|---|---|
| Basic Tm | 4(G+C) + 2(A+T) |
| Salt-Adjusted Tm | SantaLucia NN with Na+/Mg2+ correction |
| ΔG (Hairpin) | Gibbs free energy of secondary structure |
| ΔG (Dimer) | Primer-dimer formation energy (kcal/mol) |
Enter a Primer Sequence
| Parameter | Value |
|---|---|
| Sequence Length | — |
| GC Content | — |
| Melting Temperature (Tm) | — |
| ΔG (kcal/mol) | — |
| Molecular Weight | — |
What Is Primer Melting Temperature (Tm)?
Primer melting temperature (Tm) is the temperature at which half of the DNA duplex dissociates into single strands. In PCR, the annealing temperature is typically set 3–5°C below the Tm of the primers to ensure specific binding to the template.
How Is Tm Calculated?
VigyanLLM uses the SantaLucia 1998 unified nearest-neighbour thermodynamic model, which is the most accurate method for DNA oligonucleotides. The calculation accounts for:
- Nearest-neighbour base pair stacking energies (ΔG°, ΔH°, ΔS°)
- Initiation and symmetry corrections
- Monovalent salt ([Na⁺]) concentration correction
- Divalent salt ([Mg²⁺]) concentration correction
- Oligonucleotide concentration (assumed 0.25 μM)
Primer Tm Best Practices
- Optimal Tm range: 52–58°C for standard PCR
- GC content: 40–60% for consistent melting behaviour
- Primer length: 18–24 nucleotides
- Tm difference between forward and reverse primers should be ≤ 2°C
- 3' end should end in G or C (GC clamp) for tight binding
Related Tools
- AI Primer Design Tool — design complete primer pairs with BLAST specificity
- PCR Analysis Software — run full PCR simulations
- VigyanLLM Primer — complete primer design pipeline
Frequently Asked Questions About Melting Temperature (Tm) Calculator
Everything you need to know about using this tool
Is the Tm Calculator free to use?
Yes, the Melting Temperature Calculator is completely free with no usage limits. No login, account, or API key required.
How is melting temperature calculated?
The tool uses the Nearest Neighbor thermodynamic model with salt correction. Tm is calculated as Tm = ΔH/(ΔS + R·ln(C/4)) − 273.15 + salt correction, using established nearest-neighbor enthalpy and entropy values.
Why is accurate Tm important for PCR?
Accurate Tm ensures optimal annealing temperature. Too low causes non-specific binding; too high reduces efficiency. Optimal annealing temperature is typically 3–5°C below the primer Tm.
What parameters can I adjust?
You can adjust DNA concentration, monovalent salt concentration, and magnesium concentration. Both basic and salt-corrected Nearest Neighbor models are supported.
What is the difference between Tm and Ta?
Tm (melting temperature) is when 50% of DNA duplexes dissociate. Ta (annealing temperature) is the PCR cycling temperature for primer binding, typically set 3–5°C below Tm.
Is my data stored?
No. All calculations happen in your browser. Sequences are never transmitted or stored.