Free Primer Tm Calculator with SantaLucia/Owczarzy Math

Calculate primer melting temperature with SantaLucia nearest-neighbor thermodynamics, Owczarzy salt correction, adjustable Na+, Mg2+, dNTP, DMSO, formamide, and oligo concentration settings. Get Tm, ΔH, ΔS, ΔG, suggested annealing temperature, and batch analysis for up to 1000 sequences without signup or server-side sequence upload.

Matching NEB-style, IDT-style, Twist-style, or Q5 conditions? Enter the same vendor-style settings for salt, Mg2+, dNTP, polymerase, or concentration assumptions here, then use comparison pages only when you need context before calculating. This page is built for method-based, private calculation first: no sequence upload, no account gate, and no need to leave the calculator for batch Tm checks. Need more than Tm? Open the Primer Analyzer for OligoAnalyzer-style GC, MW, hairpin, and dimer checks. Need to understand a disagreement? Read why Tm calculators disagree or review the Tm method review.

Input Parameters

Length: 0 nt

Determines which nearest-neighbor thermodynamic parameters to use.

Salt Conditions

Enter sodium ion concentration in millimolar (mM), typically 50-200 mM for PCR

Enter magnesium ion concentration in millimolar (mM), typically 1.5-3 mM for PCR, or 0 if not applicable

Enter dNTP concentration in millimolar (mM), typically 0.2-0.5 mM for PCR, or 0 if not applicable

Other Conditions

Enter oligonucleotide concentration in nanomolar (nM), typically 250-500 nM for PCR

Enter DMSO percentage, typically 0-10% to reduce secondary structure, or 0 if not applicable

Results

No results yet

Enter a sequence and click "Calculate Tm"

What the calculator evaluates

Calculations run in the browser for client-side privacy and no sequence upload, with method references from SantaLucia 1998 nearest-neighbor parameters plus Owczarzy salt correction papers. Use this page for the actual primer melting temperature result, then use the NEB, IDT, method-comparison, and research pages for method notes.

Tm Calculation Method Comparison

Enter your sequence and see how 5 different Tm calculation methods produce different results. Understand which method matches NEB, IDT, or Primer3 — and why the differences matter for your experiment.

DNA only (A, T, C, G). Minimum 6 nt.

These conditions affect NN methods. Wallace ignores salt. %GC uses Na⁺ only.

What Is Melting Temperature (Tm)?

Use this Tm calculation page when you need to calculate primer melting temperature, match NEB/IDT/Twist-style reaction assumptions, account for salt, Mg²⁺, dNTP, DMSO, formamide, or concentration settings, or produce private batch Tm output in the browser. Use the comparison, guide, FAQ, and research pages when you need method notes or reference material.

The melting temperature (Tm) of an oligonucleotide primer is the temperature at which 50% of the oligo-complement duplex dissociates into single strands. The calculator uses SantaLucia 1998 nearest-neighbor thermodynamics with Owczarzy salt correction, so users can enter the sequence, Na⁺, Mg²⁺, dNTP, DMSO, formamide, and concentration assumptions that match the reaction they actually plan to run.

Tm is the single most critical parameter in PCR primer design because the annealing temperature (Ta) of your PCR reaction is derived directly from the Tm of your primers. A primer with a Tm that is too low will not bind specifically to its target, while one that is too high may cause non-specific amplification. For standard PCR, optimal primer Tm is 55-65°C with primer pairs within ΔTm ≤ 5°C of each other.

Unlike the simpler Wallace Rule (Tm = 2(A+T) + 4(G+C), accuracy ±5-10°C) or the %GC method (accuracy ±3-5°C), the nearest-neighbor method accounts for stacking interactions between adjacent base pairs, producing accuracy within ±1-2°C for oligonucleotides of 15-70 nt. The Owczarzy et al. (2008) salt correction formula handles mixed monovalent/divalent cation solutions — critical for real PCR buffers containing both Na⁺/K⁺ and Mg²⁺. All OligoPool calculations run locally in your browser via client-side JavaScript — your sequences are never transmitted to any server.

How to Use the Tm Calculator

  1. Enter your oligonucleotide sequence (5' to 3') in the input field. The calculator accepts IUPAC DNA/RNA codes.
  2. Set the reaction conditions: Na⁺ concentration (default 50 mM), Mg²⁺ concentration (default 1.5 mM), and oligonucleotide concentration (default 0.25 µM). Match these to your actual PCR buffer.
  3. If using DMSO or formamide, enter the percentage in the correction fields.
  4. For multiple primers, switch to Batch Mode and paste one sequence per line or upload a FASTA file.
  5. Click "Calculate" to see results including Tm, GC content, molecular weight, and thermodynamic parameters (ΔH, ΔS, ΔG).
  6. Use the suggested annealing temperature (Ta = Tm - 5°C) as a starting point for your PCR optimization.

Frequently Asked Questions

Which settings should I enter for a method-based Tm calculation?
Enter the primer sequence first, then match the reaction conditions you will use in the lab: Na⁺ or K⁺, Mg²⁺, dNTP concentration, DMSO or formamide percentage, and oligo concentration. If you are checking NEB-style, IDT-style, Twist-style, or Q5 conditions, use this page for the actual Tm calculation and mirror the buffer assumptions as closely as possible.
What method does this free Tm calculator use?
The result depends on the algorithm, thermodynamic parameters, and buffer assumptions used. This calculator uses published SantaLucia nearest-neighbor parameters and Owczarzy salt correction formulas, then keeps the calculation client-side so your sequences are not uploaded to a server.
When should I use the comparison pages instead of this calculator?
Use this page when you already need a Tm number. Use the NEB, IDT, or method-comparison pages when the question is which method or buffer assumption to use before calculating. After that decision is made, return here with the sequence and actual salt, Mg²⁺, dNTP, DMSO, and concentration settings.
Which Tm calculation method should I use?
For primers between 15-70 nucleotides, the nearest-neighbor (NN) thermodynamic method is usually the best practical choice because it accounts for stacking interactions between neighboring base pairs. The Wallace Rule (2°C per A/T + 4°C per G/C) is only suitable for rough estimates of short oligos (<14 nt). The %GC method is also a rough estimate and assumes standard salt conditions. This calculator uses the SantaLucia (1998) unified NN parameters.
How does salt concentration affect Tm?
Increasing monovalent cation (Na⁺, K⁺) concentration stabilizes the DNA duplex and raises Tm, typically by 12-15°C per 10-fold increase in salt. Divalent cations (Mg²⁺) have an even stronger stabilizing effect. Set the salt concentration to match your actual PCR buffer: Standard Taq buffer = ~50 mM KCl + 1.5-2.0 mM MgCl₂; Phusion HF buffer = no added monovalent salt + 1.5 mM MgCl₂. Changing Na⁺ from 50→100 mM shifts Tm by +3-5°C. Changing Mg²⁺ from 1.5→3 mM shifts Tm by +1.5-2.5°C.
What is a good Tm for PCR primers?
For standard PCR: 55-65°C (optimal: 58-62°C). Primer pairs should have Tm within 5°C of each other (ideally within 2°C). Set annealing temperature (Ta) 5°C below the lower primer Tm. For high-fidelity enzymes (Phusion, Q5): typically 65-72°C — always check the manufacturer's Ta calculator. For qPCR probes: Tm 5-10°C above primer Tm for effective 5' nuclease assays.
Why does my Tm differ from other calculators by 1-3°C?
Differences of ±2°C between nearest-neighbor calculators are normal. Common causes: 1) Different parameter sets (SantaLucia 1998 vs Breslauer 1986), 2) Different salt corrections (Owczarzy 2008 vs SantaLucia 1998 formula), 3) Different assumed oligo concentrations (0.25 µM vs 0.5 µM), 4) Initiation parameter handling. If your result differs from NEB by >2°C, check that your Na⁺ and Mg²⁺ settings match — salt concentrations cause 90% of discrepancies.
Can I calculate Tm for modified oligonucleotides?
Standard NN parameters are for unmodified DNA. Terminal modifications (5'-phosphate, biotin, fluorescent labels) have negligible effect (<0.5°C). Backbone modifications significantly affect stability: LNA (+2-6°C per substitution), phosphorothioate (-0.5°C per substitution), 2'-O-methyl RNA (+1-2°C per substitution). The calculator supports unmodified DNA/RNA — use calculated Tm as a baseline for modified oligos and adjust empirically.
Can I calculate Tm for many primers at once?
Yes. Switch to Batch Mode, paste one sequence per line or upload FASTA-style input, then use the same buffer assumptions across the set. Batch mode is the right path when you need to screen primer panels, compare Tm windows, or prepare a list for downstream QC.

Related Tools

Primer Analyzer

OligoAnalyzer-style primer review with Tm, GC%, self-complementarity, and dimer checks.

Tm Methods Comparison

Side-by-side comparison of SantaLucia, Owczarzy, Wallace, and %GC methods.

GC Content Analyzer

Analyze GC percentage and base composition. Identify extreme GC content that may affect Tm accuracy.

Secondary Structure Predictor

Predict hairpins and dimers that compete with primer-template binding and affect effective Tm.

NEB Tm Calculator Comparison

Use this support page when you need NEB-specific polymerase and buffer context before calculating.

Free Oligo Tool Comparison

Compare online tools for Tm calculation, primer analysis, structure checks, and batch QC.

Review request

Check a result or ordering detail

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Related reading

Continue with the page or tool that matches the next decision in your experiment.

Accuracy Validation: OligoPool vs NEB & IDTNEB Tm Calculator ComparisonIDT OligoAnalyzer ComparisonSalt Correction Method ComparisonΔG Threshold Database — Predict PCR SuccessTm Method ReviewValidate PCR Primers Before OrderingTm Method ReferenceQuick Answers for Tm Decisions