How to use (3 steps)
- Select an example or enter primer sequences (Forward/Reverse).
- Adjust method and conditions (Na+, primer concentration) if needed.
- Tm and the annealing temperature guide (Ta) are shown.
Tm and Ta are guides. Final conditions depend on reagents, instrument, and template quality; optimize with gradient PCR if needed.
Inputs (primer sequences)
Results (Tm / Ta guide)
| Primer | Tm | Length | GC% |
|---|---|---|---|
| Forward | — | — | — |
| Reverse | — | — | — |
Treat Ta as a starting point and fine-tune final PCR conditions with gradient PCR when needed.
Method comparison (reference)
| Primer | Nearest-neighbor | Salt-adjusted | Wallace |
|---|
Nearest-neighbor is not applied when IUPAC bases (N/R/Y, etc.) are present.
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Method and assumptions
A primer Tm result is only as reliable as the assumptions behind it. This page keeps the formula choice, salt setting, primer concentration, and annealing-temperature offset visible so you can compare methods before deciding what to test in the lab.
- Recommended (nearest-neighbor): uses SantaLucia 1998 DNA duplex nearest-neighbor parameters, Na+ correction, and primer concentration. It is applied only when the sequence contains A/C/G/T bases.
- Basic (salt-adjusted): estimates Tm from GC%, sequence length, and Na+. It is useful as a quick cross-check, but it ignores sequence-neighbor effects.
- Very simple (Wallace): counts A/T and G/C bases for a rough short-oligo estimate. Use it as a screening guide, not as the final PCR setting.
- Ta (guide): uses the lower primer Tm minus the offset (default 3°C) and shows a suggested search range of Ta ± 3°C.
Mg2+ and dNTP fields are shown for lab-note context in this version; they are not used in the Tm calculation. If IUPAC bases (N/R/Y, etc.) are present, nearest-neighbor is not applied and the page falls back to a simple method.
Use this page to compare primer pairs before wet-lab testing
Primer Tm checks are most useful when you want one place to compare forward and reverse primers, salt assumptions, and a first annealing-temperature range before gradient PCR. Use qPCR ΔΔCt Calculator after you have assay data, switch to Reagent Table when you are planning mix volumes, and open A260 Calculator when concentration or purity is the immediate question instead of annealing behavior.
Good workflow
- Paste the exact primer sequences you intend to order or test.
- Keep salt and primer concentration assumptions aligned with the PCR protocol you will actually run.
- Treat the suggested Ta as a starting point, then confirm with gradient PCR or redesign when the pair remains far apart.
Why online primer Tm calculators can disagree
Different primer Tm calculators can return different numbers for the same sequence because they do not always use the same thermodynamic table, salt model, primer concentration, Mg2+ correction, or ambiguous-base policy. The difference is expected; it does not automatically mean one calculator is wrong.
For PCR planning, use this calculator to compare the method assumptions and the forward/reverse ΔTm first. Then treat the annealing temperature as a starting guide and confirm the final condition with gradient PCR, especially when primer Tm values are far apart or the template is difficult.
FAQ
Is this a reliable primer Tm calculator for PCR?
It is reliable for transparent planning because it shows the method, salt setting, primer concentration, and Ta offset used for the estimate. It is still a guide: final PCR conditions depend on polymerase, buffer, Mg2+, template, and instrument, so confirm important assays experimentally.
What is Tm (melting temperature)?
Tm is the temperature at which roughly half of a DNA duplex is denatured. It depends on length, GC%, and salt conditions.
How should I choose annealing temperature (Ta)?
This tool shows Ta from the lower primer Tm minus an offset as a starting guide. Final PCR conditions should be optimized with gradient PCR when needed.
Why are there multiple Tm formulas?
Different formulas make different assumptions. Nearest-neighbor uses sequence context and salt correction, while simpler formulas and Wallace are faster rough estimates.
Why do different primer Tm calculators give different numbers?
They may use different nearest-neighbor parameters, salt corrections, primer concentration defaults, Mg2+ or dNTP adjustments, and rules for ambiguous bases. Compare the assumptions before comparing the final number.
Do Mg2+ and dNTP change the Tm result here?
Not in this version. Mg2+ and dNTP values are kept as visible protocol notes, but the current Tm calculation uses the selected method, Na+, primer concentration, sequence length, and base composition.
Does salt concentration affect Tm?
Yes. Higher salt typically stabilizes duplexes and raises Tm.
What if Forward and Reverse primer Tm differ a lot?
A large delta Tm usually means you should test a wider annealing range with gradient PCR or redesign one primer so the pair is closer.
Are my sequences sent anywhere?
No. All calculations run in your browser and no data is sent.
Related tools
- A260 Calculator (DNA/RNA Concentration & Purity) | CalcBEConfirm template concentration and purity before blaming primer design for weak amplification.
- Beta diversity calculator (Jaccard / Bray–Curtis) | CalcBEMove here only after primer validation if your downstream work compares sample composition or ecology data.
- Cell seeding calculator | cells/well → required volume | CalcBEUse seeding plans upstream when primer tests are tied to consistent plated cell numbers.
- Centrifuge converter (rpm ↔ RCF ×g) | by rotor radius | CalcBEConvert rotor settings separately when sample prep requires pelleting before PCR or sequencing.
- Diversity index calculator (Shannon & Simpson) | CalcBEUse diversity summaries later when the workflow moves beyond primer screening into sample-level comparison.
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