Good Primer Tm Targets
Many routine PCR primers are designed around 18 to 30 nt, 40% to 60% GC content, and similar forward/reverse Tm values. Exact targets depend on polymerase, buffer, amplicon, and protocol.
DNA Melting Temperature Calculator
Calculate an estimated DNA Tm, GC percentage, sequence length, and design warnings for short oligos and PCR primers.
DNA Melting Temperature Formula
Wallace Tm = 2(A + T) + 4(G + C)For short primers, the Wallace rule gives a quick Tm estimate from base composition. A salt-adjusted approximation adds sequence length and ionic strength effects, which can be more informative for oligos longer than very short primers.
| Symbol | Variable | Description |
|---|---|---|
| A/T/G/C | Base counts | Counts of each nucleotide in the entered DNA sequence. |
| GC% | GC content | Percentage of bases that are G or C. |
| Tm | Melting temperature | Temperature where about half of the DNA duplex is dissociated under the model assumptions. |
How to Use the DNA Tm Calculator
- 1
Paste a DNA sequence
Use only A, T, G, and C. Spaces and line breaks are ignored.
- 2
Set salt concentration
Use 50 mM if you need a general PCR-style estimate.
- 3
Review Tm and GC content
Check whether the sequence has a reasonable Tm, GC percentage, and length for your application.
When to Use a More Advanced Tool
Use a nearest-neighbor oligo analyzer when primer design is high stakes, when sequences are long, when salt/Mg/dNTP conditions matter, or when hairpins and primer dimers need to be screened.
Practical Tm Checks
A Tm estimate is most useful when paired with sequence quality checks.
- Avoid long single-base runs.
- Keep paired primers within about 3 °C when possible.
- Check the 3' end for problematic complementarity.
- Confirm product specificity with alignment or primer design software.
DNA Tm Calculator FAQ
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