Primer Melting Temperature (Tm) Calculator
Estimate primer Tm, GC% and molecular weight from a sequence.
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Estimate the melting temperature (Tm) of a PCR primer or oligo, along with its GC content and molecular weight. The tool reports both the quick Wallace rule and a salt-adjusted formula and recommends the right one for your primer length ā a fast sanity check before ordering oligos or setting up a PCR.
0 nt
0 nt
Length
0.0%
GC content
0.0 Ā°C
Tm (Wallace (2AT+4GC))
0.0
ssDNA MW
| Tm ā Wallace rule (2AT+4GC) | 0.0 Ā°C |
| Tm ā salt-adjusted | 0.0 Ā°C |
| Single-stranded MW (g/mol) | 0.00 |
| Double-stranded MW (g/mol) | 0.00 |
The Wallace rule suits primers under 14 nt; the salt-adjusted formula is better for longer primers. For high-stakes design, confirm with a nearest-neighbour calculator.
How to use the Primer Tm tool
- 1Paste a single primer or oligo sequence.
- 2Read the estimated Tm from both the Wallace rule and the salt-adjusted formula.
- 3Check GC% and molecular weight, then aim for matched Tm values across a primer pair.
Frequently asked questions
- Which Tm formula should I use?
- The Wallace rule (2Ć(A+T) + 4Ć(G+C)) is a quick estimate for primers under 14 nt. For longer primers the salt-adjusted formula 64.9 + 41Ć(GCā16.4)/N is more appropriate. We show both and recommend one based on length.
- How is molecular weight calculated?
- From standard anhydrous nucleotide monophosphate masses with an end correction for a 5'-OH oligo. Both single-stranded and double-stranded values are given.