Free DNA Melting Temperature Calculator
Calculate the melting temperature (Tm) of a DNA oligonucleotide using the Wallace rule and the nearest-neighbor approximation.
bp
Tm (Wallace Rule, short oligos)
60.0 °C
Tm (Marmur-Doty, long DNA)51.8 °C
GC Content50.0 %
Tm (Wallace Rule, short oligos) vs Oligonucleotide Length
DNA Melting Temperature
The melting temperature (Tm) is the temperature at which 50% of double-stranded DNA molecules are denatured into single strands.
Wallace Rule (for short oligos < 20 bp)
Tm = 2(A+T) + 4(G+C) °CMarmur-Doty (for longer DNA)
Tm = 64.9 + 41 × (G+C - 16.4) / N °CGC base pairs have three hydrogen bonds (vs two for AT), making them more thermally stable.
Example Calculation
A 20-mer with 10 GC bases and 10 AT bases.
- 01AT count = 20 - 10 = 10
- 02Wallace: Tm = 2(10) + 4(10) = 20 + 40 = 60°C
- 03GC content = 10/20 × 100 = 50%
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