🌡️ GC Content & Melting Temperature (Tm)
Get the GC content and melting temperature (Tm) of a DNA sequence or primer — with the method used shown, so you know which formula applies.
60.0%
GC content
(G+C) ÷ length = (12) ÷ 20
55.9 °C
Melting temperature (Tm)
Salt-adjusted GC (Marmur–Doty)
4
A
4
T/U
6
G
6
C
Longer sequence (≥14 nt): the salt-adjusted GC formula Tm = 64.9 + 41(G+C − 16.4)/length is used. For final primer design, a nearest-neighbour thermodynamic Tm is more accurate. 🔒 Computed in your browser.
How the gc content & melting temperature (tm) works
Paste a sequence and the tool counts the bases to give GC% = (G + C) / length. For melting temperature it picks the appropriate formula by length: the Wallace rule Tm = 2(A+T) + 4(G+C) for short oligos under 14 nt, and the salt-adjusted GC formula Tm = 64.9 + 41(G+C − 16.4)/length for longer sequences. It tells you which method it used.
Most calculators hide which Tm formula they used; this one shows it, because the right method depends on oligo length. For final primer design, a nearest-neighbour thermodynamic Tm (which accounts for salt and strand concentration) is more accurate — a good phase-two addition.
Frequently asked questions
How do you calculate GC content?
GC content is the percentage of bases that are guanine or cytosine: GC% = (G + C) / total bases × 100. For GGCC it is 100%; for ATGC it is 50%.
How is primer melting temperature calculated?
For short primers (under 14 nucleotides) the Wallace rule Tm = 2(A+T) + 4(G+C) is standard. For longer sequences a salt-adjusted formula Tm = 64.9 + 41(G+C − 16.4)/length is used. The most accurate method for primer design is nearest-neighbour thermodynamics.
Why does GC content affect melting temperature?
G–C base pairs have three hydrogen bonds versus two for A–T, so GC-rich sequences are more stable and melt at a higher temperature. That is why both formulas weight G and C more heavily.
Which Tm formula should I use?
Use the Wallace rule only for short oligos (<14 nt). For longer sequences use a salt-adjusted or, ideally, a nearest-neighbour method. This tool picks the length-appropriate formula and tells you which it used.
What is a good GC content for primers?
Typically 40–60% GC gives stable, specific primers with a Tm in a workable range. Very high GC can cause secondary structure; very low GC can make primers unstable.
Is my sequence uploaded?
No — the counting and Tm calculation run entirely in your browser. Nothing is sent to a server.