60
°C
46.7
°C
20
nt
50
%
60
°C
46.7
°C
20
nt
50
%
The Primer Tm Calculator estimates the melting temperature of PCR primers using two complementary methods: the Wallace rule for quick estimation and the salt-adjusted formula that accounts for monovalent cation concentration. Accurate Tm prediction is fundamental to PCR primer design, as the annealing temperature is typically set 3-5 degrees Celsius below the Tm. This calculator also reports primer length and GC content to help assess overall primer quality.
Two methods are provided:
1. Wallace Rule (for primers ≤14 nt):
Tm = 2(A + T) + 4(G + C)
2. Salt-Adjusted Formula (for primers 15–70 nt):
Tm = 81.5 + 16.6 × log10[Na+] + 41 × (GC fraction) − 675/N
Where:
The salt-adjusted formula is more accurate for typical PCR conditions and accounts for the stabilizing effect of cations on the DNA duplex.
Inputs
Results
A 20-nt primer with 50% GC content has a Wallace Tm of 60°C and a salt-adjusted Tm of 56.2°C at 50 mM Na+.
Inputs
Results
A 25-nt primer with 68% GC content shows a significant difference between Wallace (84°C) and salt-adjusted (66.5°C) Tm values. The salt-adjusted value is more reliable for this length.
For primers ≤14 nucleotides, the Wallace rule is reasonably accurate. For standard PCR primers (18–30 nt), use the salt-adjusted formula as it accounts for ionic strength. For the most accurate results, especially for primers with modified bases or in specific buffer conditions, use nearest-neighbor thermodynamic methods available in primer design software like Primer3.
As a starting point, set the annealing temperature to 3–5°C below the lower Tm of your primer pair. If you experience non-specific amplification, increase the temperature in 2°C increments. If there is no amplification, decrease the temperature. Gradient PCR (testing multiple temperatures simultaneously) is the most efficient way to optimize annealing temperature.
Ideal PCR primers have a GC content between 40% and 60%. This range provides a good balance between binding stability and specificity. Primers with very low GC content (<30%) may bind weakly, while very high GC content (>70%) can lead to secondary structures, non-specific binding, and difficulty in denaturing during PCR.
Roboculator Team
The Roboculator Team explains calculations, planning tools, and practical formulas in clear language for real-life situations.
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