Primer Melting Temperature Calculator
Estimate the melting temperature (Tm) of a PCR primer from its GC content and length using the simplified Wallace-rule approximation. Useful for quickly designing and checking primers before running PCR.
About this calculator
The melting temperature (Tm) of a DNA primer is the temperature at which 50% of the primer-template duplexes are dissociated. This calculator uses a simplified empirical formula suitable for primers longer than about 20 bases and a standard 50 mM monovalent salt concentration: Tm = 81.5 + 16.6 × log₁₀(0.05) + 0.41 × (%GC) − 675 / L, where %GC is the percentage of guanine and cytosine bases and L is the primer length in base pairs. The log₁₀(0.05) term (≈ −1.301) accounts for the fixed 50 mM Na⁺ concentration, contributing approximately −21.6 °C. GC base pairs form three hydrogen bonds versus two for AT pairs, so higher GC content raises Tm. The 675/L correction compensates for end-effects that destabilize shorter sequences. For highly accurate work, the nearest-neighbor thermodynamic method is preferred, but this formula is fast and sufficiently precise for most routine PCR design.
How to use
Consider a 25 bp primer with 52% GC content. Insert values into the formula: Tm = 81.5 + 16.6 × log₁₀(0.05) + 0.41 × 52 − 675 / 25. Step by step: 16.6 × (−1.301) ≈ −21.6; 0.41 × 52 = 21.32; 675 / 25 = 27.0. So Tm = 81.5 − 21.6 + 21.32 − 27.0 = 54.22 °C. Enter 52 for GC content and 25 for primer length in the calculator to confirm. An annealing temperature of about 49–54 °C would typically be used in PCR, set 5 °C below Tm as a starting point.
Frequently asked questions
Why does GC content affect primer melting temperature?
Guanine-cytosine base pairs are held together by three hydrogen bonds, whereas adenine-thymine pairs share only two. This makes GC pairs thermodynamically more stable and harder to separate. As GC content increases, more energy (higher temperature) is required to denature the double-stranded duplex. Primers with very high GC content can cause non-specific binding at typical annealing temperatures, while very low GC content may result in weak primer-template binding and poor PCR efficiency.
What annealing temperature should I use based on the calculated Tm?
A common starting rule is to set the PCR annealing temperature 5 °C below the calculated Tm of the lower-Tm primer in the pair. For example, a Tm of 58 °C suggests an initial annealing temperature of 53 °C. Gradient PCR allows you to test a range around this value to find the optimal condition. If two primers have significantly different Tm values, primer redesign to balance them is generally preferable to compromising the annealing temperature.
When should I use the nearest-neighbor method instead of this simplified Tm formula?
The nearest-neighbor method accounts for the specific stacking interactions between adjacent base pairs and provides more accurate Tm predictions, especially for short primers (fewer than 20 bp) or those with unusual sequences like runs of G or C. The simplified formula used here assumes average base-stacking energies and a fixed salt concentration, so it is best suited for routine primer design with standard lengths of 18–30 bp. For cloning, mutagenesis, or applications where precise melting temperatures are critical, a nearest-neighbor tool or dedicated software such as OligoCalc or Primer3 is recommended.