Annealing Temperature Calculator — PCR Primer Tm
Find the optimal annealing temperature for your PCR primer: enter the count of each nucleotide base (A, T, G, C) and the calculator applies the Wallace rule for short primers or the empirical Marmur formula for longer ones, giving Tm and the standard Ta = Tm − 5 °C.
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Recommended PCR annealing temperature (Tm − 5 °C)
- 1
GC content (%)
11 ÷ 20 × 100 = 55 % - 2
GC term (0.41 × %GC)
0.41 × 55 = 22.55 - 3
Length correction (675/N)
675 ÷ 20 = 33.75 - 4
Melting temperature Tm
81.5 + -21.6 + 22.55 − 33.75 = 48.7 °CThe −21.6 term is 16.6·log₁₀(50 mM Na⁺ buffer). - 5
Annealing temperature Ta
48.7 − 5 = 43.7
How does this calculator work?
For primers ≤18 bp: Tm = 2(A+T) + 4(G+C) °C (Wallace rule). For longer primers: Tm = 81.5 + 16.6·log₁₀[Na⁺] + 0.41·(%GC) − 675/N. Annealing temperature = Tm − 5 °C. Enter your A, T, G, C base counts and the calculator selects the right formula and reports Tm, Ta, and GC content.
Formula
How this is calculated
The melting temperature (Tm) of a DNA duplex is the temperature at which 50% of the strands are double-stranded and 50% are single-stranded. It depends on length and GC content because G–C base pairs form three hydrogen bonds (more stable) while A–T pairs form only two. For short oligomers up to 18 bases, the Wallace rule (Tm = 2(A+T) + 4(G+C)) is the standard quick estimate; it assumes 1 M NaCl and gives results accurate to ±3–5 °C for primers in that range.
For primers longer than 18 bp, the empirical formula derived from Marmur and Doty's thermodynamic data is more accurate: Tm = 81.5 + 16.6·log₁₀[Na⁺] + 0.41·(%GC) − 675/N, where N is total length. This calculator assumes a 50 mM Na⁺ buffer concentration typical of standard PCR buffers; if your buffer is significantly different, the actual Tm will shift (increasing Na⁺ raises Tm by about 16.6 °C per log-unit of [Na⁺]).
The recommended annealing temperature (Ta) is conventionally set 5 °C below Tm to allow efficient primer binding while maintaining enough specificity to avoid non-specific amplification. Touchdown PCR starts 2–3 °C below Tm and decreases each cycle to improve specificity. These are starting-point recommendations — fine-tuning with a temperature gradient PCR is always the most reliable method.
Frequently asked questions
Tm (melting temperature) is a property of the primer–template duplex: the temperature at which half the duplexes have separated. It is calculated from sequence. The annealing temperature (Ta) is the temperature you set on the PCR thermocycler — typically Tm − 5 °C to ensure most primers bind their template efficiently. Setting Ta too high reduces yield; too low causes non-specific products.
The Wallace rule (Tm = 2(A+T) + 4(G+C)) is designed for primers of 14–18 bp and is well-established for quick estimates. It systematically overestimates Tm for primers shorter than ~14 bp and underestimates for longer primers. The empirical Marmur formula performs better for primers of 19–35 bp. For mission-critical applications, use nearest-neighbour thermodynamic software such as Primer3 or OligoCalc.
A GC content of 40–60% is generally recommended for PCR primers. Primers with very high GC (>70%) tend to have high Tm and can form secondary structures (hairpins, G-quadruplexes) that reduce efficiency. Very low GC (<30%) gives low Tm, requiring very low annealing temperatures that increase non-specific binding. Balanced GC content across the primer also prevents 3' end clamp issues.
Also known as
TG we-Calculate Editorial Team. (2026). Annealing Temperature Calculator — PCR Primer Tm [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/annealing-temperature-calculator
TG we-Calculate Editorial Team. "Annealing Temperature Calculator — PCR Primer Tm." TG we-Calculate. 2026. https://we-calculate.com/calculator/annealing-temperature-calculator.
TG we-Calculate Editorial Team, "Annealing Temperature Calculator — PCR Primer Tm," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/annealing-temperature-calculator
@misc{wecalculate_annealing_temperature_calculator, title = {Annealing Temperature Calculator — PCR Primer Tm}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/annealing-temperature-calculator}}, year = {2026}, note = {TG we-Calculate} }
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