PCR Annealing Temperature Calculator
Calculate the optimal annealing temperature for PCR reactions based on your primer sequences using the nearest-neighbor thermodynamic method or the basic Wallace rule.
Formulas
1. Wallace Rule (quick estimate, primers 14–20 bp):
Tm = 2(A + T) + 4(G + C)
where A, T, G, C are the counts of each nucleotide in the primer.
2. Nearest-Neighbor Thermodynamics (SantaLucia 1998 — recommended):
Tm (K) = ΔH° / (ΔS° + R · ln(CT/4))
Tm (°C) = Tm (K) − 273.15
where ΔH° = sum of nearest-neighbor enthalpies + initiation terms (kcal/mol), ΔS° = sum of nearest-neighbor entropies + initiation terms (cal/mol/K), R = 1.987 cal/mol/K, CT = total primer strand concentration (mol/L).
Salt Correction (Owczarzy et al. 2004):
Tm(corrected) = Tm + 16.6 · log₁₀([Na⁺])
where [Na⁺] is in mol/L.
Annealing Temperature:
Ta = T̄m − 5 °C (range: T̄m − 7 to T̄m − 3 °C)
where T̄m is the average Tm of the two primers.
Assumptions & References
- The nearest-neighbor model assumes the primer forms a perfect Watson-Crick duplex with its complement under standard conditions.
- IUPAC ambiguity codes (R, Y, S, W, K, M, B, D, H, V, N) are excluded from nearest-neighbor calculations; the clean ATGC subsequence is used.
- The Wallace rule is most accurate for primers between 14 and 20 bp and does not account for salt concentration or primer concentration.
- The nearest-neighbor parameters are from: SantaLucia J. (1998). A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics. PNAS, 95(4), 1460–1465.
- Salt correction from: Owczarzy R. et al. (2004). Effects of sodium ions on DNA duplex oligomers. Biochemistry, 43(12), 3537–3554.
- The recommended annealing temperature is typically 5°C below the average Tm of the primer pair; empirical optimization (gradient PCR) is advised.
- A Tm difference >5°C between primers may reduce PCR efficiency and specificity.
- This calculator assumes non-self-complementary primers (CT/4 term in the Tm equation).
- Default conditions: 50 mM Na⁺, 250 nM primer concentration — typical for standard PCR.