PCR Annealing Temperature Calculator
Enter the less-stable primer's melting temperature Tmᵖ and the template Tmᵗ to compute the optimal annealing temperature Ta*, plus a simple Tmᵖ−5 starting reference.
Input Data
Results
At a glance:Enter the less-stable primer's melting temperature Tmᵖ and the template Tmᵗ to compute the optimal annealing temperature Ta*, plus a simple Tmᵖ−5 starting reference.
Formula
Ta* = 0.3×Tmᵖ + 0.7×Tmᵗ − 14.9 (°C).
Simple rule: Ta = Tmᵖ − 5.
$$T_a^{*} = 0.3\,T_m^{p} + 0.7\,T_m^{t} - 14.9$$$$T_a = T_m^{p} - 5$$How to Use
- Enter the less-stable primer's Tmᵖ and the template Tmᵗ.
- The calculator returns the optimal Ta* and the simple Ta.
- Use the result as the center of a gradient PCR to fine-tune.
FAQ
Why use the less-stable (lower-Tm) primer?
Both primers must bind the template at the same temperature. If you set the annealing temperature by the higher-Tm primer, the lower-Tm one may fail to bind, causing one-sided or failed amplification. Taking the lower Tm ensures both primers anneal effectively.
What happens if the annealing temperature is too high or too low?
Too high makes primers hard to bind, lowering yield or giving no product; too low relaxes binding so primers may mismatch to off-target sequences, producing non-specific bands or primer dimers. The ideal Ta balances specificity and yield.
Where do the Tm values come from?
Tm (melting temperature) is usually computed by primer-design software (e.g. Primer3, OligoCalc) or supplied by the synthesis vendor, using a nearest-neighbor thermodynamic model with salt concentration. This calculator uses the Tm values you provide directly.
Is the computed Ta the final condition?
No, it is only a starting point. Because Tm calculations vary with algorithm and buffer, a gradient PCR is recommended: center on the computed Ta and test several degrees above and below (e.g. ±5°C), then pick the temperature with the highest, most specific yield.
Which to use: the simple Tmᵖ − 5 or the weighted formula?
With only the primer Tm, Tmᵖ − 5 gives a quick estimate; if you also know the template Tm, the weighted formula Ta* = 0.3Tmᵖ + 0.7Tmᵗ − 14.9 is usually closer to optimal. Both are empirical; final conditions are always confirmed by gradient PCR.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.