PCR Annealing Temperature Calculator
Choosing the correct PCR annealing temperature is essential for specific and efficient amplification. Too high and primers cannot bind; too low and non-specific products appear. This calculator uses two established methods: the simple rule Ta = Tm(lower) - 5 degrees Celsius, and the Rychlik et al. (1990) formula: Ta = 0.3 * Tm(forward) + 0.7 * Tm(reverse) - 14.9. Enter the melting temperatures (Tm) of your forward and reverse primers. Use the DNA Melting Temperature Calculator on this site to estimate Tm from sequence if needed.
Annealing temperature formulas
Ta (simple) = Tm(lower) - 5 °C
Ta (Rychlik) = 0.3 * Tm(forward) + 0.7 * Tm(reverse) - 14.9 °C
The simple rule (Tm - 5) is widely used and reliable for most applications. The Rychlik formula (Rychlik et al., 1990, Nucleic Acids Research) is slightly more accurate when the two primer Tm values differ significantly. Always validate empirically with a gradient PCR for critical experiments.
PCR temperature cycling guide
- Initial denaturation: 95 degrees Celsius for 3 to 5 minutes to fully denature template.
- Denaturation step: 95 degrees Celsius for 30 seconds per cycle.
- Annealing step: calculated Ta for 30 seconds. Start at the recommended value and optimise.
- Extension step: 72 degrees Celsius; allow 1 minute per kilobase for Taq, 30 s/kb for Phusion.
- Final extension: 72 degrees Celsius for 5 to 10 minutes to complete all products.
- Hold: 4 degrees Celsius indefinitely until samples are retrieved.
PCR annealing temperature: frequently asked questions
What is the PCR annealing temperature?
The annealing temperature (Ta) is the temperature at which primers bind to the template DNA during a PCR cycle. It is typically set 5 degrees Celsius below the lower of the two primer melting temperatures.
How is annealing temperature calculated from Tm?
A common rule is Ta = Tm(lower) - 5 degrees Celsius. Some protocols use Ta = 0.3 * Tm(forward) + 0.7 * Tm(reverse) - 14.9, which accounts for asymmetric primer Tm values. This calculator offers both methods.
What happens if the annealing temperature is too high?
If Ta is too high, primers cannot bind efficiently to the template, resulting in little or no PCR product. Gradually lower the temperature in 1 to 2 degree steps if you get no band on a gel.
What happens if the annealing temperature is too low?
If Ta is too low, primers may bind to non-specific sites on the template, producing multiple non-specific bands or a smear on an agarose gel. Raising Ta by 1 to 2 degrees can improve specificity.
What extension temperature and time should I use?
Most thermostable polymerases (Taq, Phusion) extend at 72 degrees Celsius. Use approximately 1 minute per kilobase of target for Taq or 30 seconds per kilobase for Phusion or Q5.
Official sources
- Rychlik W., Spencer W.J., Rhoads R.E. (1990). Optimization of the annealing temperature for DNA amplification in vitro: Nucleic Acids Research, PubMed Central.
- NCBI PCR primer design guidelines: NCBI Primer-BLAST.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.