Annealing Temperature Calculator

Start with the lower primer Tm

The default calculation is starting annealing temperature = lower primer Tm − 5°C. Primer Tm describes melting of a primer–template duplex under specified conditions. Annealing temperature is a setting in your PCR program. A calculated Tm helps select that setting; it does not establish the experimental optimum.

For example, select Enter known primer Tm values and enter 60°C and 62°C. The lower value is 60°C, so the default −5°C offset gives 55°C. The primer Tm difference is 2°C. This calculator suggests five screening points: 53, 54, 55, 56 and 57°C. That ±2°C range is a planning choice, not a statistical confidence interval. Follow your enzyme protocol when deciding the actual gradient width.

IDT describes about 5°C below primer Tm as a general starting rule. Too low an annealing temperature can permit unwanted binding; too high can reduce amplification. Compare the resulting products rather than choosing a temperature solely because it gives the largest calculated number.

What the sequence estimate includes

Enter each primer in its own 5′ → 3′ direction, including only the part that binds the template during the initial cycles. Do not reverse-complement the reverse primer before entering it. Remove nonbinding tails and sequence labels. The tool accepts 14–60 unmodified DNA bases per primer and ignores whitespace and letter case. It rejects ambiguous bases such as N and RNA bases such as U.

The primer Tm calculator uses Allawi–SantaLucia DNA nearest-neighbor parameters with a SantaLucia entropy correction for sodium. Neighboring base pairs contribute differently, so equal lengths and GC percentages do not guarantee equal Tm values. Both terminal base pairs also contribute. The Biopython melting-temperature documentation describes these model choices and the concentration convention used here.

For each primer, the calculation is:

Here ΔH is in kcal/mol, ΔS is in cal/(mol·K), N is the number of bases, sodium is in mol/L, and Cₚ is the final concentration of that primer in mol/L. The controls convert mM sodium and nM primer concentration to mol/L. The formula assumes primer is in large excess over template; it uses Cₚ directly rather than dividing a total oligo concentration by four.

The sequence result is a sodium-only reference estimate. It does not model magnesium, dNTP binding, potassium, Tris, DMSO or other additives. A 50 mM sodium input does not describe a complete PCR reaction buffer. Use known Tm mode when you have values from a model suited to your actual mix. Hairpins, primer dimers, mismatches and target specificity require separate checks. The example sequences are arithmetic demonstrations, not a recommended primer pair.

Phusion, Q5 and other polymerase-specific calculations

A Phusion Tm calculator or Q5 annealing temperature calculator can use model assumptions and enzyme instructions that differ from a generic primer estimate. Do not transfer an offset between models without checking the protocol. NEB’s Phusion PCR protocol directs users to its Tm calculator. Thermo Fisher’s guidance on Tm and annealing also gives enzyme-specific instructions.

For Phusion or Q5, start with the supplier’s calculator and protocol. For DreamTaq, GoTaq, KAPA or other mixes, consult the instructions for that exact product. This page is an independent calculator and does not reproduce NEB, IDT, Thermo Fisher or other supplier models. If the supplier already reports an annealing temperature, use that recommendation directly; do not enter it as Tm and subtract another 5°C.

When the two primers disagree

A difference above 5°C triggers a screening warning here. It is a flag to inspect the pair, rather than a universal failure threshold. First confirm that both Tm values use the same model, salt conditions and concentrations. If they do, consider whether primer redesign would make the pair easier to optimize. Averaging two widely separated Tm values can hide the less stable primer’s constraint.

The calculator also flags starting temperatures outside 40–75°C, while keeping the arithmetic result visible. These thresholds help draw attention to unusual settings; they do not certify that a temperature inside that range will work. No result is silently clipped to an instrument’s range.

This tool calculates primer Tm and a PCR starting annealing temperature. It does not calculate PCR product melting temperature, annealing duration or probe-based qPCR assay performance. Those require different inputs and experimental considerations.