Primer Tm Calculator
Calculate melting temperature using nearest-neighbor thermodynamics with salt and Mg²⁺ correction under standard (50 mM Na⁺) or PCR buffer (50 mM KCl + 1.5 mM Mg²⁺) conditions. Supports IUPAC degenerate bases with Tm range estimation for PCR and qPCR.
Primer Sequence
Buffer Conditions
How Primer Melting Temperature Is Calculated
The melting temperature (Tm) of an oligonucleotide primer is the temperature at which half of the primer molecules are annealed to their complementary strand and half are in single-stranded form. This calculator uses the nearest-neighbor (NN) thermodynamic model, which sums the stacking free energies of each consecutive dinucleotide pair in the sequence.
The NN model accounts for the fact that base-pair stability depends not only on individual bases but on the identity of neighboring bases. For example, a G-C pair flanked by another G-C pair (GC/CG stack) contributes ΔH = −9.8 kcal/mol, while the same pair flanked by A-T (GA/CT stack) contributes only −8.2 kcal/mol. These stacking interactions are the primary determinant of duplex stability and Tm.
The Tm is calculated from the total enthalpy (ΔH) and entropy (ΔS) of duplex formation using the Gibbs equation: Tm = ΔH / (ΔS + R × ln(Ct/4)), where R is the gas constant and Ct is the concentration of the primer strand forming the duplex. This yields Tm at 1 M Na⁺, which is then corrected for the actual salt and Mg²⁺ conditions in the PCR buffer.
Salt and Magnesium Correction
Ionic strength significantly affects DNA duplex stability. Monovalent cations (Na⁺, K⁺) stabilize the duplex by neutralizing the negative charges on the phosphate backbone, raising the Tm. This calculator uses the Owczarzy et al. (2004) equation for monovalent salt correction, which accounts for GC content and provides accuracy within ±0.5°C for typical primer conditions.
Divalent Mg²⁺ ions have an even stronger stabilizing effect. Standard PCR buffers contain 1.5 mM MgCl₂, which typically raises the Tm by 5–8°C compared to monovalent-only conditions. The Owczarzy et al. (2008) correction uses the ratio R = √[Mg²⁺] / [mono⁺] to determine whether monovalent or divalent ions dominate the ionic environment, and applies the appropriate empirical correction accordingly.
Degenerate Primers and Tm Range
Degenerate primers contain IUPAC ambiguity codes (R, Y, S, W, K, M, B, D, H, V, N) that represent mixtures of two or more bases at specific positions. These are commonly used in consensus PCR, for amplifying gene families, or when designing primers from protein sequences.
Because each degenerate position can be resolved to multiple concrete bases, the primer pool contains variants with different Tm values. This calculator computes the Tm range (minimum and maximum) across all possible sequence realizations. For pools with up to 4,096 variants, all are enumerated exactly. For larger pools, a dynamic programming algorithm computes conservative bounds on ΔH and ΔS without full enumeration.
The recommended annealing temperature is based on the minimum Tm (worst-case variant) to ensure all primer species can anneal under the selected conditions.
Primer Concentration and Ct in the Tm Equation
The NN Tm equation includes a concentration term: Tm = ΔH / (ΔS + R × ln(Ct/4)). In PCR, the relevant reaction is each primer binding its target site independently. Therefore Ct is the concentration of that single primer — not the sum of Forward and Reverse.
At the default 250 nM (each), Ct = 250 nM and Ct/4 = 62.5 nM. This matches Primer3 and IDT OligoAnalyzer, where the "oligo concentration" field refers to one primer strand. Summing F+R (Ct = 500 nM) would only be appropriate when modeling primer–primer dimerization, which is a separate calculation. See Tm Calculation Method for the full derivation and concentration conventions for different duplex types.
Self-Dimer and Hairpin Analysis
This calculator reports whether a primer sequence is self-complementary (a complete palindrome), which affects the Tm via the symmetry correction term (ΔS += −1.4 cal/mol·K). Full self-dimer risk scoring and hairpin stability analysis require scanning all possible internal alignments and loop conformations, which is planned for a future version.
For reference, the Tm calculations for different duplex types use different Ct conventions: primer–target uses the primer concentration directly; self-dimer uses Ct with the symmetry correction; cross-dimer (F↔R) uses (CF + CR)/4. See Tm Calculation Method for details.
ℹ️ Frequently Asked Questions
What is the nearest-neighbor method for Tm calculation?
The nearest-neighbor (NN) method calculates Tm from the thermodynamic stacking parameters of each consecutive dinucleotide pair. It is more accurate than simple formulas (like Wallace or basic %GC methods) because it accounts for sequence-dependent stacking interactions. This calculator uses the SantaLucia (1998) unified parameter set, which is the same set used by Primer3 and most primer design tools.
Why is my Tm different from other tools?
Tm values vary between tools due to differences in NN parameter sets, salt correction models, and primer concentration conventions. Common sources of discrepancy include: Ct/4 vs Ct (1–2°C shift), different salt corrections (Owczarzy vs SantaLucia 1998 vs von Ahsen), and whether Mg²⁺ is included. To compare accurately, match all conditions: primer concentration, Na⁺, Mg²⁺, and method.
What annealing temperature should I use for PCR?
A common starting point is Tm − 5°C to Tm − 3°C for standard PCR and Tm − 3°C to Tm − 2°C for qPCR. Use the PCR/qPCR toggle in the results to switch between these recommendations. For primer pairs, use the lower Tm of the two primers as the basis for annealing temperature. Gradient PCR can help optimize empirically.
How do degenerate bases affect the Tm?
Each degenerate position creates multiple sequence variants in the primer pool. This calculator computes the Tm of the lowest-Tm variant (worst case) and the highest-Tm variant to give a range. The annealing temperature recommendation is based on the minimum Tm to ensure all variants can bind. Enable Advanced Details to see the full range and variant count.
What is the Wallace rule (2+4 formula)?
The Wallace rule estimates Tm as 2°C × (A+T) + 4°C × (G+C). It is a rough approximation useful only for short oligonucleotides (14–20 nt) under standard salt conditions and does not account for sequence context, salt concentration, or primer concentration. The nearest-neighbor method is recommended for accurate Tm prediction.
Why does this calculator use the primer concentration directly, not F+R combined?
In PCR, Tm describes the primer–target binding equilibrium — each primer anneals to its own target site independently. The Ct in the Tm equation refers to the concentration of the specific duplex being formed, so only that primer's concentration is used. Summing Forward + Reverse would only apply to primer–primer dimer calculations. See Calculation Method for details.
Does this calculator check for self-dimers or hairpins?
Yes. The calculator evaluates self-dimer and hairpin stability using ΔG°37 from nearest-neighbor thermodynamics and reports a risk level (Low / Moderate / High). The analysis scans for perfect contiguous complementary matches (≥ 3 bp) — internal mismatches, bulges, and internal loops are not scored in the current version. Cross-dimer (F↔R) analysis requires dual primer input and is planned for a future release.