Reaction Rate Calculator
Calculate average chemical reaction rate, reactant consumption rate, apparent rate constant k, and half-life from initial and final molarity and Δt.
Open toolDNA Melting Temperature Calculator (Nearest-Neighbor, Salt-Adjusted & Wallace Tm)
Compute PCR primer melting temperature (`Tm`) side-by-side using **SantaLucia (1998) Nearest-Neighbor thermodynamics** (`ΔH°`, `ΔS°`), **Salt-Adjusted Marmur-Doty**, and the **Wallace Rule**, complete with `GC%`, `3' GC clamp` check, and recommended annealing temperature (`Ta`).
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How SantaLucia (1998) Nearest-Neighbor Thermodynamics Works
Two primers with identical length and identical GC% can have melting temperatures that differ by 4 °C to 6 °C because base-pair stability depends on adjacent base stacking (`5'-GC-3'` vs. `5'-CG-3'`), not just individual hydrogen bonds.
Tm_NN (°C) = (1000 * ΔH°) / (ΔS°_1M + 0.368 * (N - 1) * ln([Na+_eq]) + 1.987 * ln(C_T / 4)) - 273.15
Where ΔH° (kcal/mol) and ΔS°_1M (cal/(K·mol)) sum the 10 unified SantaLucia (1998) dinucleotide step parameters plus terminal initiation penalties, R = 1.987 cal/(K·mol), and [Na+_eq] (in mol/L) includes divalent Mg2+ stabilization.
DNA Primer Melting Temperature Formulas & Default 20-nt EGFP Primer Audit
Comparison of the three standard oligonucleotide $T_m$ equations evaluated on the default `20-nt` EGFP forward primer (`5'-ATGGTGAGCAAGGGCGAGGA-3'`, `60.0% GC`, `500 nM` oligo, `50 mM Na+`, `1.5 mM Mg2+`).
| Tm Method | Thermodynamic / Empirical Equation | Default 20-nt EGFP Primer Result | Recommended Sequence Length Scope |
|---|---|---|---|
| **SantaLucia (1998) Nearest-Neighbor (`Tm_NN`)** | `1000·ΔH° / (ΔS°_salt + R·ln(C_T/4)) - 273.15` | **`68.1 °C`** (`ΔH° = -154.6 kcal/mol`, `ΔS° = -410.1 cal/K·mol`) | **Gold standard (`8 to 60 nt`)** — captures sequence-specific dinucleotide stacking |
| **Salt-Adjusted Marmur-Doty (`Tm_Salt`)** | `81.5 + 16.6·log10([Na+_eq]) + 0.41·(%GC) - 675/N` | **`60.6 °C`** (`[Na+_eq] = 197.0 mM` w/ `1.5 mM Mg2+`) | Longer oligonucleotides & probes (`20 to 80 nt`) where base composition averages out |
| **Wallace Rule (`Tm_Wallace`)** | `2 · (A + T) + 4 · (G + C)` | **`64.0 °C`** (`8 A/T` bases + `12 G/C` bases) | Short oligonucleotides only (`14 to 18 nt` at high salt); ignores `[Na+]` and `[Mg2+]` |
| **Recommended PCR Annealing (`Ta`)** | `Ta = Tm_NN - 4.0 °C` (Standard Taq rule) | **`64.1 °C`** (`3' end = GAGGA`, `3/5` GC clamp) | Starting thermal cycler annealing step (`55 °C – 65 °C` typical) |
How to Calculate Primer Melting Temperature & PCR Annealing
Follow three steps to evaluate any forward or reverse primer sequence.
Paste your 5'->3' DNA primer sequence
Enter your DNA sequence using A, T, G, and C bases (5' to 3'). Non-DNA formatting characters like spaces, dashes, or '5'-' prefixes are automatically cleaned.
Set primer concentration (nM) and buffer salt (Na+ & Mg2+)
Enter your PCR primer concentration (500 nM default) and reaction buffer cation levels (50 mM Na+/K+ and 1.5 mM Mg2+ for standard Taq buffer; set Mg2+ to 0 mM for monovalent-only comparison).
Compare Nearest-Neighbor Tm, GC%, and 3' clamp status
Click Calculate Primer Tm to view Nearest-Neighbor Tm, Salt-Adjusted Tm, Wallace Tm, total ΔH°/ΔS°, anhydrous molecular weight (Da), and the 5'<-3' reverse complement.
Worked PCR Primer Examples
Compare how primer sequence and divalent magnesium shift melting temperature.
Default 20-nt EGFP Forward Primer (50 mM Na+, 1.5 mM Mg2+)
For 5'-ATGGTGAGCAAGGGCGAGGA-3' (20 nt, 60.0% GC, 8 A/T, 12 G/C) at 500 nM with 50 mM Na+ and 1.5 mM Mg2+, SantaLucia Nearest-Neighbor Tm is 64.8 °C, Salt-Adjusted Tm is 60.5 °C, and Wallace Tm is 64.0 °C (recommended Ta = 60.8 °C).
Monovalent-Only Buffer Check (50 mM Na+, 0 mM Mg2+)
Setting Mg2+ to 0 mM reduces effective ionic strength from 197.0 mM to 50.0 mM, lowering Nearest-Neighbor Tm from 64.8 °C to 58.6 °C and Salt-Adjusted Tm from 60.5 °C to 50.6 °C.
AT-Rich 20-nt Primer Check
Replacing G/C bases with an AT-rich sequence lowers both the enthalpy magnitude |ΔH°| and Nearest-Neighbor Tm, triggering a low-Tm warning if Tm drops below 52 °C.
How to Interpret the Thermodynamic & Primer Quality Output
Use the Nearest-Neighbor $T_m$ as your primary benchmark for PCR design.
The primary result line reports SantaLucia Nearest-Neighbor Tm (°C), Salt-Adjusted Tm (°C), Wallace Tm (°C), and GC percentage.
The detailed audit lists exact base counts (A, T, G, C), total enthalpy ΔH° (kcal/mol), entropy ΔS° (cal/(K·mol)), single-stranded molecular weight (g/mol), 3'-terminal GC clamp count (number of G/C bases in the last 5 nucleotides), and the exact 5'->3' reverse complement sequence.
High-Fidelity Polymerase (Q5 / Phusion) Buffer Considerations
Note how specialized polymerase buffers differ from standard 50 mM KCl Taq buffers.
Standard Taq, Platinum Taq, and GoTaq buffers are modeled accurately by 50 mM monovalent salt + 1.5 mM Mg2+. High-processivity fusion polymerases (such as NEB Q5 or Phusion) use proprietary high-ionic-strength buffers that stabilize duplexes by an additional +3 °C to +6 °C.
When ordering lyophilized primers, divide nmol yield by 10 to find the microliters of TE buffer needed for a 100 µM master stock.
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Open toolFrequently Asked Questions About DNA Melting Temperature (Tm)
Answers about using DNA Melting Temperature Calculator (Nearest-Neighbor, Salt-Adjusted & Wallace Tm) and understanding its results.
Why do Wallace, Salt-Adjusted, and Nearest-Neighbor (SantaLucia 1998) give different Tm values?
The Wallace Rule (Tm = 2(A+T) + 4(G+C)) is a quick bench approximation valid only for short 14–18 nt oligos at ~0.9 M salt and wildly overestimates Tm for 20+ nt primers in 50 mM PCR buffer. The Salt-Adjusted formula accounts for length N, %GC, and salt concentration, while the SantaLucia (1998) Nearest-Neighbor model sums the exact base-stacking thermodynamics (ΔH° and ΔS°) of every adjacent dinucleotide step along your 5'->3' sequence.
How do sodium (Na+/K+) and magnesium (Mg2+) concentrations affect DNA melting temperature?
Positively charged cations shield the negatively charged phosphate backbones of the two DNA strands, stabilizing the duplex and raising Tm. Divalent Mg2+ ions stabilize DNA duplexes roughly 100-fold more strongly per millimole than monovalent Na+ or K+ ions; increasing Mg2+ from 0 mM to 1.5 mM raises a typical 20-mer primer's Tm by ~5.5 °C to 6.5 °C.
What is the ideal GC content, length, and 3' GC clamp for a PCR primer?
Optimal PCR primers are 18 to 24 nucleotides long with 40% to 60% GC content, a Nearest-Neighbor Tm between 58 °C and 65 °C (with forward and reverse primers matched within 2 °C), and 1 to 2 G or C bases within the final 5 bases at the 3' end (3' GC clamp) to promote specific polymerase extension.
How do I calculate the PCR annealing temperature (Ta) from primer Tm?
For standard Taq polymerase, set the initial PCR annealing temperature (Ta) to 3 °C to 5 °C below the lower Nearest-Neighbor Tm of your forward/reverse primer pair (Ta = Tm_NN - 4 °C).
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