Which Tm Calculation Method Is Most Accurate? SantaLucia vs Owczarzy vs Wallace
Trying to understand why NEB, IDT, Primer3, or another calculator gives a different Tm for the same primer? This page compares 5 common methods side by side so you can see which ones stay close, which ones drift, and which method best matches your actual buffer conditions.
5 Methods at a Glance
| Method | Accuracy | Best For | Used By |
|---|---|---|---|
| NN + Owczarzy 2008 | ±0.8°C | PCR with Mg²⁺ buffers | NEB, OligoPool |
| NN + SL98 Salt | ±1.2°C | Na⁺-only buffers | Primer3, early IDT |
| NN + von Ahsen | ±1.5°C | Clinical diagnostics | Roche LightCycler |
| %GC Method | ±3-5°C | Rough estimates | Legacy tools |
| Wallace Rule | ±5-10°C | Quick mental math | None (deprecated) |
Accuracy values from OligoPool Tm Accuracy Benchmark across 50 representative primer sequences under standard PCR conditions (50 mM Na⁺, 250 nM oligo).
Try It: Compare Tm Methods with Your Sequence
Enter your primer sequence below. The tool calculates Tm using all 5 methods simultaneously so you can see exactly how results differ.
Tm Calculation Method Comparison
Enter your sequence and see how 5 different Tm calculation methods produce different results. Understand which method matches NEB, IDT, or Primer3 — and why the differences matter for your experiment.
DNA only (A, T, C, G). Minimum 6 nt.
These conditions affect NN methods. Wallace ignores salt. %GC uses Na⁺ only.
When Methods Disagree: What to Trust
Small disagreement (≤2°C): All nearest-neighbor methods agree. Use any NN-based tool. The minor difference comes from salt correction variations.
Moderate disagreement (2-5°C): The %GC method diverges from NN methods. Trust the nearest-neighbor result. The %GC method ignores stacking interactions and is unreliable for primers with non-random sequence composition.
Large disagreement (>5°C): The Wallace Rule shows significant deviation. This rule (Tm = 2(A+T) + 4(G+C)) was designed for very short oligos (<14 nt) in 1 M NaCl. It should never be used for PCR primer design.