PCR annealing temperature calculator
The standard starting annealing temperature for PCR is 5 °C below the melting temperature of the lower-Tm primer. The more precise Rychlik formula is Ta = 0.3 × Tm(lower primer) + 0.7 × Tm(product) − 14.9, which also accounts for product stability. Primers matched within about 5 °C of each other anneal cleanly at one temperature.
Result
Both formulas use the LOWER primer Tm. Keep the pair matched within ~5 °C; if the Rychlik and rule-of-thumb values disagree, a gradient PCR across that span finds the optimum fastest.
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Formula
Ta = 0.3·Tm(lower primer) + 0.7·Tm(product) − 14.9
Rule of thumb: Ta = Tm(lower primer) − 5 °C
Worked example
Given: Primers with Tm 60 °C and 62 °C; product Tm 85 °C
- 1.Lower primer Tm = 60 °C.
- 2.Rule of thumb: 60 − 5 = 55 °C.
- 3.Rychlik: 0.3 × 60 + 0.7 × 85 − 14.9 = 18 + 59.5 − 14.9 = 62.6 °C.
- 4.Run a gradient between the two estimates if the first attempt smears or fails.
Start at 55 °C (rule of thumb) or 62.6 °C (Rychlik)
How the calculation flows
Units & constants
| Inputs | Forward & reverse primer Tm; product Tm (optional) |
|---|---|
| Primer pair matching | Keep ΔTm within ~5 °C |
| Too low Ta | Mispriming, smears, nonspecific bands |
| Too high Ta | Weak or no product |
| Product Tm estimate | For long products, ≈ 81.5 + 16.6·log[Na⁺] + 0.41·%GC − 675/N |
| Price | Free |
Which primer Tm sets the annealing temperature?
The lower one. Both primers must anneal in the same cycle step, so the less stable primer is the constraint. If the pair differs by more than ~5 °C, the high-Tm primer misprimes at a Ta chosen for the low one — redesign toward matched Tms rather than splitting the difference.
Rule of thumb or Rychlik — which to trust?
Both are starting points, and they usually land within a few degrees of each other. The Tm − 5 rule is robust for routine primers; Rychlik's formula rewards a stable product with a higher, more specific Ta. In practice the fastest resolution is a single gradient PCR spanning both estimates — most thermocyclers run one natively.
When should you deviate from the calculated Ta?
Common special cases:
- Tailed cloning primers: use the Tm of the template-binding region for the first 5 cycles, then step Ta up.
- High-GC templates: additives (DMSO, betaine) lower effective Tm — drop Ta 2–4 °C.
- Fast 2-step protocols (e.g. with hot-start proprietary enzymes): annealing merges with extension at 60–72 °C per vendor guidance.
- Touchdown PCR: start ~5 °C above the calculated Ta and decrease 0.5 °C per cycle to enforce specificity.
Frequently asked questions
- What annealing temperature for primers with a 60 °C Tm?
- Start at 55 °C by the Tm − 5 rule. If the product is stable (high Tm), the Rychlik formula will suggest something higher — often 60–63 °C — which usually improves specificity. A gradient across 55–65 °C settles it in one run.
- Why does NEB recommend a different Ta for the same primers?
- NEB's Tm calculator applies polymerase-buffer-specific salt corrections, and some of its enzymes (notably proprietary hot-start formulations) are validated for annealing well above generic recommendations. Follow the vendor's tool for that enzyme; the formulas here are enzyme-agnostic.
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