PCR Primer Tm Calculator (Wallace / Nearest-Neighbor)

PCR Primer Tm Calculator (Wallace / Nearest-Neighbor) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Education
Input types: text, select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Calculate a PCR primer’s melting temperature (Tm) from a 5–60 nucleotide DNA sequence using the Wallace rule or the SantaLucia 1998 nearest-neighbor model. The tool also reports GC content and a suggested annealing-temperature starting point of Tm − 5°C.

When to use

Estimate Tm during initial PCR primer screening.
Compare the quick Wallace estimate with the nearest-neighbor result.
Check GC content and account for primer and sodium concentrations with the nearest-neighbor method.

How it works

1Enter a DNA primer sequence in the 5′→3′ direction using only A, C, G, and T, with 5–60 nucleotides.
2Choose Wallace rule for a quick estimate or SantaLucia nearest-neighbor for a thermodynamic calculation.
3For nearest-neighbor calculations, enter the primer concentration and sodium concentration of the reaction buffer.
4Review the selected Tm, the other reported Tm value, GC content, sequence length, and suggested Ta start.

Use cases

Screening candidate forward and reverse PCR primers before laboratory testing.
Comparing a simple Wallace estimate with a concentration- and salt-adjusted nearest-neighbor Tm.
Reviewing primer GC content and selecting an initial annealing-temperature range.

Examples

1. M13 forward primer with closely matching Tm estimates

Molecular biology student

Background

A student is checking the classic M13 forward primer before a PCR setup and wants both a quick estimate and a nearest-neighbor calculation.

Problem

Determine whether the Wallace and nearest-neighbor methods give similar values for this 17 nt primer.

How to use

Enter `GTAAAACGACGGCCAGT`, select nearest-neighbor, and use 500 nM primer concentration with 50 mM sodium concentration.

Method: nearest-neighbor; primer concentration: 500 nM; sodium concentration: 50 mM; decimal places: 1.

Outcome

The calculator reports Wallace Tm 52°C, nearest-neighbor Tm 52.8°C, GC content 52.94%, and a suggested Ta start of about 47.8°C.

2. Alternating oligo showing a Wallace and nearest-neighbor difference

PCR assay developer

Background

A developer is comparing methods for the 12 nt sequence `ACGTACGTACGT`, which has 50% GC content and self-complementary symmetry.

Problem

See how much the simple Wallace estimate differs from the thermodynamic nearest-neighbor result.

FAQ

What sequence format does the calculator accept?

Enter a 5–60 nt DNA sequence in the 5′→3′ direction using only A, C, G, and T. Input is case-insensitive.

Which Tm methods are available?

The calculator supports Wallace’s 2°C·AT + 4°C·GC rule and the SantaLucia 1998 unified nearest-neighbor model.

Which method is better for primer design?

Nearest-neighbor is the standard choice for primer design because it considers sequence stacking, terminal initiation, concentration, and salt.

Do primer and sodium concentrations affect both methods?

No. These concentrations are used by the nearest-neighbor method only. Wallace uses base counts without concentration or salt correction.

What does the suggested Ta value mean?

It is a suggested annealing-temperature starting point calculated as the selected Tm minus 5°C. Use it for initial screening, not as a final PCR setting.

Method: Wallace; primer concentration: 250 nM; sodium concentration: 100 mM; decimal places: 1.

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. M13 forward primer with closely matching Tm estimates

2. Alternating oligo showing a Wallace and nearest-neighbor difference

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