Tm Calculator

How to Calculate DNA Melting Temperature (Tm)

The Tm calculator on this page computes DNA melting temperature, GC content, and AT content from a primer sequence using the Wallace rule. The melting temperature (Tm) of a DNA duplex is the temperature at which half the DNA strands are in single-stranded form — the Wallace rule is the simplest and most widely used formula for short oligonucleotides:

Tm = 2(A + T) + 4(G + C)

Each A-T base pair contributes 2°C (two hydrogen bonds) and each G-C pair contributes 4°C (three hydrogen bonds). This formula is valid for oligonucleotides up to about 13–14 bp in near-1 M NaCl conditions.

Example Calculation

For the primer ATCGATCGATCG (12 bp): 4 A, 4 T, 2 G, 2 C. Tm = 2(4+4) + 4(2+2) = 2(8) + 4(4) = 16 + 16 = 32°C. This would be a short, low-Tm primer — too short for most PCR applications.

GC Content and Its Importance in PCR

GC content is the percentage of G and C bases in a primer. It directly affects Tm, secondary structure risk, and primer performance:

• 40–60% GC — optimal range for PCR primers
• <40% GC — low Tm, possible non-specific binding, longer primers may be needed
• >65% GC — very high Tm, risk of secondary structure, GC-clamping, hairpin formation

A "GC clamp" — one or two G/C bases at the 3' end of a primer — improves binding stability at the crucial 3' end where DNA synthesis initiates.

Salt-Adjusted Tm

The Wallace rule assumes 1 M NaCl. Real PCR buffers contain 50–100 mM KCl (roughly equivalent to Na⁺). To correct for actual salt concentration:

Tm (adjusted) = Tm (Wallace) + 16.6 × log₁₀([Na⁺])

At 50 mM Na⁺: correction = 16.6 × log₁₀(0.05) = 16.6 × (−1.301) ≈ −21.6°C. This means the Wallace rule overestimates Tm for typical PCR conditions by about 20°C. This is why manufacturers specify annealing temperatures much lower than Wallace-rule Tm values.

When to Use the NEB Tm Calculator

The Wallace rule is a quick approximation. For accurate nearest-neighbor Tm calculations (better for oligos 14–60 bp), use the NEB Tm calculator or the SantaLucia 1998 nearest-neighbor parameters. This calculator is designed for quick benchwork estimates — especially useful for oligonucleotide ordering, primer pair matching, and understanding relative primer strengths.

Applying Tm to PCR Setup

In PCR, the annealing step temperature determines primer specificity:

• Set annealing temperature 3–5°C below the lower primer's Tm
• Both forward and reverse primers should have similar Tm (within 5°C)
• Use a gradient PCR first if you're unsure of optimal annealing temp
• Touchdown PCR (decreasing temperature gradient over cycles) improves specificity for difficult templates

For solution preparation in molecular biology labs, also see our dilution calculator for making working concentrations from stock solutions. For experiments that also require analyzing inheritance patterns of the genes your primers target, our Punnett square calculator predicts genotype and phenotype ratios for monohybrid and dihybrid crosses.

RNA Sequence Support

This calculator accepts RNA sequences (A, U, G, C) by automatically converting U (uracil) to T (thymine) before applying the Wallace rule. This is useful for calculating hybridization temperatures for siRNA, microRNA, or other RNA oligos. The resulting Tm reflects the RNA:DNA hybrid duplex stability approximation.