GC Content Calculator
Enter the counts of A, T, G, C bases in a DNA sequence to compute the GC percentage and an estimated melting temperature (Tm), for primer design and sequence analysis.
Input Data
Results
At a glance:GC content is the percentage of guanine (G) and cytosine (C) in a DNA or RNA sequence: %GC = (G+C) ÷ (A+T+G+C) × 100. Because G–C pairs have three hydrogen bonds while A–T has two, higher GC content makes the duplex more stable and raises the melting temperature (Tm). GC content affects PCR primer design, annealing temperature, and DNA thermal stability, and is also a key genomic trait of species.
Formula
GC content: %GC = (G + C) ÷ (A + T + G + C) × 100.
Basic melting temperature (Marmur–Doty): Tm = 64.9 + 41 × (G + C − 16.4) ÷ total length.
This Tm formula suits longer sequences for rough estimates; for short primers use the Wallace rule.
$$\%GC = \dfrac{G + C}{A + T + G + C} \times 100$$$$T_m = 64.9 + 41 \times \dfrac{(G + C) - 16.4}{N}$$How to Use
- Count the numbers of A, T, G, C bases in the sequence and enter them.
- The tool instantly shows GC%, total length, and a basic Tm estimate.
- For RNA sequences, enter U as T.
Common GC content and duplex stability
| GC content | Duplex stability | Note |
|---|---|---|
| < 40% | Lower | AT-rich, low Tm, easy to denature, primers prone to non-specific binding |
| 40–60% | Moderate | Ideal range for PCR primer design |
| > 60% | Higher | GC-rich, high Tm, PCR often needs DMSO to aid denaturation |
| 50% | Balanced | Half GC and half AT, annealing conditions easier to manage |
PCR primers are generally recommended at 40–60% GC, and avoid consecutive G/C at the 3' end to reduce primer dimers and non-specific amplification.
Case Studies
Primer GC content check
A 26 bp sequence has A6, T6, G7, C7.
GC content = (7 + 7) ÷ 26 × 100 ≈ 53.85%, within the ideal 40–60% range.
Basic Tm = 64.9 + 41 × (14 − 16.4) ÷ 26 ≈ 61.1 °C, a reference for setting the annealing temperature.
Comparing two species' genomic GC
Equal-length fragments: species A A5T5G5C5 (GC 50%), species B A2T3G10C10 (GC 80%).
Species B has markedly higher GC, its DNA is more thermally stable and needs higher temperature to denature.
GC differences often reflect genomic traits and environment (e.g. thermophiles tend to higher GC).
FAQ
Why does higher GC content raise the melting temperature?
G and C pair with three hydrogen bonds, A and T with only two. GC pairs are harder to break, so sequences with higher GC content have more stable duplexes and need higher temperature to denature (melt), giving a higher Tm.
Is this Tm formula accurate?
This calculator uses the Marmur–Doty formula (Tm = 64.9 + 41×(G+C−16.4)/N) for a rough estimate, suitable for longer sequences as a quick reference. Precise primer Tm should use the nearest-neighbor thermodynamic model and consider salt and primer concentration, ideally with a dedicated primer Tm calculator.
How to compute GC content for RNA sequences?
RNA replaces thymine (T) with uracil (U). For GC content, enter U in the T field, because GC content only looks at the G and C proportion; A/U (or A/T) does not affect the GC percentage.
What GC range should PCR primers target?
Generally 40–60%. Too low (AT-rich) gives too low Tm and non-specific binding; too high (GC-rich) gives high Tm and secondary structures. Also avoid 3 or more consecutive G/C at the 3' end to reduce primer dimers and mis-priming.
What is the significance of GC content in genomics?
GC content is a key genomic trait: different species, even different regions of the same genome (e.g. isochores, CpG islands), differ markedly in GC. Thermophilic microbes tend to higher GC for DNA stability; GC content also affects sequencing coverage and PCR amplification efficiency.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.