GC content calculator

What is GC content?

GC content is the percentage of guanine (G) and cytosine (C) bases in a nucleic acid sequence. It relates to sequence stability, gene density, and how a sequence behaves in molecular biology experiments.

GC base pairs form three hydrogen bonds compared to two for AT/AU pairs, so GC-rich regions are more thermally stable. This affects DNA melting temperature, PCR primer design, and sequencing success rates.

How is GC content calculated?

GC content is calculated by dividing the sum of guanine and cytosine bases by the total number of nucleotides, then multiplying by 100:

$$GC\% = \frac{G + C}{A + T + G + C} \times 100$$

For RNA sequences, uracil (U) replaces thymine (T) in the denominator:

$$GC\% = \frac{G + C}{A + U + G + C} \times 100$$

Ambiguous bases like N are excluded from both the numerator and denominator to ensure accurate calculations.

How does our GC content calculator work?

The calculator identifies each sequence as DNA, RNA, or Unknown based on nucleotide content:

Input sequences are cleaned before analysis. Whitespace, numbers, and newlines are removed. Ambiguous bases (N and other IUPAC degenerate codes such as R, Y, S, W) are excluded from both the numerator and denominator of every percentage, so GC% and AT% always sum to 100%. Characters that are not valid nucleotide codes are counted separately and reported in a warning.

Paste sequences or upload a file in FASTA format. The calculator accepts single sequences or multi-FASTA files, and .txt, .fasta, .fa, .fas, and .seq extensions. Results are shown per sequence and as a combined total across all sequences.

Understanding the results

The calculator provides nucleotide statistics for each sequence and a combined total:

GC and AT skew

Skew measures the imbalance between complementary bases on a single strand. GC skew is calculated as (G − C) / (G + C) and AT skew as (A − T) / (A + T). Values range from −1 to +1, where 0 means the two bases occur equally. Strand-specific skew shifts near replication origins and termini, so skew is often used to locate these features in bacterial genomes.

Melting temperature

The calculator reports an estimated melting temperature (Tm) for each sequence. Sequences shorter than 14 bases use the Wallace rule (Tm = 2(A+T) + 4(G+C)); longer sequences use the basic GC formula (Tm = 64.9 + 41 × (G+C − 16.4) / N). These are quick approximations and do not account for salt concentration or nearest-neighbor thermodynamics. For precise PCR design, use Primer3 to design primers with salt-corrected melting temperatures.

Sliding-window GC plot

Open any sequence to see GC content plotted along its length using a sliding window. The window size scales with sequence length, and the dashed line marks the overall GC content. This reveals GC-rich and AT-rich regions that a single average would hide, such as promoters, isochores, and CpG islands.

Related tools

To work with the same sequences in other ways, use Reverse complement to generate the complementary strand, DNA to RNA to transcribe a DNA sequence, and RNAfold to predict secondary structure, where GC content drives stability.

Interpreting GC content

GC content varies widely across organisms and genomic regions:

For PCR primers, optimal GC content is 40-60%, with 50-55% preferred for balanced annealing temperatures.

CpG islands

A CpG island is a GC-rich DNA region where the CpG dinucleotide frequency is significantly higher than the surrounding genome. These regions are defined as being at least 200 bp long, with GC content ≥50% and a CpG observed-to-expected ratio >0.6.

CpG islands are often associated with promoter regions and play important roles in gene regulation. The calculator flags any sequence that meets these criteria as a predicted CpG island, using the Gardiner-Garden and Frommer observed-to-expected ratio. This is a whole-sequence test, so a long sequence with a localized island may not be flagged; use the sliding-window plot to spot GC-rich stretches within it.

Frequently asked questions

How much GC content is best?

Optimal GC content depends on the application. Most eukaryotic genomes range from 30-50%, with humans averaging 41%. For PCR primers, 40-60% GC is recommended to ensure proper annealing. Coding regions typically have higher GC content than intergenic sequences. There is no universal ideal, since appropriate GC content varies by organism, genomic region, and experimental context.

What is considered high GC content?

Sequences with GC content above 60% are generally considered high. These GC-rich regions form stronger secondary structures due to three hydrogen bonds per GC pair versus two for AT pairs. High GC content is common in promoter regions, CpG islands, and certain bacterial genomes like Streptomyces (over 70%). Such regions can pose challenges for PCR amplification and DNA sequencing.

Is high or low GC content better?

Neither is inherently better, since each has distinct properties. High GC content increases thermal stability and melting temperature, which helps thermostable primers but complicates sequencing. Low GC (AT-rich) sequences denature more easily and may form fewer secondary structures. The right balance depends on your application: primer design favors 40-60%, while genomic studies must account for natural variation across organisms.