GC Content Calculator

GC content is the percentage of nucleotides in a DNA or RNA sequence that are either guanine (G) or cytosine (C). Because guanine pairs with cytosine through three hydrogen bonds while adenine pairs with thymine through only two, the GC content of a sequence directly shapes its thermal stability, melting behaviour, and many downstream molecular biology properties. The GC content calculator on this page reads a raw DNA or RNA sequence, counts each base, and reports the guanine-cytosine percentage instantly.

This GC content calculator cleans your input first. It converts every character to uppercase and strips anything that is not one of A, T, G, C, or U, so spaces, line breaks, numbers, and FASTA-style headers are ignored automatically. The remaining valid bases form the sequence length used in every calculation. Because the tool accepts U (uracil), it works equally well for RNA transcripts and DNA templates, making it a flexible nucleotide composition calculator for genomics, primer design, and sequence-analysis workflows.

Typical genomes span a wide GC range. Human DNA averages roughly 41% GC, while organisms such as Streptomyces can exceed 70% and the malaria parasite Plasmodium falciparum falls below 20%. Knowing the GC content of a region helps you predict primer annealing temperatures, anticipate PCR difficulty, interpret codon usage bias, and identify functional features such as CpG islands. This calculator turns those questions into a single number plus a full nucleotide breakdown.

The core of the GC content calculator is a simple ratio. After the sequence is cleaned, the tool counts guanine and cytosine bases, adds them together, divides by the total number of valid nucleotides, and multiplies by 100 to express the result as a percentage. The matching AT content is the percentage of adenine plus thymine (or uracil), and the two values always add up to 100% for a sequence made only of standard bases.

The page also computes GC skew, which measures the strand asymmetry between guanine and cytosine. A positive GC skew means guanine outnumbers cytosine on the analysed strand, a value often associated with the leading strand of replication; a negative skew indicates cytosine enrichment. The same logic produces the AT skew from adenine and thymine counts. These skew metrics range from -1 to +1 and are widely used to locate replication origins and terminators in bacterial genomes.

Paste or type your sequence into the DNA/RNA Sequence box. You can include spaces and line breaks freely because the GC content calculator discards every non-base character before counting. Then set the sliding window size, which controls how the tool scans the sequence for locally GC-rich or AT-rich regions. The default window is 10 bases, and you can choose any size from 5 to 100.

As soon as you enter a sequence, the results panel updates with the overall GC content, a classification label, the count and percentage of each base, the AT content, both skew values, and a sliding-window summary. The quick-fill buttons load sample sequences so you can see how a balanced, GC-rich, or AT-rich sequence changes the output.

Reading the output

• GC Content - the headline percentage and its classification.
• Nucleotide Composition - counts and percentages for A, T/U, G, and C.
• Nucleotide Skew - GC skew and AT skew, each shown to four decimal places.
• Sliding Window Analysis - the maximum and minimum window GC%, plus how many windows are GC-rich (at least 60%) or AT-rich (at most 40%).

The GC content calculator labels each sequence so you can interpret the percentage at a glance. The thresholds used by the tool are shown below. Sequences below 30% GC are flagged as AT-rich, those above 70% as GC-rich, those between 40% and 60% as balanced, and everything else as moderate.

The skew readouts add directional information. A GC skew above zero (G greater than C) is highlighted as guanine-enriched, while a value below zero marks cytosine enrichment; a skew of exactly zero means the strand is balanced. AT skew works the same way for adenine versus thymine. Researchers plot cumulative GC skew across a genome to pinpoint the origin and terminus of replication, where the skew sign flips.

GC content influences nearly every step of working with nucleic acids. Because G-C base pairs are held by three hydrogen bonds versus two for A-T pairs, higher GC content raises the melting temperature of a duplex. This is why primer design tools weight GC content heavily and why GC-rich templates often need higher denaturation temperatures, DMSO, or specialised polymerases to amplify cleanly.

In genomics, GC content varies systematically across chromosomes, forming long isochores in vertebrate genomes that correlate with gene density and recombination rate. Promoters and CpG islands are characteristically GC-rich, so scanning a sequence with this nucleotide composition calculator can hint at regulatory regions. In molecular cloning, knowing the GC content of restriction sites and synthetic constructs helps you predict secondary structure, codon optimisation outcomes, and oligo synthesis difficulty.

The sliding window feature makes the GC content calculator useful beyond a single average. By scanning fixed-length windows along the sequence, you can spot local GC-rich stretches that may form stable hairpins or AT-rich stretches that melt early. This local view complements the global percentage and helps explain mapping artefacts, sequencing coverage dips, and primer-binding problems in awkward regions.