GC Content Calculator
Paste a DNA or RNA sequence to calculate GC content, AT content, sequence length, and base composition. Accepts raw sequence or FASTA, counts IUPAC ambiguity codes separately, and runs entirely in your browser.
Calculated in this page. Your sequence is never uploaded.
| Base | Share | Count | % |
|---|---|---|---|
| G | 79 | 26.3 | |
| C | 88 | 29.3 | |
| A | 64 | 21.3 | |
| T/U | 67 | 22.3 | |
| N* | 2 | 0.7 |
What the calculator counts
GC content is the proportion of a nucleic-acid sequence made up of guanine and cytosine. It is a basic descriptor of a sequence or genome: GC-rich regions melt at higher temperatures, behave differently in PCR, and often mark coding or regulatory features.
The calculator reads your input, discards line breaks, numbers, and FASTA header lines, then counts each letter. Uracil (U) is counted with thymine so RNA and DNA report the same way. Ambiguity codes are counted separately rather than guessed at — by default they are excluded from the GC denominator, which you can change above.
Multi-record FASTA input is summarized as a whole and broken out per record, so a file of amplicons or contigs can be scanned in one pass.
With “exclude ambiguous” switched off, the denominator becomes A + T + G + C + ambiguous, and the reported GC percentage falls accordingly.
| Sequence length | 20 |
| G + C | 7 + 5 = 12 |
| A + T | 4 + 4 = 8 |
| GC content | 12 / 20 × 100 = 60.00 % |
| AT content | 8 / 20 × 100 = 40.00 % |
It depends entirely on the organism and the region. Escherichia coli K-12 sits near 50.8 %, the human genome near 41 %, Plasmodium falciparum near 19 %, and many Streptomyces species above 70 %. Within a genome, coding sequence, isochores, and repeats can each differ by several points, so compare like with like.
- Cornish-Bowden, A. (1985). Nomenclature for incompletely specified bases in nucleic acid sequences. Nucleic Acids Research 13(9), 3021–3030. — the IUPAC ambiguity codes recognized here.
- SantaLucia, J. (1998). A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics. PNAS 95(4), 1460–1465. — why GC content alone is a rough proxy for melting temperature.
- NCBI. FASTA format description, BLAST user documentation. — the header and sequence-line conventions this parser follows.