GC Content Calculator
Find the percentage of G and C bases in a DNA sequence.
AT content 48.3% · length 60 nt (A=15, T=14, G=16, C=15).
GC content is the share of bases in a DNA sequence that are guanine (G) or cytosine (C), written as GC% = (G + C) ÷ (A + T + G + C) × 100. For example, ATGCATGC has 2 A, 2 T, 2 G and 2 C, so its GC content is 4 ÷ 8 × 100 = 50%. The rest, 50%, is AT content.
What GC content measures
GC content (also called the GC ratio) is the proportion of a DNA or RNA sequence made up of the two strong-pairing bases, guanine and cytosine. Because a G always pairs with a C across the double helix, counting one strand is enough — the percentage is the same on both strands. The remaining bases are adenine and thymine, so AT% = 100 − GC%.
The calculator counts each base case-insensitively, ignoring spaces, line breaks, and digits (so you can paste FASTA-style sequences or numbered blocks). GC content matters because G-C pairs are joined by three hydrogen bonds while A-T pairs share only two, so a sequence with more G and C is more thermally stable and melts at a higher temperature.
How the calculation works
Every letter is read once and sorted into A, T, G, C, or “other”. The four canonical bases are summed to give the sequence length; then GC% is the count of G plus C divided by that total, times 100. Ambiguous letters such as N are counted separately and left out of the total so they can’t distort the ratio.
- 1 Count each base. For ATGCATGC: A = 2, T = 2, G = 2, C = 2. Ignore spaces, line breaks, and digits.
- 2 Add up the canonical bases. Total = A + T + G + C = 2 + 2 + 2 + 2 = 8 bases.
- 3 Divide G + C by the total. (G + C) ÷ total = (2 + 2) ÷ 8 = 0.5.
- 4 Multiply by 100 for a percentage. 0.5 × 100 = 50%, so GC content is 50% and AT content is 100 − 50 = 50%.
GC content ranges and what they imply
Higher GC means more three-hydrogen-bond pairs, so higher melting temperature and greater duplex stability. Ranges are typical guides, not hard cut-offs.
| GC content | Character | Implication |
|---|---|---|
| Below 40% | AT-rich | Lower melting temperature; strands separate more easily. Common in AT-rich genomes and regulatory regions. |
| 40–60% | Balanced | Typical of the human genome (≈ 41%) and many primers; predictable, well-behaved melting. |
| 60–70% | GC-rich | Higher melting temperature and duplex stability; may need additives or higher annealing temperatures in PCR. |
| Above 70% | Very GC-rich | Very stable and prone to secondary structure; found in thermophiles and some housekeeping genes. |
Why GC content matters in the lab
Melting temperature and PCR. Because G-C pairs are held by three hydrogen bonds and A-T pairs by only two, GC content is a first-order predictor of a duplex’s melting temperature (Tm). Primers are usually designed for roughly 40–60% GC so their Tm values match; the tool reports a quick Wallace-rule Tm for oligos under 14 nucleotides, while longer sequences need a nearest-neighbour model.
Genome variation. GC content varies widely between organisms — from under 30% in some AT-rich bacteria and parasites to over 70% in certain actinobacteria — and even between regions of a single genome. It correlates with gene density, codon usage, and how easily a region can be amplified or sequenced, which is why GC content is one of the first numbers biologists check for a new sequence.