GC Content Calculator
Calculate the GC content and AT content of a DNA sequence.
Only A, T, G, and C bases are counted (case-insensitive). Spaces, numbers, and other letters are ignored.
GC Content
AT Content
Sequence Length
GC Count (G + C)
AT Count (A + T)
A / T
G / C
What Is GC Content?
GC content (or GC-content) is the percentage of bases in a DNA or RNA molecule that are either guanine (G) or cytosine (C). It is calculated as the number of G and C bases divided by the total number of bases, multiplied by 100. The remaining percentage is the AT content - the fraction of adenine (A) and thymine (T) bases.
For example, the sequence ATGC has one of each base, so its GC content is 50%. A sequence like GGCC is made entirely of G and C, giving a GC content of 100%.
Why GC Content Matters
DNA Stability
G and C pair with three hydrogen bonds, while A and T pair with only two. DNA regions with a higher GC content are therefore more thermally stable and more resistant to denaturation.
Melting Temperature
Because GC pairs are stronger, higher GC content raises the melting temperature (Tm) of a DNA duplex. This is critical when designing PCR primers and probes, where Tm must be tuned for reliable annealing.
Organism Classification
GC content varies between species and even across genomic regions. It is used as a taxonomic and phylogenetic marker - for instance, bacterial genomes can range from roughly 20% to 75% GC, which helps distinguish organisms.
GC vs AT Base Pairing
In double-stranded DNA, bases pair according to Chargaff's rules: guanine always pairs with cytosine, and adenine always pairs with thymine. The two pairing types differ in strength:
- G ≡ C: three hydrogen bonds - a stronger, more stable pairing.
- A = T: two hydrogen bonds - a weaker, more easily separated pairing.
Because of this complementarity, GC content and AT content always add up to 100% of the valid bases counted in a sequence.
Related Calculators
Educational Disclaimer: This GC content calculator is intended for educational and informational purposes. It counts only the standard DNA bases A, T, G, and C and ignores ambiguous or non-standard characters. For research, diagnostic, or clinical work, validate results with established bioinformatics tools and consult qualified professionals.