GC content formula
GC content is the fraction of a DNA sequence made up of guanine (G) and cytosine (C) bases: %GC = (G + C) ÷ total bases × 100. Because G pairs with C using three hydrogen bonds, a higher GC content gives DNA a higher melting temperature and greater stability.
Since every base is A, C, G or T, the AT content is always 100 − %GC. That complementarity is why reporting one figure is enough to describe the base composition of a sequence. The result is a share of the total bases, so the percentage calculator does the same arithmetic on any other count.
How to use this calculator
Paste or type a DNA sequence using only the letters A, C, G and T. Spaces are ignored and case does not matter. The tool counts G and C bases, divides by the total length, and reports the percentage along with the AT content. For sequence growth, see the cell doubling calculator.
Because it works on a single strand, the answer is the same for a sequence and its reverse complement — G on one strand is always C on the other, so the combined count never changes. Line breaks copied from a sequence file are treated as whitespace and removed automatically.
Worked example
For GGCCATAT, there are 2 G and 2 C bases out of 8 total. GC content = (2 + 2) ÷ 8 × 100 = 50%, so the AT content is also 50%.
In practice this number guides primer design and PCR conditions. Primers in the 40–60% GC range usually anneal reliably, while GC-rich templates may need higher denaturation temperatures. Comparing G+C against A+T is often clearer as a ratio, which the ratio calculator will reduce for you.
Frequently asked questions
- What is GC content?
- The percentage of bases in a DNA or RNA sequence that are guanine or cytosine. It is a basic measure of sequence composition.
- Why does GC content matter?
- G-C pairs form three hydrogen bonds versus two for A-T, so GC-rich DNA is more thermally stable and has a higher melting temperature.
- Does case or spaces matter?
- No. The calculator converts everything to uppercase and removes spaces, so formatting differences do not change the result.
- What if my sequence has letters other than ACGT?
- The tool only accepts A, C, G and T. Remove any ambiguous bases or non-DNA characters before calculating.
- Can I paste an RNA sequence or a FASTA header?
- Not as-is. RNA uses U in place of T, so replace every U with T before pasting. FASTA headers beginning with ">" must also be deleted, since only the letters A, C, G and T are accepted.