About this tool
Paste a DNA or RNA sequence to measure its base composition. The sequence is uppercased and reduced to A, C, G, T and U, then each base is counted. Everything is calculated locally in your browser.
GC content is the fraction of bases that are guanine or cytosine: GC% = (G + C) / total × 100. AT content is simply 100 − GC%. GC skew, defined as (G − C) / (G + C), indicates strand bias and is often used to locate replication origins. Base counts and total length are reported alongside these figures.
An optional window size splits the sequence into consecutive segments and lists the local GC% of each, which helps you spot GC-rich or GC-poor regions. For example the sequence GGCC is 100% GC, while a balanced sequence sits near 50%.
Frequently asked questions
How is GC content calculated?
It is the number of G and C bases divided by the total number of bases, expressed as a percentage: GC% = (G + C) / length × 100. AT content is the remaining 100 − GC%.
What is GC skew?
GC skew is (G − C) / (G + C). A value above zero means more guanine than cytosine on that strand; it is commonly used to help identify replication origins and terminals.
What does the window size do?
It divides the sequence into consecutive blocks of that length and reports the GC% of each block, so you can see how GC content varies along the sequence.
Does it work for RNA?
Yes. Uracil (U) is counted together with thymine (T), so RNA and DNA sequences both give correct GC and AT percentages.