GC content calculator
Paste a DNA or RNA sequence, plain or in FASTA format, to see its GC content — the share of guanine and cytosine among its bases — along with the length and the count of every base.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Paste the sequence. Upper or lower case, line breaks and numbered GenBank lines are all fine.
- Read GC content and AT content in percent and the length in nucleotides.
- Check the base counts; letters other than A, C, G, T and U are counted under “other letters”.
GC content = (G + C) ÷ (A + C + G + T/U) × 100 %. AT content is the rest of the definite bases: 100 % − GC. The length counts every letter, including ambiguous ones.
Worked example
The sample sequence of 39 bases has A 9, C 8, G 14 and T 8. GC = (14 + 8) ÷ 39 × 100 = 56.41 %, shown as 56.4 %; AT is 43.6 %.
Tips
- With many N letters the percentage describes only the known bases; check the “other letters” count before comparing samples.
- For a whole genome or a large FASTA file the counting still runs in your browser; nothing is uploaded.
- Primer melting temperature depends on GC content and length — see the primer Tm calculator.
Frequently asked questions
How is GC content calculated?
GC % = (G + C) ÷ (A + C + G + T) × 100, with U counted as T for RNA. Ambiguous letters such as N, R or Y are left out of both the top and the bottom, because they may or may not be G or C; they are shown separately as other letters.
Why does GC content matter?
G–C pairs are held by three hydrogen bonds and A–T pairs by two, so GC-rich DNA melts at a higher temperature. GC content is used in PCR primer design (often around 40–60 %), in comparing genomes and in checking sequencing data.
What is ignored in the input?
FASTA header lines starting with > or ;, spaces, line breaks, digits and punctuation such as gap dashes. Every other letter counts toward the length.