How is GC content calculated?

GC content is the number of G and C bases divided by the total number of bases, expressed as a percentage.

How are ambiguous bases counted?

Ambiguous bases (N and other IUPAC codes) are counted separately and excluded from the GC and AT totals so the percentages reflect only unambiguous bases.

GC Content & Base Composition Calculator

Calculate GC%, AT% and per-base composition of a sequence.

🔒 Runs entirely in your browser — nothing is uploaded

Calculate the GC content, AT content and full base composition of any DNA or RNA sequence. GC% influences primer melting temperature, PCR conditions and overall sequence stability, so it is one of the first things to check when designing an experiment. Ambiguous bases are counted separately so the percentages stay accurate.

Sequence (DNA or RNA)

Length

0.0%

GC content

0.0%

AT content

Ambiguous

How to use the GC Content tool

1Paste a DNA or RNA sequence, or a FASTA record.
2Read the GC% and AT% along with per-base counts for A, C, G and T/U.
3Use the composition to inform primer design, PCR annealing temperatures or quality checks.

Frequently asked questions

How is GC content calculated?: GC content is the number of G and C bases divided by the total number of bases, expressed as a percentage.
How are ambiguous bases counted?: Ambiguous bases (N and other IUPAC codes) are counted separately and excluded from the GC and AT totals so the percentages reflect only unambiguous bases.

Learn more

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