ToolerWork
Biology

GC Content Calculator

Calculate DNA GC percentage, AT percentage and base counts with clear handling of ambiguous bases.

Your sequence

Paste raw letters or one FASTA record

Up to 100,000 bases or residues. Whitespace and a single FASTA header are accepted. Invalid characters are flagged.

Sequences are processed locally and are not uploaded or saved by this tool.

Your result, clearly explained

Paste a sequence or load the example, then run the tool. Your sequence stays in this browser.

How to use

  1. Enter your DNA

    Paste a sequence or one FASTA record.

  2. Calculate composition

    Count G and C alongside the other bases.

  3. Check the denominator

    Review known and ambiguous base counts before using the percentage.

Why ToolerWork?

Private by design

Sequence calculations run inside your browser.

Clear assumptions

Strand direction, code table and approximation limits are explained.

Related Tools

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DNA Sequence Analyzer

Inspect DNA length, nucleotide counts, GC content, GC skew and approximate molecular size.

RNA Sequence Analyzer

Measure RNA length, A/U/G/C composition, GC content and ambiguous nucleotide counts.

Reverse Complement Generator

Generate the reverse complement of DNA, including IUPAC ambiguity codes.

Why use this tool?

  • See the exact counts behind the GC percentage.
  • Keep unknown bases visible instead of counting them as A or T.
  • Export a compact composition report for notes or comparisons.

Method & assumptions

GC% = 100 × (G + C) / (A + C + G + T). Ambiguous IUPAC positions are excluded from this denominator and reported separately. Base-composition bars use the full sequence length. An all-ambiguous sequence has no defined GC percentage.

References: NCBI genetic codes

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Frequently asked questions

How is GC content calculated?

Add the counts of G and C, divide by the number of A, C, G and T bases, then multiply by 100.

Does N count in the GC denominator?

No. N and other ambiguous IUPAC letters are excluded. The report shows both total positions and known bases so you can inspect the difference.

Why does GC content show a dash?

A sequence with no unambiguous A, C, G or T has no denominator. The tool reports an undefined percentage instead of a misleading zero.

Is GC percentage a melting-temperature prediction?

No. GC content is one contributor to duplex stability. Temperature estimates also depend on length, sequence, salt and concentration.

Can I calculate RNA GC content?

Use the RNA Sequence Analyzer, which accepts U instead of T and reports RNA GC content using known RNA bases.