GC content is the percentage of nucleotides in a DNA or RNA sequence that are either guanine (G) or cytosine (C). It is calculated as (G + C) / total bases × 100. GC pairs form three hydrogen bonds versus two for AT/AU pairs, making GC-rich sequences more thermally stable.

Does the tool accept FASTA format?

Yes. Any line beginning with ">" is treated as a FASTA header and ignored. All remaining lines are concatenated and analyzed, so you can paste multi-line FASTA sequences directly.

How are ambiguous IUPAC bases handled?

Bases outside the core set (A, T, U, G, C) — such as R, Y, S, W, K, M, B, D, H, V, N — are counted and shown in the breakdown table under "Ambiguous (IUPAC)". They are excluded from the GC% denominator so the percentage reflects only unambiguous bases.

What is a typical GC content for a gene?

Human genes average around 41–43% GC. GC content varies widely: AT-rich organisms like Plasmodium falciparum average ~19%, while some Streptomyces bacteria exceed 72%. Primers are typically designed to be 40–60% GC for optimal amplification.

Can I analyze RNA sequences?

Yes. Switch to RNA mode and any T in the input is treated as U. The GC% formula remains the same: (G + C) / (A + U + G + C) × 100.

Is my sequence data sent anywhere?

No. All calculation happens entirely in your browser using JavaScript. Your sequence never leaves your device.

DNA/RNA GC Content Calculator

Paste a DNA or RNA sequence (or FASTA) and instantly get GC%, AT%, and per-base counts with a visual composition bar.

Enter Nucleotide Sequence

Mode:

0 bp

Paste a nucleotide sequence on the left to calculate GC content.

GC Content 0.0% G + C bases

AT Content 0.0% A + T bases

Length 0 bp

Total Bases 0 unambiguous

A T G & C

A T G C

Base Composition Breakdown

Base	Count	% of Total	Bar

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Summary

The GC Content Calculator computes the proportion of guanine (G) and cytosine (C) bases in any DNA or RNA sequence. It strips FASTA headers, whitespace, and digits, then counts every IUPAC nucleotide and reports GC%, AT/AU%, sequence length, and an interactive per-base breakdown table — all in your browser with no data sent to any server.

How it works

Paste a raw sequence or FASTA-format text into the input box.
FASTA header lines starting with ">" are automatically removed.
Whitespace, digits, and dashes are stripped; the sequence is uppercased.
G, C, A, T/U bases are counted; ambiguous IUPAC codes are tallied separately.
GC% = (G + C) / (total unambiguous bases) × 100.
Results update live as you type, with a visual composition bar and per-base table.

Use cases

Estimate PCR primer melting temperature — higher GC content raises Tm.
Assess cloning insert stability before vector ligation.
Check genome or gene GC bias for phylogenetic comparisons.
Validate synthesized oligonucleotides against specification sheets.
Screen promoter regions for GC-rich regulatory motifs.
Quickly characterize unknown sequences pasted from NCBI BLAST results.
Identify AT-rich regions prone to melting during stringent hybridization.
Compare GC content across multiple amplicons in a single session.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu