🧬GC Content Calculator
Paste a DNA/RNA sequence or FASTA file to get GC%, base composition, sliding-window analysis, and Tm.
Free GC Content Calculator
Our free GC content calculator analyzes any DNA or RNA sequence instantly. Paste in raw text or a full FASTA file, and get GC%, AT/AU%, total length, and exact base counts in one view — no manual counting required.
For deeper analysis, use the sliding-window mode to see how GC content varies across the length of your sequence, plotted as a simple chart — handy for spotting GC-rich or GC-poor regions. The tool also estimates melting temperature using the Wallace rule for short primers or a GC%-based formula for longer sequences, with the formula shown so you know exactly how it was calculated.
Multi-sequence FASTA files are supported too, with per-sequence stats summarized in a table. Free, fast, and fully private — your sequence never leaves your browser.
About the GC Content Calculator
Features
FASTA support
Header lines are stripped automatically; multiple sequences get per-sequence stats.
Sliding-window analysis
A configurable window/step chart reveals GC-rich or GC-poor regions.
Melting temperature
Wallace rule for short sequences, GC%-based formula for longer ones.
Validation
Ambiguous bases are counted separately; empty input is blocked.
Frequently Asked Questions
How does the GC content calculator work?
Paste in a DNA or RNA sequence (or FASTA file), and it instantly calculates the percentage of G and C bases along with full base composition.
What is GC content and why does it matter?
It's the proportion of guanine and cytosine bases in a sequence; higher GC content generally means higher thermal stability, which matters for primer design and genome analysis.
Can I paste a FASTA file directly?
Yes, header lines starting with > are automatically stripped, and multiple sequences in one file are analyzed separately.
Does it work with RNA sequences too?
Yes, it auto-detects RNA (uracil) vs DNA (thymine) and calculates GC content accordingly.
What is sliding window analysis?
It calculates GC content across a moving window of the sequence, revealing regions with unusually high or low GC content that a single overall percentage would hide.
How is melting temperature calculated?
Short sequences use the Wallace rule (2°C per A/T, 4°C per G/C); longer sequences use a GC%-based approximation formula, both shown alongside the result.
What if my sequence has ambiguous bases like N?
They're counted and reported separately rather than being silently included in the GC percentage.
Is there a sequence length limit?
No practical limit for typical use; very large genome files may be slow since everything runs in your browser.
Is my sequence data uploaded anywhere?
No; all calculations happen locally in your browser.
Is this tool free?
Completely free, no sign-up or downloads.