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Biology Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

Codon Usage Bias Calculator

You can analyze codon usage bias in a coding sequence, with codon counts and frequencies, per-amino-acid RSCU, GC1/GC2/GC3, Wright's effective number of codons and separate terminal-stop reporting.

Runs in your browserFree · No sign-up · No watermark
CDS → CODON USAGERuns in your browser
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Everything runs locally in this tab. No file is uploaded while the tool loads.

This tool runs in your tab. Nothing is uploaded and no copy is stored.

How files are handled

At a glance

Works with
CDS → CODON USAGE
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Biology Tools

About Codon Usage Bias Calculator

Codon usage describes how often each synonymous codon appears in a coding sequence. Because several codons can encode the same amino acid, a gene can show clear preferences, and those preferences are shaped by mutation, selection and translation efficiency. Measuring them is a useful first step when comparing genes or designing a sequence for a chosen host.

This calculator reads a coding DNA or RNA sequence and reports codon counts and frequencies, per-family RSCU, GC1/GC2/GC3 and Wright's effective number of codons. A terminal stop is reported separately, ambiguous codons are excluded with a warning, and the result is explicitly a descriptive analysis of one sequence.

What this tool does

The details that matter when you use Codon Usage Bias Calculator for real work.

  • Codon counts and frequencies

    Every sense codon observed in the sequence is listed with its count, share of analyzed codons and synonymous family.

  • RSCU by amino acid family

    Relative synonymous codon usage shows which codons are over- or under-represented within each family, with 1 meaning equal usage.

  • GC1, GC2 and GC3

    GC content is reported at each codon position, separating silent-site composition from the first and second positions.

  • Wright's effective number of codons

    ENC summarises overall bias on a 2 to 61 scale, with a short interpretation and a note on the limitations for short sequences.

  • Stop codons handled separately

    A terminal stop is reported and excluded from the synonymous analysis, and internal stops are flagged as unexpected.

  • Strict sequence validation

    Length must be a multiple of three, RNA is converted to DNA, and invalid letters or ambiguous codons are reported rather than silently included.

How to use Codon Usage Bias Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Paste the coding sequence

    Use raw DNA or RNA, or a FASTA record; headers, spaces and punctuation are removed automatically.

  2. 02

    Check that the length is a multiple of three

    The sequence is read from the first base in triplets, so a partial codon produces a clear error instead of a shifted analysis.

  3. 03

    Read the codon summary

    The table gives each observed codon's count, frequency and RSCU within its amino acid family.

  4. 04

    Compare GC positions and ENC

    Use GC3 to see silent-site composition and ENC to judge how strongly codons are biased across the sequence.

  5. 05

    Check the warnings

    Ambiguous codons, internal stops and converted RNA letters are reported so you can judge whether the sequence is suitable.

How this tool works

Works with
CDS → CODON USAGE
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Describes a single sequence only; it does not compare against reference codon-usage tables, test for selection or infer phylogeny.
  • ENC and RSCU are sensitive to sequence length, and amino acid families with few observations give unstable values.
  • Ambiguous codons and stop codons are excluded, so a sequence with many N bases yields statistics over a smaller set of codons.

Privacy and file handling

The coding sequence you paste stays in this browser tab. Parsing, translation and all codon statistics run with JavaScript on your device, so nothing is uploaded, stored or logged, and closing the page discards the input.

Read the full privacy policy

Codon Usage Bias Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
What does RSCU mean in codon usage analysis?
RSCU is the relative synonymous codon usage: the observed count of a codon divided by the count expected if every codon in its amino acid family were used equally. A value of 1 means equal usage, above 1 indicates a preferred codon and below 1 indicates an underrepresented one.
How is the effective number of codons (ENC) calculated?
ENC is derived from Wright's homozygosity statistic for each synonymous family: F = (n Σp² − 1)/(n − 1), combined as ENC = 2 + 9/F̄₂ + 1/F̄₃ + 5/F̄₄ + 3/F̄₆. Values near 61 mean little bias, while values closer to 2 mean strong bias.
Why must the coding sequence length be a multiple of three?
Codons are read in non-overlapping triplets from the first base, so a sequence with one or two extra bases has a partial codon at the end. The calculator rejects that input so the counts are not shifted or based on an incomplete codon.
Are stop codons included in codon usage statistics?
No. Stops are not translated into amino acids, so they are excluded from the synonymous families, RSCU and GC calculations. A terminal stop is reported separately, and internal stops are flagged because they are unusual in a coding sequence.
What happens to codons containing N or another ambiguity code?
They cannot be assigned to a specific codon, so they are excluded from every statistic and counted in a warning. That keeps the frequencies and RSCU values based on known codons, though it also means ambiguity reduces the amount of usable sequence.
Can ENC be used to compare sequences of different lengths?
Only cautiously. ENC is affected by sampling noise in short sequences, and families with few observations produce unstable F values. For short coding sequences treat ENC as a rough descriptive number, and compare complete genes rather than fragments.
Does a biased codon usage prove that selection is acting?
No. Codon bias can come from mutational patterns, GC content, drift or selection for translation efficiency, and a single sequence cannot separate those causes. This calculator describes the pattern only; it does not test selection or compare against a reference table.
Is my coding sequence uploaded anywhere?
No. Parsing, translation and all statistics run with JavaScript in this browser tab, so the sequence is never uploaded, stored or logged, and closing the page discards it.

Background reading

Format guides that explain the trade-offs behind Codon Usage Bias Calculator.