Codon Optimization Calculator

Example data table

Construct	Host	Length	Original CAI	Optimized CAI	GC shift	Comment
Kinase fragment	E. coli	318 nt	0.61	0.87	44% to 53%	Rare arginine codons were reduced.
Secreted peptide	Human	204 nt	0.68	0.90	47% to 59%	GC3 increased without changing protein sequence.
Enzyme domain	B. subtilis	429 nt	0.52	0.81	56% to 41%	AT-rich codons improved host preference fit.

Formula used

1. Codon Adaptation Index: CAI = exp[(1/L) × Σ ln(w_i)], where w_i is the host weight for each codon and L is codon count.

2. GC percentage: GC% = ((G + C) / N) × 100, where N is total nucleotide length.

3. GC3 percentage: GC3% = (third-position G or C codons / total codons) × 100.

4. Rare codon rate: Rare rate = (rare codons / total codons) × 100. This calculator treats host weights below 0.30 as rare.

5. Optimization index: Index combines CAI, GC distance from target, rare codon burden, restriction hits, and longest homopolymer run into a 0–100 summary score.

How to use this calculator

Paste a coding DNA or RNA sequence in the sequence field.
Select the host organism that best matches your planned expression system.
Choose an optimization mode based on CAI emphasis or GC control needs.
Set GC boundaries, optional motif screening, and maximum homopolymer length.
Submit the form to generate an optimized synonymous coding sequence.
Review CAI, GC, GC3, rare codon counts, warnings, and the per-codon table.
Use the CSV export for spreadsheet review and the PDF export for reports.
Confirm final constructs with organism-specific databases and wet-lab checks.

FAQs

What does codon optimization improve?

It improves host compatibility by swapping synonymous codons, reducing rare codons, balancing GC, and avoiding sequence patterns that can lower transcription or translation efficiency.

Does optimization change the protein sequence?

It should not. This calculator keeps the translated amino acid sequence constant while changing nucleotide choices. Always verify the final translated sequence before synthesis.

Why is CAI important?

CAI estimates how closely a coding sequence matches preferred host codons. Higher values often indicate better translational compatibility, though expression also depends on promoter design, mRNA structure, and context.

Why track GC and GC3 together?

Overall GC affects stability and synthesis behavior, while GC3 reflects third-base codon bias. Reviewing both helps avoid extreme nucleotide composition during synonymous redesign.

What are rare codons in this report?

Rare codons are those assigned low host weights in the selected preset. They can slow translation or stress tRNA pools when present too often.

Can I use custom restriction motifs?

Yes. Add motifs separated by commas or spaces. The calculator scans the optimized sequence and flags any remaining matches in the final report.

Is this enough for gene synthesis ordering?

No. Use it as a design aid. Final orders should also check mRNA structure, repeats, cloning strategy, regulatory motifs, and the synthesis vendor’s screening rules.

Why might warnings still appear after optimization?

Some proteins have limited synonymous choices, especially around methionine, tryptophan, or required motifs. Tight GC limits and motif removal can also compete with each other.