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Codon & Amino Acid Count Calculator

Convert an mRNA length in nucleotides into the number of codons and the length of the encoded protein, or work backwards from a known protein length to the coding nucleotides needed.

Convert from

mRNA length (nucleotides)

Protein length (amino acids)

Include a stop codon

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How it works

Codons = ⌊nucleotides ÷ 3⌋ · Amino acids = codons − 1 (when a stop codon is present) · Nucleotides = (amino acids + stop) × 3

The genetic code is read in non-overlapping triplets called codons, so every three mRNA nucleotides specify one codon. Sixty-one codons encode amino acids and three are stop signals; a typical open reading frame ends in one stop codon that adds no residue. That makes the protein length equal to the number of codons minus the stop, and any nucleotides beyond the last full triplet are left over.

Worked example

A 300-nucleotide open reading frame has 300 ÷ 3 = 100 codons. With one stop codon, it encodes 99 amino acids. Working backwards, a 99-residue protein needs 99 + 1 = 100 codons = 300 nucleotides of coding sequence.

Sources: Genetic code — Wikipedia

Frequently asked questions

How many nucleotides code for one amino acid?

Three. Each amino acid is specified by a codon of three consecutive mRNA nucleotides, so coding length is always a multiple of three.

Why subtract one amino acid for the stop codon?

Stop codons (UAA, UAG, UGA) terminate translation without adding a residue. So a reading frame of 100 codons produces a 99-amino-acid protein when one codon is the stop signal.

What do leftover bases mean?

If the mRNA length is not a multiple of three, the extra one or two bases cannot form a complete codon. In real messages this usually reflects untranslated regions rather than a partial codon in the reading frame.

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Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific organism or protocol.