How many amino acids are there?

There are 20 standard amino acids used to make proteins. Humans also use a rare 21st amino acid, selenocysteine. Across all known organisms, pyrrolysine brings the number of genetically encoded amino acids to 22.

All three answers are correct because they count different amino-acid sets.

How many amino acids are there?

There are 20 standard amino acids in the protein alphabet: the set used throughout biology textbooks, nutrition references, and most protein sequence analyses.^[1]

What are the 22 genetically encoded amino acids?

The names and codes follow the standard protein sequence notation used by UniProt. U represents selenocysteine and O represents pyrrolysine.^[6] The property labels are broad; amino-acid acidity and basicity also depend on pH and the local environment inside a protein.

The nutrition column applies to healthy adults: an asterisk marks arginine, cysteine, glutamine, glycine, proline, and tyrosine, which may become conditionally essential in some stages of life or health conditions.^[7] Selenocysteine is made during translation rather than obtained as a dietary amino acid, while humans do not encode pyrrolysine.^[2][3][4]

These amino acids can be linked in almost any order. Even a chain only 100 residues long, far shorter than the 300 to 400 amino acids typical of many proteins, has 20¹⁰⁰ possible sequences before accounting for chemical changes made after translation. This small alphabet is enough to produce the enormous variety of proteins found in nature.

The chart combines the 20 standard amino acids, the 9 that are essential in a healthy adult diet, and the two rare genetically encoded additions. The 11 nonessential standard amino acids are calculated as 20 minus 9.^[1][2][3][4]

How many amino acids are in the human body?

The human body uses all 20 standard amino acids to make proteins and also genetically encodes selenocysteine, so 21 is the most complete protein-synthesis count.^[2]

Selenocysteine is rare. Human cells insert it when a UGA codon, normally read as a stop signal, appears with the molecular signals needed to recode it. The human genome contains 25 genes for selenocysteine-containing proteins, many involved in redox chemistry and thyroid hormone metabolism.^[2]

Most sequence work still uses a 20-letter alphabet because the standard amino acids account for nearly every residue, although their frequencies in proteins are far from equal. Selenocysteine uses the one-letter code U. Pyrrolysine, code O, is not part of human protein synthesis.

This distinction also explains why a protein tool may expect 20 canonical residue codes. An amino acid composition analysis usually reports the standard 20 separately from rare or ambiguous symbols, while reverse translation must decide how to handle U as well as the standard genetic code.

“How many amino acids are in the human body?” can also be read as a question about individual molecules rather than types. There is no fixed molecule count. Amino acids are continually absorbed, made, incorporated into proteins, released by protein breakdown, and metabolized.

How many amino acids are essential for humans?

Healthy adults require nine essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.^[4]

“Essential” does not mean more important. It means the body cannot make enough of that amino acid, so it must come from food. The other 11 standard amino acids are classed as nonessential because a healthy adult can synthesize them, sometimes using an essential amino acid as a precursor.

The categories can change with age or health. For example, cysteine and tyrosine can become conditionally essential when the body cannot make enough from methionine and phenylalanine. The 9-versus-11 split is therefore a nutritional classification for healthy adults, not a second protein alphabet.^[4]

Why do sources say 20, 21, 22, or hundreds?

Sources report different amino-acid counts because “amino acid” can mean a standard protein building block, a genetically encoded building block, or any molecule with amino and carboxylic acid groups.

The standard protein alphabet has 20 amino acids. Selenocysteine is the 21st genetically encoded amino acid and is used by humans. Pyrrolysine is the 22nd; researchers found that some archaea and bacteria translate UAG as pyrrolysine, but humans do not.^[2][3]

Nature contains many nonprotein amino acids as well. A 1981 review described more than 200 nonprotein amino acids made by plants alone.^[5] Ornithine and citrulline, for example, take part in human metabolism but are not among the 20 standard residues directly specified in ordinary protein sequences.

This is why 20 is the best short answer to the general question. Use 21 for the genetically encoded amino acids in humans, 22 for the known genetically encoded set across life, and “hundreds” only when the chemical definition includes nonprotein amino acids.

Sources▼

1. Amino Acids National Human Genome Research Institute · July 23, 2026. https://www.genome.gov/genetics-glossary/Amino-Acids
2. Selenoprotein Gene Nomenclature Journal of Biological Chemistry · 2016. https://pmc.ncbi.nlm.nih.gov/articles/PMC5104929/
3. Pyrrolysine encoded by UAG in Archaea: charging of a UAG-decoding specialized tRNA Science · 2002. https://pubmed.ncbi.nlm.nih.gov/12029131/
4. Protein and Amino Acids Recommended Dietary Allowances, National Research Council · 1989. https://www.ncbi.nlm.nih.gov/books/NBK234922/
5. Non-protein amino acids of plants Food Chemistry · 1981. https://doi.org/10.1016/0308-8146(81)90009-1
6. Sequences UniProt · July 23, 2026. https://www.uniprot.org/help/sequences
7. Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake? Nutrients · 2023. https://pubmed.ncbi.nlm.nih.gov/36839196/