How this calculator works
A peptide bond removes water from free amino acids. Published residue masses already account for that loss, so summing residue masses and adding one terminal water gives a single linear chain. Average isotopic masses are used, not monoisotopic masses. Sequence order does not change this unmodified mass, allowing composition mode. Disulfides, modifications, bound cofactors, salt forms and multiple chains require additional chemistry.
Formula
Average molecular weight (g/mol) = sum of standard residue masses × their counts + 18.01524 for terminal water. kDa = mass in Da / 1,000. This assumes one linear chain with unmodified termini.
Worked example
AG has 71.0788 + 57.0519 + 18.01524 = 146.14594 g/mol. GG has 2 × 57.0519 + 18.01524 = 132.11904 g/mol. A single alanine has 89.09404 g/mol after adding terminal water.
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Frequently asked questions
Why add water when peptide bonds remove it?
The table contains residue masses, each already missing water relative to the free amino acid. A chain has one fewer bond than residues, leaving one water to add back.
Can I enter B, Z, X or a stop symbol?
No. Ambiguous or unknown residues do not have one exact mass in this model. Resolve the sequence before calculating.
Will this match a mass-spectrometry peak?
Not necessarily. Peaks may use monoisotopic mass, charge states, adducts or modified protein. This output is the neutral average mass of the stated unmodified chain.