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Peptide mass calculator

Enter a sequence in one-letter codes to get its formula, its monoisotopic and average mass and the m/z of the common charge states. The m/z values are what a mass-spectrometry result on a certificate of analysis reports.

The 20 standard amino acids (GASPVTCLINDQKEMHFRYW), up to 200 residues. Spaces and hyphens are ignored.
Modifications (optional)

How it is calculated

  • Each residue contributes its elemental formula; one water closes the chain. The formula is summed, then converted to mass with IUPAC atomic masses: monoisotopic (the most abundant isotope of each element) and average (natural isotope mixture).
  • An N-terminal acetyl adds C2H2O, a C-terminal amide replaces OH with NH2, and each disulfide bond removes two hydrogens.
  • m/z for [M+zH]z+ is (M + z × 1.007276) ÷ z, with the proton mass.

Mass spectrometry confirms a mass, not by itself the sequence or the amount in a vial; read what a peptide CoA proves and does not and the checklist. The calculator does not know any product; it only turns the sequence you enter into numbers.

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