Mass & m/z calculator.

Paste a one-letter peptide sequence to compute its molecular formula, average and monoisotopic mass, positive-mode ESI-MS m/z series, and isoelectric point. Standard 20 L-amino acids only — the calculator refuses non-standard or modified residues rather than return a fabricated value.

Peptide sequence (one-letter, standard 20 L-amino acids)

Method

Formula: derived from standard residue compositions plus one water (free termini).
Mass / m/z: the same element-arithmetic engine used on the compound pages — monoisotopic from most-abundant-isotope masses, average from IUPAC 2021 atomic weights, m/z = (M + n·1.007276) / n.
pI: Henderson–Hasselbalch net-charge bisection, EMBOSS pKa set, free termini assumed. Analytical/characterization only — no efficacy, dosing, or human-use meaning.