Peptide Molecular Weight Calculator
Average and monoisotopic molecular weight plus molecular formula for any peptide sequence, with terminal modifications and disulfide bonds.
Average molecular weight
Working
About this tool
Paste a sequence in one-letter (GHK) or three-letter (Gly-His-Lys) notation. The calculator adds up the elemental formula of every residue, adds one water for the free termini, applies any modifications you choose, and reports both the average and the monoisotopic mass.
How to use it
- Enter the sequence. Spaces, numbers and line breaks are ignored, and
H-,Ac-,-OHand-NH2written on the ends are recognised. - Choose the N- and C-terminal groups and the number of disulfide bonds.
- Read the average mass (for weighing and molarity work) or the monoisotopic mass (for mass spectrometry).
Formula f(x)
M = Σ residue masses + M(H2O) + Δ(modifications)
Residue masses are calculated from elemental formulas (amino acid minus H2O), so the molecular formula and both masses come from the same arithmetic. N-acetylation adds C2H2O, C-terminal amidation replaces –OH with –NH2 (+HN −O), and each disulfide bond removes H2.
Worked example check your setup
GHK (Gly-His-Lys, free acid): Gly C2H3NO + His C6H7N3O + Lys C6H12N2O + H2O = C14H24N6O4.
Average mass = 14(12.011) + 24(1.008) + 6(14.007) + 4(15.999) = 340.38 g/mol. Monoisotopic mass = 340.1859 Da.