Peptide Molecular Weight Calculator & Sequence Analyzer
Enter a standard one-letter amino-acid sequence to calculate its theoretical neutral monoisotopic mass, common protonated m/z values, sequence length and residue composition. The calculation is intended for laboratory research and analytical planning.
Enter standard one-letter amino-acid codes. Spaces, line breaks, numbers and punctuation are ignored.
How the calculation works
Residue masses plus terminal water
The neutral monoisotopic mass is calculated by summing the monoisotopic residue mass of each amino acid and adding the mass of water for the free N- and C-termini. Protonated m/z values then add one proton for each charge state before dividing by the charge.
This gives a theoretical reference value. Experimental spectra may contain different charge states, adducts, isotopic envelopes, modifications, salts or other species depending on the sample and analytical method.
Interpretation
Use theoretical mass as context—not identity proof.
A calculated molecular mass is a reference derived from a sequence. Matching a theoretical value to an observed mass-spectrometric signal can support interpretation, but complete analytical identity depends on the experiment, instrument, sample and supporting evidence.
Read the peptide mass spectrometry guide