Protein Fragment Molecular Weight Calculator

Paste a protein/peptide sequence or import a FASTA file. Select a subsequence either by highlighting directly in the sequence box or by entering start/end positions. All calculations happen entirely in your browser on your local machine.

100% local processing. All calculations run entirely in your browser using plain JavaScript. No sequence data is ever transmitted, uploaded, or sent to any server or third-party service. This tool makes no network requests after the page loads.

① Protein Sequence

Sequence

Supported residue codes: ACDEFGHIKLMNPQRSTVWY. Whitespace, line-breaks, numbers, and FASTA headers are ignored. Unknown letters are flagged.

② Selection & Masses

Start position

End position

Selected sequence

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Sequence length

Selected length

Cysteine modification

Custom added mass (Da)

At residue number

The arbitrary mass is added once at the specified 1-based residue position when that residue falls within the selected window.

Average molecular weight

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Residue sum + one water molecule (18.01528 Da)

Monoisotopic mass

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Residue monoisotopic sum + one water molecule (18.01056 Da)