Protein molecular weight is one of the most frequently needed numbers in a molecular biology or biochemistry workflow — for SDS-PAGE gel interpretation, for converting between mass and molar concentration, and for comparing a purified sample against its expected size. This calculator gives you both a fast estimate from residue count alone and an exact result from a pasted amino acid sequence.

How the Protein Molecular Weight Calculator works

When you paste a full sequence, the calculator looks up each amino acid's standard average residue mass (the widely used ExPASy/ProtParam values) and adds them together, then subtracts one water molecule (18.015 Da) for every peptide bond formed — a chain of n residues has (n − 1) bonds, since forming each bond during protein synthesis releases a water molecule via condensation. When you only have a residue count, the calculator instead uses the well-known shortcut of ~110 Da per residue, which is simply the practical average of the 20 amino acids' individual residue masses.

Inputs and what they mean

Residue count is just the number of amino acids in the chain — use this when you know the protein's approximate length but not its exact sequence. The sequence field accepts a single-letter amino acid code string (e.g. MKTAYIAK...); whitespace and line breaks are stripped automatically, and any character outside the 20 standard codes is rejected with a message pointing at its position. Your sequence is processed entirely in your browser — it is never written to the page URL or to local storage, unlike the residue count and mass inputs, which are safe to share in a link. Mass (µg) is optional and only feeds the Mass ↔ Moles conversion tab.

Limits and edge cases

The residue-count estimate is inherently approximate — a protein unusually rich in heavy residues (tryptophan, 186 Da) or light residues (glycine, 57 Da) will deviate more from the ~110 Da/residue average than a protein with typical amino acid composition. The exact-sequence calculation does not account for post-translational modifications (glycosylation, phosphorylation, disulfide bonds, signal-peptide cleavage, and similar), all of which change a real protein's actual measured mass. Ambiguous or non-standard codes (X, B, Z, U, and lowercase modified residues) are not recognized — only the 20 standard single-letter amino acid codes are supported.