Protein Molecular Weight Calculator
Estimate a protein’s molecular weight in daltons and kilodaltons from its number of amino acid residues.
How it works
MW ≈ (residues × average residue mass) + 18.02 Da (terminal water)
Each amino acid joined into a chain loses a water molecule at the peptide bond, so residue masses (not free amino-acid masses) are summed and a single water is added back for the free N- and C-termini. The average residue mass of about 110 daltons comes from the typical amino-acid composition of proteins, which is why the quick "residues × 110" rule lands close to sequence-based calculators. For an exact figure, sum the individual residue masses of the real sequence.
Worked example
A 300-residue protein at 110 Da per residue is 300 × 110 + 18 = 33,018 Da, or about 33 kDa — close to the size of a typical single-domain enzyme.
Sources: Proteins — average residue mass
Frequently asked questions
Why 110 Da per residue?
It is the average residue mass across the 20 amino acids weighted by how often each appears in real proteins. Sequences unusually rich in tryptophan (204 Da) or glycine (57 Da) will drift from this average.
Does this include glycosylation or other modifications?
No. This is the mass of the bare polypeptide. Sugars, phosphate groups, lipids and other post-translational modifications add mass and must be counted separately.
How does molecular weight relate to migration on an SDS-PAGE gel?
SDS-PAGE separates denatured proteins roughly by log of molecular weight, much like DNA on agarose, so kDa estimates from a protein ladder use the same log-distance idea.
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This calculator is for educational purposes. Double-check important results before acting on them.