Protein Molecular Weight Calculator
A protein's molecular weight from its length in amino acids — or from a sequence — with the moles in a sample and the concentration a solution gives.
The average amino acid residue weighs about 110 daltons, so a 300-residue protein is roughly 33 kDa.
How the protein molecular weight calculator works
The average amino acid residue weighs about 110 daltons, so a 300-residue protein is roughly 33 kDa. From a sequence the calculation sums the actual residue masses plus one water for the termini, which is exact for the polypeptide backbone.
The practical use is converting between mass and moles: 1 mg of a 33 kDa protein is 30 nanomoles, and that is what a binding assay needs to know.
Formula: MW ≈ residues × 110 Da; moles = mass / MW
Worked examples
| Inputs | Molecular weight (kDa) | Note |
|---|---|---|
| 300 residues, 1 mg in 1 mL | 33.018 | 33 kDa, 30 µM |
| From a sequence | 36.618 | exact mass |
| A small peptide | 1.338 | 1.3 kDa |
FAQFrequently asked questions
How do I estimate a protein's molecular weight?
Multiply the residue count by 110 daltons. It is within a few per cent for most proteins; the sequence method is exact for the polypeptide.
Why 110?
The average residue mass weighted by how often each amino acid occurs in proteins. Glycine-rich proteins come in lighter, tryptophan-rich ones heavier.
Does this include modifications?
No — glycosylation, phosphorylation and prosthetic groups all add mass. A glycoprotein can weigh half again what its sequence predicts.
How do I convert mass to moles?
Divide mass by molecular weight. One milligram of a 33 kDa protein is 30 nanomoles — the number a stoichiometric assay needs.
What is a dalton?
One twelfth the mass of a carbon-12 atom — the same as the unified atomic mass unit. Kilodaltons are the working unit for proteins.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- National Human Genome Research Institute — the US genomics research institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.