Protein Molecular Weight Calculator

A Protein's Weight Is the Sum of Its Residues, Minus the Water

Every peptide bond that links two amino acids together releases one water molecule. So a protein's molecular weight isn't quite the sum of its free amino acids — it's the sum of their residue masses (the amino acid minus the water lost in bonding), plus one water molecule added back for the two free ends of the finished chain.

The Formula

Each of the twenty standard amino acids has a defined average residue mass; this calculator sums the mass for every letter in the entered sequence and adds one water molecule to account for the free amino and carboxyl termini.

Where This Calculation Matters

• Mass spectrometry interpretation — a protein's predicted molecular weight is the reference point for identifying and validating an observed mass spec peak.
• SDS-PAGE gel analysis — predicted molecular weight determines where a protein band is expected to migrate relative to size standards.
• Molar concentration conversions — converting a protein concentration from mg/mL to a molar unit requires its molecular weight.
• Recombinant protein design — predicting the size of a construct before expression helps plan purification and detection strategies.

Residue Masses for Selected Amino Acids

Tryptophan's bulky indole ring makes it the heaviest standard residue by mass, more than three times glycine, the lightest.

How to Use This Calculator

1. Enter the protein's Amino Acid Sequence using standard one-letter codes (e.g. MVLSPADKTNVK).
2. Select Calculate to sum the residue masses and add the terminal water molecule.
3. Review the total molecular weight in daltons (Da).

Related Calculations

Need the melting behavior of the gene that encodes this protein? See the DNA Melting Temperature Calculator. For the enzyme kinetics of a protein once expressed, use the Enzyme Rate Calculator.