Protein Molecular Weight Calculator

How to use this protein molecular weight calculator

What this calculator does

This tool estimates the neutral molecular mass of a short, unmodified peptide assembled from the 20 standard proteinogenic amino acids. It adds the molecular masses of the selected free amino acids and subtracts the mass of water released as each peptide bond forms. The result is shown in unified atomic mass units, written u, and in kilodaltons, kDa. It is useful for classroom calculations, quick checks of short synthetic peptides, and rough comparison with bands or peaks in laboratory workflows. It does not account for disulfide formation, terminal modifications, isotopic labeling, phosphorylation, glycosylation, other post-translational modifications, salts, adducts, charge state, or sequence-specific isotope distributions.

When to use it

Use the calculator to verify a hand calculation for an introductory biochemistry exercise, estimate the mass of a short peptide before ordering or analyzing it, compare closely related sequences after an amino-acid substitution, or create a transparent calculation sheet for a lab notebook. For long proteins or modified sequences, use a specialist sequence-analysis service such as EMBL-EBI EMBOSS Pepstats.

How to calculate

1. Choose the 1st amino acid. The default is Alanine (Ala, A), so the calculator begins in a valid state.
2. Choose the 2nd amino acid, 3rd amino acid, and any later positions in sequence order. Use Add amino acid to reveal more positions, up to 20.
3. Read Molecular weight in kDa and u, then check Your protein, Average residue mass, Sum of free amino acids, and Condensation loss.
4. Review the Residue detail table. Its running value shows how each newly added residue increases the chain mass after one water loss.
5. Select Download Excel to export the current inputs, outputs, residue rows, and calculation notes. Select Reset to restore the single-alanine default.

Input guide

Each control labeled 1st amino acid through 20th amino acid is an optional sequence position except the first, which is required for a valid result. Select one standard amino acid by full name, three-letter code, or one-letter code shown in the list. Positions are interpreted from top to bottom. A realistic example is Glycine (Gly, G) followed by Alanine (Ala, A) and Serine (Ser, S). Adding a heavier residue usually raises the molecular weight, while substituting a lighter residue lowers it. Avoid leaving an internal blank followed by later selected residues: the calculator reports a visible sequence-gap error rather than silently joining nonadjacent positions. Add amino acid only reveals another position; it does not change mass until a residue is selected. Remove last clears and hides the final visible position, but never removes the first three visible controls.

Output guide

Molecular weight is the primary estimate. kDa is the same numerical mass in thousands of daltons; u and Da are equivalent unit names for molecular-scale mass, as explained by the NIST Guide to units outside the SI. Your protein is the one-letter sequence. Average residue mass divides the final peptide mass by residue count. Sum of free amino acids is the uncondensed total before bond formation. Condensation loss equals 18.0153 u times the number of peptide bonds. The summary pills report residue count, bond count, and water loss. In the table, Position, Amino acid, and Code identify each residue; Free amino-acid mass gives its isolated mass, and Running peptide mass gives the chain total through that row. These are formula-based estimates, not experimental measurements.

Worked example

For the sequence Gly-Ala-Ser, the free amino-acid masses are 75.0669 u, 89.0935 u, and 105.0930 u. Their sum is 269.2534 u. Three residues form two peptide bonds, so the condensation loss is 2 × 18.0153 u = 36.0306 u. The estimated peptide molecular weight is therefore 269.2534 – 36.0306 = 233.2228 u, displayed as 233.22 u and 0.233 kDa. The detailed table reaches the same final running mass.

Learn more

Peptide chains form through condensation reactions in which amino acids become residues after loss of water; the NCBI Bookshelf overview of peptides explains this biological context. Protein database records often report both sequence length and molecular weight, as described in the EMBL-EBI guide to UniProt entry views.