Protein Molecular Weight Calculator
Estimates the molecular weight of a protein from its amino acid count and average residue mass. Use it when designing proteins, interpreting SDS-PAGE results, or planning purification workflows.
About this calculator
A protein is a chain of amino acids linked by peptide bonds. Each time two amino acids join, a water molecule (18 Da) is released. The total molecular weight therefore equals the sum of all residue masses minus the water lost at each peptide bond. With a known average amino acid molecular weight, the formula becomes: MW = (aminoAcids × averageMW) − ((aminoAcids − 1) × 18). The average molecular weight of an amino acid residue is approximately 110 Da, though this varies by composition. For precise work, sum the individual residue masses from a table; this calculator gives a rapid estimate when the exact sequence is unknown.
How to use
Suppose you have a protein of 300 amino acids and an average residue MW of 110 Da. Step 1 — Enter 300 in the 'Number of Amino Acids' field. Step 2 — Enter 110 in the 'Average Amino Acid MW' field. Step 3 — The calculator computes: MW = (300 × 110) − ((300 − 1) × 18) = 33,000 − 5,382 = 27,618 Da (≈ 27.6 kDa). This estimate helps you predict where the band should appear on a gel or which membrane to choose for a Western blot.
Frequently asked questions
Why do we subtract 18 Da for each peptide bond in the molecular weight formula?
When two amino acids form a peptide bond, a molecule of water (H₂O, 18 Da) is released through a condensation reaction. A chain of N amino acids forms N−1 peptide bonds, so N−1 water molecules are lost in total. Failing to subtract this water would overestimate the protein's mass. This correction is why the formula uses (aminoAcids − 1) × 18 rather than N × 18.
What average molecular weight should I use for a typical amino acid?
The commonly accepted average is approximately 110 Da per residue, which reflects the natural abundance of all 20 standard amino acids in most proteins. However, if your protein is unusually rich in large residues like tryptophan (186 Da) or small ones like glycine (57 Da), the average will shift accordingly. For the most accurate result, use the known sequence and sum individual residue masses from an amino acid table. This calculator is best for quick estimates when the full sequence is unavailable.
How does protein molecular weight affect SDS-PAGE band migration?
In SDS-PAGE, proteins are denatured and coated with the detergent SDS, giving them a uniform negative charge proportional to their mass. Smaller proteins migrate faster through the polyacrylamide gel, while larger ones move more slowly. By knowing your protein's estimated molecular weight, you can predict where it will appear relative to a molecular weight ladder. This helps confirm a successful expression or purification step and detect degradation products or unexpected isoforms.