Protein Molecular Weight Calculator | Free MW Tool

Protein Molecular Weight Calculator

A protein molecular weight calculator finds the mass of a protein from its amino acid sequence. The result is given in Daltons (Da), the standard unit for molecular mass. One Dalton is close to the mass of a single hydrogen atom.

Molecular weight matters in everyday lab work. It helps a researcher choose gel percentages for SDS-PAGE, pick a column for gel filtration, and check whether a mass spectrometry result matches the expected protein. The value the calculator gives is a theoretical weight based only on sequence. Real proteins can weigh more because of chemical additions made after the protein is built, called post-translational modifications. These range from a few Daltons for a single phosphate group to tens of thousands of Daltons for heavy sugar chains.

How to calculate protein molecular weight

The formula

$MW_{protein} = \sum_{i=1}^{n} MW_{residue_i} + MW_{water}$

Each amino acid in a chain contributes its residue weight, not the weight of the free amino acid. When two amino acids join in a peptide bond, the reaction releases one water molecule. Residue weights already have that water removed, so the sequence sum accounts for all the internal peptide bonds automatically.

After summing the residues, one water molecule (18.010565 Da) is added back. This covers the two free ends of the chain: a hydrogen atom at the start (the N-terminus) and a hydroxyl group at the end (the C-terminus).

A common mistake is to subtract water for each peptide bond and then add a water molecule back separately. That double-counts the water loss, because the residue weights below already exclude it. The only water term in the calculation is the single one added at the end.

Amino acid residue weights

These are monoisotopic residue masses, in Daltons, for the 20 standard amino acids:

Example calculation

Take the short chain Alanine-Glycine-Serine, written as AGS.

1. Add the residue weights: 71.03711 + 57.02146 + 87.03203 = 215.0906 Da.
2. Add one water molecule for the two chain ends: 215.0906 + 18.010565 = 233.101165 Da.
3. Rounded to two decimal places, the molecular weight of AGS is 233.10 Da.

No extra water is subtracted for the two peptide bonds in this tripeptide. That loss is already built into the three residue weights used in step 1.

How to use the protein molecular weight calculator

1. Enter a sequence using standard one-letter amino acid codes, such as MVKMDVYKGSSIGDSMSRSM. Uppercase and lowercase both work.
2. The calculator checks the sequence as it is typed and flags any character that is not one of the 20 standard codes.
3. It displays the molecular weight in Daltons, the sequence length, and a breakdown of amino acid composition.
4. Results can be copied with one click for lab notebooks or reports.

Before pasting a sequence from a database such as UniProt, remove the FASTA header line (the line starting with >) and any position numbers from alignment software. The letters X, B, Z, and J are not standard amino acid codes and will be rejected; replace them with a specific amino acid before calculating.

Why protein molecular weight matters

• Gel electrophoresis (SDS-PAGE): small proteins under 20 kDa need a higher-percentage gel for good separation; large proteins over 100 kDa need a lower percentage.
• Gel filtration: the molecular weight range of a protein determines which column resin will separate it properly.
• Mass spectrometry: the calculated weight is compared against the measured weight to confirm identity or detect unexpected changes.
• Peptide synthesis: molecular weight sets the exact amount of starting material needed for each coupling step.
• Buffer and dose calculations: converting between molarity and concentration by mass requires the molecular weight.

Limitations

The calculator gives a theoretical weight based only on the sequence. It does not include:

• Post-translational modifications, such as the sugar chains added in glycosylation or the phosphate groups added in phosphorylation. These must be added manually or measured with mass spectrometry.
• Disulfide bonds. Each bond between two cysteine residues removes two hydrogen atoms, about 2.02 Da per bond. The calculator assumes all cysteines are unbonded.
• Non-standard amino acids, such as selenocysteine or pyrrolysine, and chemically modified residues like hydroxyproline.
• Cofactors, such as heme groups or metal ions bound to the protein.

Frequently asked questions

What is protein molecular weight? It is the total mass of a protein, usually given in Daltons (Da) or kilodaltons (kDa), where 1 kDa equals 1,000 Da. It equals the sum of the amino acid residue weights in the sequence plus one water molecule for the two chain ends.

Why does the formula add water instead of subtracting it? Forming a peptide bond releases a water molecule, and the residue weights used in the sum already have that water removed. A single water molecule is added back at the end only to account for the free hydrogen and hydroxyl groups at the two ends of the finished chain.

How accurate is the calculated weight? For a protein with no chemical modifications, the calculation is accurate to the precision of the amino acid masses used, typically within a fraction of a Dalton. Real proteins with modifications, such as glycosylation or disulfide bonds, will weigh a different amount than the calculated value.

Can the calculator handle modified or non-standard amino acids? No. It supports only the 20 standard amino acids. Selenocysteine, pyrrolysine, and other modified residues are not recognized and must be handled with a manual correction or a specialized tool.

How do I convert Daltons to kilodaltons? Divide by 1,000. A protein of 25,000 Da is 25 kDa. Papers commonly use Da for small peptides and kDa for larger proteins.

Why does my gel or mass spec result not match the calculated weight? The most common causes are post-translational modifications, disulfide bonds, or a protein that has been cut down to a shorter mature form after translation. SDS-PAGE can also show a band at a misleading position for very acidic or very basic proteins, so it is only a rough estimate of molecular weight.