Biology & Life Science

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Protein Molecular Weight Calculator.

Calculate sequence residue mass with an explicit terminal or modification adjustment.

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Molecular mass: 1,219.55 Da

Molecular mass

1,219.55Da
Residues: 10

Residues

10.00

How to Use

The result sums standard amino-acid residue masses for the entered sequence and adds the explicit terminal or modification adjustment. No sequence database or modification is inferred.

Use measured values and a documented convention. This calculator does not provide medical, veterinary, phenotype, or laboratory-method advice.

Use Cases

Protein Characterization

Determine the molecular weight of a protein for experimental design, such as SDS-PAGE analysis or mass spectrometry calibration.

Example: Calculate the MW of a 50-residue peptide with an N-terminal acetylation.

Quality Control in Biotech

Verify the molecular weight of recombinant proteins or synthetic peptides to ensure correct synthesis or expression.

Example: Check the MW of a monoclonal antibody fragment after adding a His-tag.

Frequently Asked Questions

How does the Protein Molecular Weight Calculator work?
The calculator sums the atomic masses of all amino acid residues in your protein sequence. It also accounts for the terminal group (e.g., hydrogen and hydroxyl) and any specified modifications, providing an accurate molecular weight.
What is the 'terminal or modification adjustment'?
This adjustment accounts for the extra atoms at the protein's termini (like H and OH) or any post-translational modifications (e.g., acetylation, phosphorylation) that alter the mass. You can specify these to get a more precise molecular weight.
Can I use this calculator for any protein sequence?
Yes, you can input any amino acid sequence using standard one-letter codes. The calculator will compute the molecular weight based on the sequence and any adjustments you provide.

Tips & Common Mistakes

Tips

  • Use standard one-letter amino acid codes (e.g., A, C, D) for accurate calculations.
  • If you have a modification, specify it clearly (e.g., 'acetylated N-term') to get the correct mass.
  • Double-check your sequence for typos; a single wrong residue can significantly alter the molecular weight.
  • For proteins with disulfide bonds, note that the calculator may not account for the loss of hydrogens unless specified.

Common Mistakes to Avoid

  • Forgetting to include terminal adjustments, leading to a mass that is off by ~18 Da (water molecule).
  • Using three-letter amino acid codes instead of one-letter codes, causing errors.
  • Not specifying modifications like phosphorylation or glycosylation, which add significant mass.

Last updated: August 13, 2026