molecular weight protein calculator

What this molecular weight protein calculator does

This tool estimates the molecular weight of a protein directly from its amino acid sequence. It sums residue masses, adds terminal water, and applies optional adjustments like disulfide bonds and terminal modifications. The output is shown in Daltons (Da) and kilodaltons (kDa), which are commonly used for SDS-PAGE planning, mass spectrometry prep, and protein purification workflows.

Why protein molecular weight matters in the lab

Knowing protein MW helps you make better decisions before running experiments. Typical use cases include:

• Gel electrophoresis: Estimate where your band should migrate.
• Western blot validation: Check if observed size matches expected sequence-based size.
• Mass spectrometry: Compare measured intact mass with theoretical mass.
• Protein expression: Predict size shifts caused by tags or cleavage events.
• Formulation and dosing: Convert between molar concentration and mass concentration.

How the calculation works

1) Residue mass summation

Each amino acid contributes a residue mass. The calculator adds all residue masses from the sequence. You can choose average masses (good for many routine estimates) or monoisotopic masses (common in high-resolution MS contexts).

2) Addition of terminal water

A complete peptide/protein chain includes terminal groups equivalent to one water molecule: +18.01528 Da (average) or +18.01056 Da (monoisotopic).

3) Optional chemical adjustments

• Disulfide bonds: subtracts hydrogen mass for each bond formed.
• N/C-terminal modifications: add or subtract known masses.
• Extra mass shift: useful for labels, adduct assumptions, or custom corrections.

Step-by-step usage guide

1. Paste your protein sequence in one-letter format (raw or FASTA).
2. Select Average or Monoisotopic mass mode.
3. Enter oligomer chain count (1 for monomer, 2 for homodimer, etc.).
4. Add disulfide count and modification masses if needed.
5. Click Calculate MW to view the result summary.

Example interpretation

If a monomer is predicted at 27,540 Da and your protein forms a homodimer, the complex mass is approximately 55,080 Da before additional PTMs or non-covalent ligands. In practice, apparent gel migration can differ due to protein shape, charge, detergent binding behavior, and glycosylation.

Common pitfalls and accuracy notes

• Sequence-only MW does not automatically include glycosylation or phosphorylation.
• Signal peptides and propeptides can inflate predicted size if not removed from input.
• Ambiguous characters (X, B, Z, J) are rejected to avoid misleading output.
• SDS-PAGE apparent MW may deviate from true molecular mass.

Quick FAQ

Should I use average or monoisotopic mass?

Use average for general biochemistry estimates and routine gel planning. Use monoisotopic for high-resolution mass spectrometry workflows.

Does this calculator support FASTA input?

Yes. Lines starting with > are ignored as headers.

Can I account for tags like His-tag or FLAG?

Yes. Include the full tagged sequence directly, or enter a known added mass using the extra mass shift field.