Protein Molecular Weight Calculator
Paste a plain amino acid sequence or FASTA to calculate molecular weight, residue composition, theoretical ε280, and estimated A280 at 1 mg/mL.
Use the Protein Molecular Weight Calculator
Amino Acid Sequence
Enter single-letter codes or paste FASTA (e.g. MKVLWAALLVTFLAGSKATA). FASTA header lines, spaces, digits, hyphens, and * are ignored.
Theoretical absorbance at 280 nm
Sequence estimate for a 1 cm path. A280 (0.1%) assumes 1 g/L = 1 mg/mL protein.
All cysteines reduced
ε280 (M−1 cm−1)
A280 (0.1%):
Maximum cystines
ε280 (M−1 cm−1)
A280 (0.1%):
Note:
Residue Composition
| Residue | Code | Count | Residue Mass (Da) | Total Mass (Da) |
|---|
Enter a sequence above and click Calculate to see the molecular weight.
Method, sources, and privacy
Mass is the sum of average residue masses plus 18.0153 Da for the free termini. This excludes signal-peptide processing, disulfide mass loss, labels, and other post-translational modifications.
ε280 uses the sequence coefficients reported by Pace et al. (1995). Average-mass and modification scope follows the documented approach of ExPASy Compute pI/Mw. FASTA header handling follows the UniProt FASTA format.
Privacy: the sequence is processed only in your browser; this calculator does not upload or store it.
Summary
The Protein Molecular Weight Calculator determines the mass of any protein or peptide from its amino acid sequence written in standard single-letter codes (e.g., MAST...). It sums the average residue masses plus one water molecule for the free termini, and displays the total in both Daltons (Da) and kiloDaltons (kDa). A per-residue composition table shows the count and mass contribution of each amino acid present.
How it works
- Paste a plain amino acid sequence or FASTA. Lines beginning with > are treated as FASTA headers and excluded.
- Invalid characters (spaces, digits, ambiguous codes) are flagged but ignored in the calculation.
- The tool looks up the average residue mass for each amino acid.
- All residue masses are summed, then one water molecule (18.02 Da) is added to account for the free N- and C-termini.
- The total molecular weight is displayed in Da and kDa alongside a composition breakdown table.
- Theoretical ε280 is estimated from tryptophan and tyrosine, with a second value assuming all possible cysteine pairs form cystines. A280 at 1 mg/mL and a 1 cm path is ε280 divided by molecular weight.
Use cases
- Estimate the expected mass of a recombinant protein before SDS-PAGE or mass spectrometry.
- Verify that a synthesized peptide matches its theoretical mass.
- Calculate mass for gel filtration column calibration.
- Quickly compare molecular weights of protein variants or mutants.
- Support biochemistry coursework and laboratory planning.
- Check the mass contribution of individual residues in a peptide sequence.