Protein Molecular Weight Calculator
Calculate the molecular weight of a protein or peptide from its one-letter amino acid sequence.
Enter a sequence above and click Calculate.
Formula
Average Molecular Weight:
MW = Σ(residue average masses) + H₂O − (2 × nSS × MH)
where H₂O = 18.01524 Da accounts for the free N- and C-termini, and each disulfide bond removes 2 hydrogen atoms (2 × 1.00794 Da).
Monoisotopic Molecular Weight:
MWmono = Σ(residue monoisotopic masses) + 18.01056 − (2 × nSS × 1.007825)
Molar Extinction Coefficient at 280 nm (Pace et al.):
ε₂₈₀ = (nTrp × 5500) + (nTyr × 1490) + (nSS × 125) [M⁻¹cm⁻¹]
Absorbance at 280 nm (1 mg/mL):
A₂₈₀ = ε₂₈₀ / MWavg
Assumptions & References
- Residue masses are the masses of amino acids minus water (peptide bond formation releases H₂O).
- Average masses use IUPAC atomic weights; monoisotopic masses use the most abundant isotope of each element.
- One water molecule (18.01524 Da average) is added to account for the free N-terminus (H) and C-terminus (OH).
- Each disulfide bond removes 2 hydrogen atoms (2 × 1.00794 Da average).
- Ambiguous residues: B (Asx) = average of Asn/Asp; Z (Glx) = average of Gln/Glu; X = 111.1 Da (average unknown); J (Xle) = Leu/Ile mass.
- Selenocysteine (U) and Pyrrolysine (O) are supported.
- Extinction coefficients: Trp ε = 5500 M⁻¹cm⁻¹, Tyr ε = 1490 M⁻¹cm⁻¹, disulfide ε = 125 M⁻¹cm⁻¹ at 280 nm.
- Pace, C.N. et al. (1995) Protein Science 4:2411–2423. doi:10.1002/pro.5560041120
- Residue masses from: Sigma-Aldrich / ExPASy ProtParam tool reference tables.
- Post-translational modifications (glycosylation, phosphorylation, etc.) are not accounted for.