Protein Molecular Weight Calculator — Residues to Da & kDa
Estimate a protein's molecular weight from its amino acid residue count. Enter the number of residues and any disulfide bonds — the calculator gives approximate MW in Da and kDa plus an estimated Stokes radius.
Approximate MW based on average residue mass of 110 Da
- 1
Total residue mass
300 × 110 = 33,000 DaAverage residue mass after peptide bond condensation is 110 Da. - 2
Add terminal water
33,000 + 18.02 = 33,018 Da - 3
Convert to kDa
33,018 ÷ 1000 = 33
How does this calculator work?
Protein MW (Da) ≈ residue count × 110 + 18 − (disulfide bonds × 2). For a 300-residue protein with no S-S bonds: ≈ 33,018 Da ≈ 33 kDa. Accurate to ~10%; for exact mass use mass spectrometry or ProtParam from the full sequence.
Formula
How this is calculated
The mass of a protein chain is the sum of its amino acid residue masses minus the water molecules lost during peptide bond formation. Because each peptide bond releases one water molecule (18.02 Da), the average residue mass after condensation is approximately 110 Da — the widely cited textbook average over the 20 standard amino acids at typical occurrence frequencies (the exact value ranges from 110 to 112 Da depending on the source and weighting). One water molecule is added back for the free N- and C-termini, giving: MW ≈ n × 110 + 18.02.
Disulfide bonds form when two cysteine side chains each lose a hydrogen atom to form a covalent S–S link. Each bond therefore reduces the mass by 2 × 1.008 ≈ 2.02 Da, a small but sometimes measurable correction for proteins with multiple disulfide bridges.
The estimated Stokes radius (hydrodynamic radius) uses the empirical power-law r ≈ 0.066 × MW^(1/3) nm, valid for compact globular proteins; intrinsically disordered proteins are larger. This is an approximation — actual values depend on shape, glycosylation and solution conditions. For precise MW, use mass spectrometry; for Stokes radius, use dynamic light scattering or size-exclusion chromatography.
Frequently asked questions
The 20 standard amino acids have free-acid masses ranging from 75 Da (glycine) to 204 Da (tryptophan). After forming peptide bonds (losing 18 Da of water per bond), their residue masses range from 57 to 186 Da. The weighted average across typical protein compositions is approximately 110 Da, making it a convenient single-number approximation for sequence-length-to-mass conversion.
For a protein whose composition is close to the average, the 110 Da rule predicts MW to within about ±10%. Proteins rich in large residues (tryptophan, tyrosine, phenylalanine) will be underestimated; glycine-rich proteins will be overestimated. Post-translational modifications such as glycosylation (adds hundreds to thousands of Da), phosphorylation (+80 Da per site) and acetylation (+42 Da) are not included.
Databases such as UniProt (uniprot.org) list the canonical sequence length and mass for annotated proteins. For custom or engineered proteins, count the residues from the FASTA sequence (excluding the signal peptide if it is cleaved). ExPASy ProtParam can calculate exact MW from the full sequence when you need higher accuracy.
Also known as
TG we-Calculate Editorial Team. (2026). Protein Molecular Weight Calculator — Residues to Da & kDa [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/protein-molecular-weight-calculator
TG we-Calculate Editorial Team. "Protein Molecular Weight Calculator — Residues to Da & kDa." TG we-Calculate. 2026. https://we-calculate.com/calculator/protein-molecular-weight-calculator.
TG we-Calculate Editorial Team, "Protein Molecular Weight Calculator — Residues to Da & kDa," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/protein-molecular-weight-calculator
@misc{wecalculate_protein_molecular_weight_calculator, title = {Protein Molecular Weight Calculator — Residues to Da & kDa}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/protein-molecular-weight-calculator}}, year = {2026}, note = {TG we-Calculate} }
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