🧫 Protein Molecular Weight Calculator
Get the molecular weight of a protein in daltons and kilodaltons straight from its amino-acid sequence, with the residue composition.
6,865.9 Da
molecular weight (6.9 kDa)
65
residues
Sum of average residue masses + one water (18.02 Da). This is the unmodified-chain mass — post-translational modifications and disulfide bonds shift it. 🔒 Computed in your browser.
How the protein molecular weight calculator works
The molecular weight is the sum of the average residue masses of each amino acid plus one water molecule (18.02 Da) for the free N- and C-termini. Paste a one-letter sequence (FASTA headers are ignored) and the tool sums the residues and reports the mass in daltons and kilodaltons, along with how many of each amino acid the protein contains.
Average residue masses are fixed physical constants, so the result is exact for the unmodified chain. Post-translational modifications, bound cofactors and disulfide bonds shift the real mass — this is the polypeptide backbone mass.
Frequently asked questions
How do you calculate the molecular weight of a protein?
Add up the average residue mass of each amino acid in the sequence, then add one water molecule (18.02 Da) for the two free ends. For example, a 100-residue protein is roughly 11 kDa, since the average residue is about 110 Da.
What is the average mass of an amino acid residue?
About 110 daltons on average, though it ranges from glycine (57 Da) to tryptophan (186 Da). Multiplying the residue count by ~110 gives a quick estimate; this tool sums the exact residue masses.
Why daltons and kilodaltons?
Protein masses are reported in daltons (Da); 1000 Da = 1 kilodalton (kDa). Most proteins fall in the 10–150 kDa range. The tool shows both.
Does this include modifications?
No — it computes the mass of the unmodified polypeptide chain. Phosphorylation, glycosylation, disulfide bonds and bound cofactors change the real mass, so a gel or mass-spec value can differ from the sequence mass.
What sequence format does it accept?
One-letter amino-acid codes (A, R, N, D, C, …). FASTA header lines starting with ">" and any spaces or numbers are ignored, and unrecognised characters are skipped and reported.
Is my sequence uploaded?
No — the calculation runs entirely in your browser, so your protein sequence is never sent to a server.