# Protein Molecular Weight Calculator (from Sequence) MW = Σ ProtParam average residue masses + H₂O, with ambiguity-code handling, kDa/mean-residue output, and unmodified-chain caveats. Educational use only. > Canonical page: https://elysiatools.com/en/tools/protein-molecular-weight - **Category:** Education - **Keywords:** protein molecular weight calculator, amino acid sequence mass, average residue mass table, protparam molecular weight, polypeptide mass calculator, protein kda from sequence, peptide molecular weight, expasy protparam equivalent, protein length to mass, biochemistry calculator, lab calculator ## Overview The Protein Molecular Weight Calculator estimates the molecular weight of an unmodified polypeptide from its one-letter amino acid sequence. It reports the result in daltons and kilodaltons, along with sequence length and mean residue mass, using average residue masses and adding H₂O. ## Inputs - **Amino Acid Sequence** (textarea): One-letter amino-acid sequence (case-insensitive; spaces and * - . separators are ignored). - **Decimal Places** (number) ## When to use - Estimate a protein or peptide molecular weight directly from its amino acid sequence. - Convert a calculated sequence mass from daltons to kilodaltons for experimental planning. - Check the expected size of affinity-tagged or short synthetic peptide sequences. ## How it works - Enter a one-letter amino acid sequence; uppercase and lowercase letters are accepted. - Spaces and the separators *, - and . are ignored when the sequence is read. - The calculator sums average residue masses and adds the mass of H₂O to represent the complete chain. - The JSON result includes molecular weight in Da and kDa, residue count, and mean mass per residue; ambiguity codes are handled and reported. ## Use cases - Estimate the expected mass of recombinant proteins before laboratory analysis. - Calculate the contribution of a short affinity tag when interpreting a tagged construct. - Verify amino acid mass calculations with a sequence containing all 20 standard residues. ## Frequently asked questions ### What does this calculator calculate? It estimates the molecular weight of a polypeptide from its amino acid sequence. ### What units are included in the result? The result includes daltons (Da), kilodaltons (kDa), residue count, and mean mass per residue. ### Are spaces and separators allowed in the sequence? Yes. Spaces and *, - and . separators are ignored, and input is case-insensitive. ### How are ambiguity codes handled? Ambiguity codes such as B, J, O, U, X and Z are not added to the mass calculation but are counted and reported. ### Does the result include modifications or disulfide bonds? No. It represents an unmodified chain and does not account for signal-peptide cleavage, disulfide bonds, post-translational modifications, or tags. ## Related tools - [Amino Acid Composition Analyzer](https://elysiatools.com/en/tools/amino-acid-composition): Per-residue counts and mol%, group tallies, Kyte–Doolittle GRAVY score, and ProtParam MW from a sequence. 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