# Protein Isoelectric Point Calculator (pI by Composition) Theoretical pI via the simplified ExPASy/Bjellqvist pKa set with bisection to zero net charge, plus net charge at pH 7.4, ProtParam average molecular weight, and charged composition. Bjellqvist 1993 / Gasteiger 2003 derived. Educational use only. > Canonical page: https://elysiatools.com/en/tools/protein-isoelectric-point - **Category:** Education - **Keywords:** protein isoelectric point calculator, pi calculator protein, theoretical pi expasy, bjellqvist pka values, net charge at ph, protein molecular weight sequence, acidic basic protein classification, ion exchange chromatography ph, isoelectric focusing, molecular biology, lab calculator ## Overview Calculate a protein’s theoretical isoelectric point (pI) from its amino acid sequence, along with net charge at pH 7.4 and average molecular weight. The calculator uses a simplified ExPASy/Bjellqvist pKa set and is intended for educational use. ## Inputs - **Protein Sequence (one-letter codes)** (textarea): One-letter amino-acid codes, 20 standard residues only (FASTA headers, whitespace, and '*' are stripped). - **Decimal Places** (number) ## When to use - Estimate whether a protein sequence is acidic or basic from its theoretical pI. - Check the approximate net charge of a protein at pH 7.4. - Calculate the average molecular weight of a sequence before planning educational or laboratory workflows. ## How it works - Enter a protein sequence using the one-letter codes for the 20 standard amino acids. - FASTA headers, whitespace, and asterisks are removed before calculation. - The calculator estimates net charge for ionizable groups using the simplified ExPASy/Bjellqvist pKa values. - It uses bisection to find the pH where the calculated net charge reaches zero, then reports pI, pH 7.4 charge, and average molecular weight. ## Use cases - Classify protein sequences as relatively acidic or basic using their theoretical pI. - Compare peptide or protein charge behavior at pH 7.4. - Estimate sequence-based molecular weight for molecular biology coursework and early experiment planning. ## Frequently asked questions ### What sequence format does the calculator accept? Enter a sequence using one-letter codes for the 20 standard amino acids. FASTA headers, whitespace, and asterisks are stripped. ### What does the pI result mean? The pI is the estimated pH at which the protein’s calculated net charge is zero. ### What additional results are provided? The calculator also reports net charge at pH 7.4, average molecular weight in daltons, and an acidic or basic interpretation. ### Can I change the number of decimal places? Yes. Set Decimal Places from 0 to 8; the default is 2. ### Are the results suitable for experimental decisions? They are theoretical estimates based on an unfolded-chain pKa model and exclude post-translational modifications, so they are intended for education and preliminary planning. ## Related tools - [Enzyme Activity Unit Converter (U ↔ katal)](https://elysiatools.com/en/tools/enzyme-unit-converter): Converts activity values exactly between U/mU/µU and katal/mkat/µkat/nkat/pkat (1 U = 16.67 nkat, 1 kat = 6×10⁷ U). 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