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Isoelectric Point Calculator

Estimate the theoretical isoelectric point of a peptide or protein, the pH where its modeled net charge is zero. Use the estimate to compare unmodified sequences under the same pKa model, and keep the model name with any result you copy.

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Sequence input

Paste plain text or one FASTA record with up to 10,000 standard amino acid residues and 100,000 input code units. Calculation runs in your browser.

Results

Calculate a valid sequence to see its theoretical isoelectric point.

How the isoelectric point is calculated

The Henderson–Hasselbalch equation estimates fractional charges for both termini and seven ionizable side chains. Bisection finds the pH where those charges sum to zero. Results display two decimal places.

Read the calculation method →

Worked example

PETER

Theoretical pI 4.53 with the Bjellqvist parameters in Biopython 1.86.

Limitations

  • One linear, unmodified chain with free N and C termini and reduced cysteines.
  • Fixed pKa values approximate independent ionization. Structure, solvent conditions, and neighboring charges can shift experimental pI.
  • Treat short-peptide estimates with care because sequence context can have a large effect on ionization.
  • A root above pH 14 is labeled as a mathematical extrapolation.

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