Isoelectric Point Calculator
The isoelectric point of an amino acid — the pH at which it carries no net charge — from its pKa values, for the twenty standard amino acids or any pKa set you enter.
An amino acid is a zwitterion between the pKa of its carboxyl group and that of its amino group; the isoelectric point is the average of the two pKa values either side of the neutral form.
How the isoelectric point calculator works
An amino acid is a zwitterion between the pKa of its carboxyl group and that of its amino group; the isoelectric point is the average of the two pKa values either side of the neutral form. For neutral side chains that is (pKa₁ + pKa₂) ÷ 2. An acidic side chain (Asp, Glu) moves the neutral form lower, so pI averages the two acidic pKas; a basic one (Lys, Arg, His) averages the two basic pKas.
Formula: pI = (pKa of the group that loses the last positive charge + pKa of the group that gains the first negative charge) ÷ 2
Worked examples
| Inputs | Isoelectric point (pI) | Note |
|---|---|---|
| Glycine | 5.97 | 5.97 |
| Aspartic acid | 2.76 | 2.77 |
| Lysine | 9.74 | 9.74 |
FAQFrequently asked questions
Why is the pI an average of two pKas?
Halfway between the pKa where the molecule loses its last positive charge and the one where it gains its first negative, the two charged forms are equal and cancel; that is where there is no net charge and no movement in an electric field.
Why does cysteine average the carboxyl and thiol pKas?
Its thiol (pKa about 8.2) is an acidic group that ionises before the amino group (10.3), so at the neutral form the flanking pKas are the carboxyl and the thiol — the same logic as aspartate.
Do the pKa values vary between sources?
By a tenth or two: tables differ by measurement conditions. The values here are a widely used biochemistry set. For a peptide or protein, side chains in the sequence shift things further and the pI needs a full titration calculation.
What is pI used for?
Isoelectric focusing and 2-D gels separate proteins by pI; solubility is lowest at the pI, which is used in precipitation; ion-exchange chromatography is run at a pH that gives the protein a known charge.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.