What this calculator does
Isoelectric point (pI) works out pH at which a molecule such as an amino acid carries no net electric charge. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulapI = (pKa1 + pKa2) / 2, averaging the two pKa values that flank the neutral (zwitterionic) form
The inputs explained
| Field | What to enter |
|---|---|
| Lower pKa (e.g. α-carboxyl group) | A number. Starts at 2.34. |
| Upper pKa (e.g. α-amino group) | A number. Starts at 9.6. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with lower pka (e.g. α-carboxyl group)
Every other input is held at the calculator’s starting values while lower pka (e.g. α-carboxyl group) varies. Select any row to load that scenario into the calculator.