Isoelectric Point Calculator

Work out isoelectric point instantly with clear inputs, formula shown and shareable results.

About Isoelectric Point Calculator

Sums the Henderson-Hasselbalch charge contributions of ionisable side chains and termini, then finds the pH at which net charge is zero.

Formula

Net charge and pI

Charge(pH) = sum of basic groups / (1 + 10^(pH - pKa)) - sum of acidic groups / (1 + 10^(pKa - pH)); the pI is the pH where the sum is zero

Worked example

With the default values below, Isoelectric Point Calculator returns Estimated isoelectric point of 5.451. Change any input above and every figure here is recalculated in your browser.

Inputs used

Aspartate and glutamate residuesaccepts 0–500: 10
Lysine and arginine residuesaccepts 0–500: 8
Histidine residuesaccepts 0–200: 2
pH for the charge calculationaccepts 0–14: 7

Results

Estimated isoelectric point: 5.451 pH
Net charge at the chosen pH: -1.813
Net charge at pH 7: -1.813
Basic minus acidic residues: 0

Step by step

1Values used
2Net charge and pI
3Estimated isoelectric point= 5.451 pH
4Net charge at the chosen pH= -1.813
5Net charge at pH 7= -1.813
6Basic minus acidic residues= 0

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Which pKa values are used?

Approximate side-chain values: aspartate and glutamate 4.1, histidine 6.0, lysine and arginine 11.5, with termini at 3.6 and 9.1.

Why does the real pI differ?

Local structure shifts side-chain pKa values by a unit or more, so sequence-based estimates are approximate.