Isoelectric Point Calculator
Find the pH at which a molecule carries no net charge.
Enter any two of pKa, pKb, or pI to solve for the third.
Updated October 19, 2026
Introduction
A molecule that carries both an acidic and a basic group has one pH at which its charges cancel out completely. That pH is the isoelectric point, written , and this page finds it from the two dissociation constants sitting either side of the neutral form.
The arithmetic is short: the page averages the two constants you enter. . Both boxes take logarithmic constants rather than concentrations, so there is no unit to choose and no conversion to do — the answer is a pH.
Charge is what decides how a molecule behaves in a separation. If you are picking a pH for ion-exchange chromatography so a protein binds or lets go, reading a band from an isoelectric-focusing strip, or precipitating something at the pH where it is least soluble, this is the number you need. Because any two of the three values determine the third, the page also runs the other way: a measured plus one known constant is enough to recover the other one. If the next step is holding a working pH away from that point, the buffer pH calculator works out the buffer that gets you there.
3 boxes, 1 equation
Two constants always determine the third value
Any two of the three
Solve forwards for pI or backwards for a constant
No pH ceiling
Below 0 and above 14 are valid, flagged with a note
Quick start
Type the two dissociation constants
Put the acidic group's constant in the first box and the basic group's constant in the second. Both are logarithmic and dimensionless, so a value like 2.34 goes in as it stands — nothing to convert into a working unit first.
Read the third box
As soon as both constants are in, the box you did not type in fills itself in. The two values you entered stay as you left them; the calculated one is the isoelectric point.
Or start from the isoelectric point you already have
Enter the together with either constant, and the page solves for the remaining one instead. That is useful for checking a table entry, or for recovering a constant you never measured directly.
Follow one calculation through
Glycine is the simplest case: the pair most tables print is 2.34 for the carboxyl group and 9.60 for the ammonium group. Enter both and the page returns exactly the value those same tables list for glycine:
Read the result as a pH, not a concentration: at pH 5.97 glycine carries no net charge, while a more acidic solution leaves it positive and a more alkaline one leaves it negative.
Read the note under the boxes if the answer looks unusual
A result below 0 or above 14 raises a short note rather than an error, because those are real pH values — concentrated hydrochloric acid sits near pH −1 and concentrated sodium hydroxide near 15. The note is a reminder that the pH scale has no hard ends; the answer itself is still the midpoint of the two constants.
Calculation method
One equation does all the work, and every other direction is that same equation rearranged. The is the midpoint of the two constants, which is why the formula is an average rather than a difference or a product.
Halving the sum is the entire calculation. A table pair such as 2.34 and 9.60 returns 5.97 rather than a rounded 6.0, so what you read back carries every digit you typed in.
Solving for either constant
Both are the same rearrangement: double the isoelectric point, then subtract the other constant. Because the three values live on one equation, two of them are always enough — the page simply uses whichever two you filled in.
Choosing the two constants that belong in the boxes
The average is only the isoelectric point when the two numbers bracket the neutral form — the pH region in which the molecule carries no net charge. In a table of amino-acid constants that means the two values immediately on either side of the zwitterion, and a charged side chain takes the place of whichever constant it sits next to. The table below shows the choice, using the page's own arithmetic:
| Amino acid | First box (pKa) | Second box (pKb) | pI the page returns |
|---|---|---|---|
| Glycine — neutral side chain | 2.34 (α-carboxyl) | 9.60 (α-ammonium) | 5.97 |
| Glutamate — acidic side chain | 4.25 (side-chain carboxyl) | 9.67 (α-ammonium) | 6.96 |
| Lysine — basic side chain | 8.95 (α-ammonium) | 10.53 (side-chain ammonium) | 9.74 |
| Histidine — weakly basic side chain | 6.00 (imidazole) | 9.17 (α-ammonium) | 7.585 |
Read that table as a rule, not as data to copy. Published constants differ in the last digit with temperature, ionic strength and measurement method, so the midpoint inherits that spread — but the rule survives it: find the two constants on either side of the pH at which the net charge is zero, and average those two. A third constant you happen to have is simply not used here.
If you want to see how far the charge has actually moved from zero at some other pH, the Henderson–Hasselbalch calculator turns a constant and a ratio of protonated to deprotonated forms into a pH, which is the same comparison read from the other end.
pKa and pKb are two views of one equilibrium
A conjugate acid–base pair describes the same reaction from opposite sides, so the two constants are linked: in water at 25 °C. That means a pKb of 4.40 is a conjugate-acid pKa of about 9.60 — the same equilibrium, a different scale.
Most amino-acid tables already print the basic group's constant on the acid scale (often labelled pKa2). If your source prints a true pKb instead, convert it before typing: . The page averages exactly what you give it and cannot tell which scale a number came from, so mixing the two scales shifts the midpoint without any warning.
Worked examples
An acidic side chain: glutamate
Situation: Glutamic acid has three constants — 2.19 for its α-carboxyl group, 4.25 for the carboxyl group on the side chain and 9.67 for its α-ammonium group. Which two belong in the page? The neutral form is the one that dominates between 4.25 and 9.67, so that is the pair to enter.
Inputs
Result
In practice: pH 6.96 is where glutamate carries no net charge, so it is the pH to aim for in an isoelectric-focusing run or a precipitation step. The 2.19 constant is real, but it describes a region where the molecule is already positive and plays no part in this midpoint.
Running backwards: a focusing result for lysine
Situation: An isoelectric-focusing strip puts your lysine sample at pH 9.74, and your table gives the α-ammonium constant as 8.95. Enter those two values and the page recovers the constant you did not have.
Inputs
Result
In practice: The page returns 10.53, the side-chain ammonium constant. Agreement with your table is a quick transcription check; a large disagreement usually means one of the numbers came from a different source or a different temperature, not that the arithmetic failed.
Reading the decimals: histidine
Situation: Histidine has a weakly basic imidazole side chain, with constants of 6.00 and 9.17. The midpoint does not land on a tidy two-decimal value.
Inputs
Result
In practice: The page keeps the exact midpoint, 7.585, while printed tables round it to 7.59. Enter the pre-rounded pair 6.0 and 9.2 instead and you get 7.6 — a difference of 0.015 that explains most last-digit disagreements between sources. Round at the end, not at the input.
Questions
Why is my isoelectric point negative, or higher than 14?
Because a pH is not capped in either direction. A negative midpoint means both constants are small — a molecule whose groups are strongly acidic, so it is already neutral in a very acidic solution. A value above 14 means both constants are large, and the molecule only sheds its charge in strongly alkaline solution.
Both are legitimate answers, which is why the page shows a note instead of a red warning. If the value surprises you, check the two constants rather than the arithmetic: a constant typed on the wrong scale, or with the wrong sign, moves the midpoint by half of that error.
Can I enter a pKb in the box labelled pKb?
Yes — that is what the box is for — but only once the number is on the acid scale. A pKa and a pKb describe the same equilibrium from opposite sides, and in water at 25 °C they add up to about 14. So a source that reports a true pKb of 4.40 is reporting a conjugate-acid pKa of about 9.60.
Many amino-acid tables sidestep this by printing the basic group's constant as a pKa already, often as pKa2. Check which one your source gives: if it is a true pKb, convert it with before typing it in, so both boxes sit on one scale.
Does this give the isoelectric point of a whole protein?
No. A protein carries many ionizable groups — every aspartate, glutamate, lysine, arginine, histidine and terminal residue contributes — and its isoelectric point is the pH at which all of those charges sum to zero. That is found by sequence-based tools or measured by isoelectric focusing, not by averaging two numbers.
What this page covers is the single acid–base pair: one acidic group and one basic group, or the one pair of a larger molecule that brackets its neutral form. Feed it the right pair and the midpoint rule applies; feed it two constants from different parts of a complex molecule and the average is just a number.
How far to trust it
- •The midpoint rule assumes one acidic and one basic group. Molecules with several ionizable groups — peptides, proteins, polyprotic acids — need the pH where the total charge is zero. Use the pair that brackets the neutral form, and use a sequence-based tool when the whole molecule is the question.
- •Tabulated constants vary. Temperature, ionic strength and the method used to measure a constant all move its value, and the page applies no correction. A 0.1 shift in either constant moves the midpoint by 0.05, so treat the last digit as an indication rather than a measurement.
- •Watch the scale of the second number. The page averages what you type, so an acid-scale constant and a base-scale constant average to something that is not a . Convert a true pKb before entering it.
- •An isoelectric point is not a solubility prediction. A molecule is often least soluble near its , which is exactly why precipitation is done there, but salts, excipients, aggregation and time all change the outcome. Treat the result as the pH worth testing.
- •A value far outside the usual window deserves a second look. Answers below 0 and above 14 are handled correctly, but they are uncommon enough that a misplaced decimal or a mixed scale is the more likely explanation.
Nothing here replaces a measurement. If a purification or formulation decision turns on the , confirm it by isoelectric focusing or titration before you commit to it.
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