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Use our Isoelectric Point Calculator to find amino acid or protein pI. Learn the formula, follow easy steps, and see clear examples.
| Formula | Meaning |
|---|---|
| Acidic charge(pH) = -1 / (1 + 10^(pKa-pH)) | Group charge approaches -1 above its pKa, 0 below it |
| Basic charge(pH) = 1 / (1 + 10^(pH-pKa)) | Group charge approaches +1 below its pKa, 0 above it |
| Q(pH) = sum of all group charges | Net charge at a given pH |
| pI: Q(pI) = 0 | Found by bisection, not a fixed-category shortcut |
An Isoelectric Point Calculator helps you find the pH at which a molecule has no net charge. This value is called the isoelectric point, or pI. It helps you study amino acids, peptides, and proteins.
You can use our online Isoelectric Point Calculator to get a quick result. Choose an amino acid or enter a peptide sequence, based on the options available. The tool uses pKa values to estimate the pI. It also shows the key steps behind the result.
Think of pI as a balance point. The molecule has both positive and negative charges, but their sum is zero.
The formula depends on the type of molecule.
For an amino acid with no ionizable side chain:
Isoelectric Point (pI) = (pKa1 + pKa2) / 2
For an acidic amino acid, use the two pKa values around its zero-charge form. For a basic amino acid, do the same with the two pKa values around that form.
For a peptide or protein, the calculator can estimate the net charge at different pH values. It then finds the pH where the net charge reaches zero.
This method uses the Henderson–Hasselbalch equation. The result may vary with the pKa values and the molecule's chemical setting.
Follow these five easy steps to find the pI.
Choose a mode. Select an amino acid or peptide mode.
Enter your data. Pick an amino acid or type a valid sequence.
Check the pKa values. If you use custom values, make sure they're correct.
Click Calculate. The tool checks your input and runs the formula.
Read your result. View the pI, net charge check, and key steps.
You don't need to solve each step by hand. The tool does the math for you.
Let's find the pI of glycine. Glycine is a simple amino acid with no ionizable side chain.
Its pKa values are 2.34 and 9.60.
Step 1: Add both values.
2.34 + 9.60 = 11.94
Step 2: Divide the sum by two.
11.94 / 2 = 5.97
Result: The isoelectric point of glycine is 5.97.
At this pH, the positive and negative charges balance out. The net charge is zero.
For a peptide, the calculation is more complex. The tool checks the charge of each ionizable group. It then finds the pH where the total charge is close to zero.
Our Isoelectric Point Calculator makes pI calculations easy. It helps you find the pI of amino acids and estimate the pI of peptide chains. You can also review the main formula and key results.
Use the result as a guide for study or lab work. For proteins, the pI can vary with the pKa data and test conditions. Always check the method when you need high accuracy.
The isoelectric point is the pH at which a molecule has zero net charge. It is often shown as pI.
For an amino acid with no ionizable side chain, add the two main pKa values and divide by two. For other amino acids, choose the two pKa values around the zero-charge form.
Yes, if the tool supports protein or peptide input. It can estimate pI from the sequence and the pKa values of its ionizable groups.
No. pH tells you how acidic or basic a solution is. pI is the pH at which a given molecule has zero net charge.
The pI can help you study protein charge, solubility, and separation. It is useful in biochemistry and lab work.
No. Results depend on the pKa values and the calculation method. A predicted protein pI may differ from a lab result.