Peptide Isoelectric Point (pI) Calculator
Enter a single-letter amino acid sequence to estimate the pH at which the peptide carries no net charge — its isoelectric point. The chart shows the full net-charge curve the estimate is drawn from, not just the final number.
Single-letter amino acid codes only (A–Z, standard 20).
Results
- Isoelectric point (pI)
- —
Charge vs pH
How this calculator works
Each ionizable group (the N-terminus, C-terminus, and side chains of Asp, Glu, Cys, Tyr, His, Lys, and Arg) contributes charge according to the Henderson–Hasselbalch equation at a given pH. The calculator searches for the pH where those contributions sum to zero.
Different tools use different literature pKa scales, which can shift the reported pI by a few tenths of a pH unit — that is a property of which scale is used, not an error.
How to calculate the isoelectric point of a peptide
- List every ionizable group in the sequence: the free N-terminus, the free C-terminus, and any Asp, Glu, Cys, Tyr, His, Lys, or Arg side chains.
- Assign each group a pKa value from a standard literature scale.
- At a given pH, use the Henderson–Hasselbalch equation to find the fractional charge each group carries.
- Sum the charges across every group to get the peptide's net charge at that pH.
- Search for the pH where that net charge sums to zero — that pH is the isoelectric point.
Worked example: a short peptide made only of glycine, alanine, and leucine has no ionizable side chains, so only its N-terminus (pKa ≈ 9.0) and C-terminus (pKa ≈ 2.0) carry charge. Its pI falls roughly at the midpoint of those two values: (2.0 + 9.0) ÷ 2 ≈ 5.5. Adding an acidic residue like Asp or Glu pulls the pI lower; adding a basic residue like Lys, Arg, or His pulls it higher.
Frequently asked questions
Does this work as a peptide net charge or amino acid charge calculator?
Yes. Finding the pI means computing the peptide's net charge across a range of pH values first — this calculator shows that full net-charge-vs-pH curve, built from each ionizable amino acid's charge contribution, not just the single pI number.
What is a peptide's isoelectric point (pI)?
The isoelectric point is the pH at which a peptide's ionizable groups balance out to a net charge of zero. Below that pH the peptide carries a net positive charge; above it, a net negative charge.
How do I calculate the pI of a peptide?
Identify every ionizable group in the sequence — the N-terminus, C-terminus, and the side chains of Asp, Glu, Cys, Tyr, His, Lys, and Arg — then use the Henderson–Hasselbalch equation to find each group's charge at a given pH, sum them, and search for the pH where that sum is zero. Enter your amino acid sequence above and the calculator runs this search for you.
Is "pI" different from "isoelectric point"?
No — "pI" is simply the standard abbreviation for isoelectric point. They refer to the same value.
Why do different peptide pI calculators give different answers for the same sequence?
Because they use different literature pKa scales for the ionizable groups. That shift is a property of which scale a calculator uses, not a sign that one tool is wrong — see the note below the chart for the scale this calculator uses.
Does this calculator need the full protein structure?
No — it works from the amino acid sequence alone, using standard pKa values for each residue type. It does not account for pKa shifts caused by three-dimensional folding or the local chemical environment, so treat the result as a first-pass estimate.
Is this isoelectric point calculator a substitute for experimental determination?
No. It's a computational estimate for research and study purposes. Techniques like isoelectric focusing remain the standard way to experimentally determine a peptide's actual pI.
Related calculators
Educational tool only, not medical advice — verify with a licensed clinician. See the full medical disclaimer.