How to use the isoelectric point calculator
Prepare the values for this calculation
Estimate the isoelectric point and net charge of an unmodified peptide/protein sequence using the published EMBOSS pKa set. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- One-letter amino-acid sequence: use the same separators and value order as the example in the input table. Replace the example data with your own complete dataset.
- Also report net charge at pH: enter a value at least 0 and no more than 14. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Estimated isoelectric point first. With the unchanged example inputs, it is 3.424712. Compare the additional metrics with the primary result; each has its own label and unit.
Compare and keep a useful result
The result diagram shows the inputs, method and output. A numeric chart is shown only when the calculation supports a meaningful numeric relationship.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Isoelectric Point Calculator formula
Net charge is the Henderson–Hasselbalch sum of the N-terminus, C-terminus and ionizable C/D/E/H/K/R/Y side chains. pI is the pH where the modeled net charge crosses zero, found by bisection.
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| One-letter amino-acid sequence | Text / numeric list | PEPTIDE |
| Also report net charge at pH | minimum 0; maximum 14 | 7 |
Worked example
Using the example inputs above, the estimated isoelectric point is 3.424712.
- Estimated isoelectric point
- 3.424712
- Modeled net charge at pH 7
- -3.020797
- Sequence length
- 7
- Ionizable groups modeled
- 5
Results and assumptions
Uses the EMBOSS default pKa set for free termini and C/D/E/H/K/R/Y side chains. Sequence context, post-translational modifications, blocked termini, disulfides, ionic strength and conformational effects are not modeled.
Sources and method references
Fix an input or unexpected result
Also report net charge at pH is required. Enter also report net charge at ph using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.