Isoelectric Point Calculator
Calculate the isoelectric point of an amino acid from its pKa values.
Which two pKa values to average
For a neutral side chain, average the carboxyl and amino pKa values. For an acidic side chain, average the two acidic groups; for a basic one, average the two basic groups. Choosing the wrong pair is the most common error, and it moves the answer by several pH units — which matters, because proteins are least soluble at their pI and that is exactly how isoelectric precipitation works.
What this page needs from you
This page asks for pka₁ (carboxyl), pka₂ (amino) and pka of the side chain (0 if none). From those it returns isoelectric point (pi), together with simple average of pka₁ and pka₂, charge at ph 7, difference between the pka values and net charge is zero at. Every figure updates as you type, and nothing you enter leaves your browser.
Where lab results and calculated results part company
These relations describe ideal behaviour: ideal gases, complete reactions, fully dissociated strong acids, no side products. Real yields fall short, real gases deviate near condensation, and real solutions show activity effects at higher concentrations. The calculated figure is the target to compare a measurement against, not a prediction of what the flask will contain.
The formula
pI = the average of the two pKa values either side of the neutral species
Every result on this page comes from this formula. Nothing is rounded until the final display, so figures match a spreadsheet to the cent.
How to use the Isoelectric Point Calculator
- Fill in each field with your own figures — the defaults are only there as an example.
- The result updates as you type, so there is no button to press.
- Check the breakdown under the main result for the supporting numbers.
- Use the copy button to take the result with you, or share the link to reload the same inputs.
Isoelectric Point Calculator — frequently asked questions
Is this isoelectric point calculator free to use?
Yes. Every calculator on the site is free, needs no sign-up, and runs entirely in your browser, so your figures never leave your device.
How accurate is the result?
The maths is exact for the formula shown on the page. Accuracy in the real world depends on your inputs and on assumptions the formula cannot see, so treat the output as a well-grounded estimate rather than a quote.
Can I use it on my phone?
Yes. The page is responsive, weighs a few kilobytes and works offline once loaded, so it behaves the same on a phone as on a desktop.
What people search for
This page answers each of these searches. They are the exact phrases visitors use to find the isoelectric point calculator.
- isoelectric point calculator
- amino acid charge calculator
- isoelectric point formula
- pka to pi
- pi calculator amino acid
About this calculator
Written and reviewed by The Samsung Calculator editorial team. Every calculator is written against a published formula, reviewed against at least one independent reference implementation, and dated when it changes.
Last reviewed September 14, 2026 · Calculation runs client-side · No data collected · Report an error