Use this amino-acid isoelectric point calculator
This amino-acid isoelectric point calculator is designed for students and routine checking. Choose one of the 20 standard amino acids to see the ionizable groups, ordered pKa values, formal charge changes, neutral interval, and calculated pI. The workspace keeps the selected method, known values, result, formula, and substituted arithmetic together so you can see what produced the answer instead of receiving an unexplained number. Change one input at a time when checking homework, and confirm that the direction of change agrees with the stated relationship.
The result uses the model and assumptions stated on this page together with the values shown in the active form. It does not infer values, identities, or conditions that the active method does not ask for. If your course, data sheet, or laboratory method specifies a different reference state or sign convention, use that source consistently before comparing answers.
Amino-acid isoelectric point formula and input guide
The calculation uses pI=(pKa below the neutral form+pKa above the neutral form)/2. pKa, formal charge, pH, and pI are dimensionless. The table makes the two values averaged for the selected amino acid visible instead of applying one pair to every molecule. Inputs that must be positive, integral, dimensionless, or above absolute zero are checked before a result is shown. A hidden default is never used for a selected unknown, and changing a method or known value invalidates the previous result until you calculate again.
Read each visible field label as part of the active formula. A label distinguishes quantities that may share a symbol or unit but use a different basis. Keep every input on the basis named by the selected method, and use the visible substitution steps as a unit audit before accepting the result.
Work through an amino-acid isoelectric point example
Glycine is formally neutral between its carboxyl pKa 2.34 and amino pKa 9.60, so the educational estimate is pI 5.97. The page displays the intermediate scale or correction before the final answer, which makes sign, exponent, and unit mistakes easier to spot. Recalculate the example independently and compare the displayed substitution rather than matching only the last rounded digits.
A useful reverse check substitutes the reported answer back into the original relationship. The reconstructed known quantity should agree at the displayed precision. If it does not, inspect the active fields, unit prefixes, selected method, and stated model limits before recalculating.
Interpret the amino-acid isoelectric point result
The numerical answer describes only the quantity named in the result card under the selected method and its stated assumptions. Do not treat one calculated quantity as proof of a different property, mechanism, or experimental outcome. Preserve the sign and unit when copying the result because removing either can reverse or erase its physical meaning.
Report significant figures that match the least precise input or source constant required by your work. Additional digits are shown when they are needed to keep the visible equation internally consistent, not to claim laboratory accuracy. For consequential research, manufacturing, medical, environmental, or safety decisions, use validated measurements and a method approved for that application.
Model limits, sources, and private use
The data version covers free standard amino acids in an aqueous educational convention. Peptide termini, multiple residues, post-translational modifications, temperature, solvent, and ionic strength change the ionization problem, so this page does not accept or predict protein sequences. The principal reference basis is versioned 2026-09 free-amino-acid aqueous pKa teaching table shown in each result. When the page uses a named correlation or constant, it keeps that source within the range stated here; unsupported inputs are rejected instead of silently extrapolated or completed with invented values. A classroom approximation can be useful without being a universal property prediction.
The values are calculated in the current browser tab and are not uploaded as part of the calculation. No account is required. Copying is a deliberate action after the answer and steps are visible, while Reset restores the example and Clear removes the current result. Refreshing or leaving the page clears the working state, so save only the information you need for later study.