How do I calculate isoelectric points?

Isoelectric points are calculated differently depending on the type of amino acids. To calculate the isoelectric point (pI) for amino acids with neutral side chains, one must take average the pKa values of their carboxylic acid and ammonium groups. Amino acids with basic side chains have a pI calculated by averaging the pKa values of their 2 least acidic groups. For amino acids with acidic side chains, the pl is calculated by averaging the pKa values of their 2 most acidic groups.

pI = (pka1 + pka2) /2