How is Isoelectric Point computed?

The isoelectric point, pI, is the pH at which the net charge of the peptide is zero. We first computes the net charge for pH 7.0.
If the charge is > 0, the next pH to check is 7 + 3.5.
If the charge is < 0 then pH 7 - 3.5 is checked.
This is repeated, using increments/decrements half the size of the previous, until the modulus of the charge found is less than or equal to 0.0001.
pKa values used can be chosen between the following two sources with ExPASy being the default one.

1) ExPASy - ProtParam 1


2) A.Lehninger, Principles of Biochemistry, 4th Edition (2005), Chapter 3, page 78, Table 3-1. 2