The pKa table is a crucial tool for any organic chemist when ranking the strength of acids or bases.
Check out this very comprehensive pKa table
The pKa values given in this table represent the -log[Ka] of an acid (pKa = -log[Ka]). The Ka describes the extent to which a specific acid dissociates to its conjugate base. A large Ka represents a strong acid that dissociates easily. SO, mathematically a lower pKa value represents a stronger acid.
If two acids are in equilibrium with a >= 3 pKa unit difference between them, the lower pKa acid will almost completely dissociate. Organic chemists also categorize compounds by their pKa value into four groups: strong acid, weak acid, very weak acid, extremely weak acid. Those groups are described below:
Strong acid: pKa < H3O+
Weak acid: pKa > H3O+ and < H2O
Very weak acid: pKa > H2O and < NH3
Extremely weak acid: pKa > NH3