Although the acid catalyzed and base catalyzed hydrolysis of esters gives transition states for the rate determining steps that have differing charge densities, their structures differ only by two hydrogen atoms.  Taft thus assumed that steric effects would influence both reaction mechanisms equally.  Due to this, the steric substituent constant E was determined from solely the acid catalyzed reaction, as this would not include polar effects.  E was defined as:
where k is the rate of the studied reaction and  is the rate of the reference reaction (R = methyl).  δ is a reaction constant that describes the susceptibility of a reaction series to steric effects.  For the definition reaction series δ was set to 1 and E for the reference reaction was set to zero.  This equation is combined with the equation for σ* to give the full Taft equation.
From comparing the E values for methyl, ethyl, isopropyl, and tert-butyl, it is seen that the value increases with increasing steric bulk.  However, because context will have an effect on steric interactions some E values can be larger or smaller than expected.  For example, the value for phenyl is much larger than that for tert-butyl.  When comparing these groups using another measure of steric bulk, axial strain values, the tert-butyl group is larger.