Theoretical thickness = (C/S Area of the section ) / (Semi perimeter in contact with the atmosphere.)

Where, semi perimeter = perimeter /2

For example, Consider a 150 mm side cube, and assume these three sides are exposed to the atmosphere

Then,

Thus the theoretical thickness for the given cube and given exposure conditions is 100 mm.

**Where is it used?**

Theoretical thickness is generally used in the study of creep or drying shrinkage of a structure. During dry shrinkage, the water is expelled outside of the concrete. As a result, the water travels a path. This path is expressed in terms of theoretical thickness.