Section Modulus
In the formula
$(f_b)_{max} = \dfrac{Mc}{I} = \dfrac{M}{I/c}$
the ratio $I/c$ is called the section modulus and is usually denoted by $S$ with units of mm3 (in3). The maximum bending stress may then be written as
$(f_b)_{max} = \dfrac{M}{S}$
This form is convenient because the values of $S$ are available in handbooks for a wide range of standard structural shapes.