Calculating the section modulus

To calculate the section modulus, the following formula applies:

An equation: Z = I /y.  

where I = moment of inertia,
           y = distance from centroid to top or bottom edge of the rectangle, here d/ 2. 

For symmetrical sections the value of Z is the same above or below the centroid.

For asymmetrical sections, two values are found: Z max and Z min.

To calculate the value of Z for a simple symmetrical shape such as a rectangle:

An equation: Zxx = I xx/y.  where An equation: Ixx = bd3/12 mm4.

and y=d/2

This gives the formula for  Z = bd2/6 mm3.