How do you calculate the building column size??

Usually final size will be obtained based on iteration process.

To start with, columns can be given preliminary dimensions for analysis. After arriving at analysis results i.e., Axial forces and moments, columns can be designed to arrive at area of reinforcement required. If percentage of reinforcement is within limits prescribed by codes, section can be considered as final.

Guidelines to choose column dimensions:

1. In rectangular or or square columns, one side will be usually equal to width of the wall usually 230mm or 300mm

2. Other side will be usually provided based on form work available usually 230mm, 300mm, 375mm, 450mm, 600mm