For Menswear, you need to measure your around chest and refer to the available size chart in the desired item page. It is important to refer to the size chart because there might be variations in he same size based on the design and type of item you are opting for.
Example: If your around chest is 36 inches, you must select size 36 of the desired item.
If you refer to the size chart, for size 36, you will find the top chest as 40 inches. This extra room of 4 inches is provided for a comfort fit.