Brother, Here you are only calculating the Size of total Root File Directory i.e.
1 block is of 4KB and we are using 4 such blocks so total 16KB.
===> Is the size of total file system = Only size of root file Directory ???
Not the blocks Pointing by the UFDs
Because a file system has :- Boot Block , Super Block , Inode List , Data Block (Unix Structure)
It also contains data blocks right ?? so why aren't we adding that ??? or am i missing somethin'