disk space usage

I w'd like to know how the DB growth impacts the actual disk
space. For example, I have a table with one textfield,
char(20) and I insert 1000000 records. My DB size w'd
be approximately,
20+28(Overhead)=48*1000000=48MB. What is the actual
effect on the physical HDD. I w'd appreciate your
help.
Thanks,
Uma