How do I calculate total hours worked from Time in/Time out?

I am tracking hours for work using an excel sheet. Currently I input my time-in in column C and Time out in Column D. I have Column E set to calculate total hours for each day using "=HOUR(Dx-Cx)" but it rounds the hours (time-in: 9:00, time-out: 15:15, total hours calculated: 6). How do I formulate it to include the fractions of hours (6.25 instead of 6). I have already tried formatting it with extra decimals but that doesn't change anything.