Concrete is measured in terms of volume (in cubic yards, usually). That means you need to find the total volume for the floor.
To calculate the volume in cubic yards, the easiest thing to do is convert all your dimensions to yards first.
Then for each slab you multiply the length times the width times the depth (all in yards) and you will get a number which is the amount of cubic yards of concrete needed for that slab.
Add up all the slabs and you have your total amount of concrete.
Hope this helps,
Stephen La Rocque.