Here's a simple, easy to use concrete volume calculator to figure the amount of cubic yards or cubic meters of concrete you will need for your project.
Enter your dimensions into the boxes and change to the metric system if you need to. Always check your thickness after the dirt has been graded. If you're supposed to have a 4 inch thick slab and it's actually 4.5 inches thick, that's a difference of almost 1 yard of concrete for a 24 foot x 24 foot slab.