Here is a great concrete yardage calculator for estimating how much concrete to order to complete your job. Just remember to add some extra, maybe a half yard extra, because running short is not an option.
When you calculate concrete for your job it's important to check the level of the sub-grade and come up with an average thickness. If the dirt isn't perfectly level you're going to have some areas thicker and some thinner than you expected.
Most of the time when I calculate cubic yardage, the dirt grade is rarely level. Using a laser level I can easily determine the average thickness of a concrete floor. I usually spray paint the thickness onto the sub-grade every four to six feet until I have covered the entire area. Then I average those numbers together to get my average concrete thickness.
On most basement concrete floors, I try to get as close to a 4 inch average as possible. Sometimes I need to rake out the dirt grade a little, I don't want any areas less than 3.5 inches thick.
Gravel is much less expensive than concrete, try to get the sub-grade level by using a laser. This also makes it easier to calculate the amount of concrete you need to order from the ready mix concrete company.
If you make a mistake and don't order enough concrete, just call the concrete dispatcher and he will send another concrete truck with the balance on it.
Just be prepared to wait if the concrete company is busy and doesn't have a truck waiting. I order anywhere from a half yard to a full yard extra depending on how the sub-grade is prepared.
Once you know the average thickness, it's a great idea to use a concrete calculator, it takes all the guess work out of figuring how many cubic yards to order. Enjoy!