Try the handy tool below to calculate concrete yards for your project. Not only does it calculate yardage, but it will also give you the number of 40, 60 and 80 lb bags for those smaller jobs.
When pouring concrete, you should always order at least 10% more than the exact amount the project calls for. Just take the cubic yardage figure and multiply it by 1.10. It's better to have too much versus running short. It's a real pain in the you know what to run out and have to order more. You will probably get charged extra delivery fees and fuel charges. So save yourself some money by making sure you have enough.
Also, keep in mind that most ready mix companies will deliver a minimum of 1 yard and will take orders in 1/4 yard increments above the 1 yard. As an example, if you have a 20x20 slab that's 4 inches deep, the calculator will show 4.94 cubic yards needed. Then add 10% and you get 5.4 yards. Using the closest 1/4 yard increment, you want to order 5 1/2 yards. This is assuming that the subgrade is consistently 4 inches thick or as close as possible.