How much concrete do you need? Careful measurement is the first step. Often your dimensions may vary and accurate measurement is not possible. If this is the case, measure as best as you can and take your best guess on what the average thickness or width is.
Concrete is sold by volume. In this country that is by cubic yards. The easiest way to come up with how many cubic yards you need is to convert all dimensions to feet. Multiply length x width x height to get cubic feet and then divide by 27, the number of cubic feet in one cubic yard. A typical sidewalk 30 feet long, 4 feet wide and 4 inches (1/3 foot) thick would be 30 x 4 x 1/3 = 40 cubic feet divided by 27 = 1.5 cubic yards. If you are pouring on top of crushed stone remember to allow for some of the concrete to penetrate the stone base, more so if the stone is loose and less if the stone is well compacted or has a layer of plastic vapor barrier over it.
If you are working with a complicated shape, break it down into smaller sections, do the math for each section and then total up all the sections.
For your convenience, below is a link for a website with a concrete calculate. You simply input the dimensions for the job you want to pour.