Measure the dimensions of your site--length, width, depth--in feet. These measurements are all that are required for a typical site, ie. slabs, but take all relevant measurements for irregular shapes. Measure twice, to ensure accuracy.
Multiply together the length, width and depth--this will determine your site's volume, in cubic feet. Irregularly shapes sites, however--spheres, cones, and so on - have different formulas for determining their volume.
Divide the number of cubic feet by 27; this converts your figure to cubic yards.
Repeat Steps 2 and 3 once or twice more, to double-check your math. A mistake in multiplication could easily leave you coming up short, or overwhelmed with excess concrete.
Multiply your final figure by 1.05, to determine 105% of the concrete minimally required. You should have this extra 5% of concrete to account for waste, spillage, over excavation, or mistakes.