Accurately measuring the amount of concrete you need is essential to any building project--running low on concrete can throw you off schedule, and having too much left over can sink your budget. Luckily, measuring the amount of concrete required demands only some simple multiplication and division.
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.