How to Calculate Concrete Required
Q. I’d like to build a backyard concrete patio to replace a rotting deck. The patio would be about 18 by 10 feet. How do I figure out how much concrete to order?
A. Concrete is sold by the cubic yard, and there’s no real secret to determining volume. Like anything else, it’s measured as length times width times height.
Remember, 1 cubic yard is 3-by-3-by-3 feet, or 27 cubic feet. And slabs are generally poured to 4-inch (0.33-foot) or 6-inch (0.5-foot) depths. It sounds like you know your length and width, so for a 6-inch-thick slab, you get 18 x 10 x 0.5 feet = 90 cubic feet. 90 divided by 27 is 3.33. So you would need to order 4 cubic yards of concrete for the slab.
By the way, if you’re doing this yourself, you should also use a gravel or crushed-rock base, rebar reinforcement, and provide an expansion joint against the foundation. And don’t forget to slope the slab away from the house.