What is a yard of concrete is the first question a homeowner asks and the last one a contractor thinks about, because the yard is the unit the whole trade runs on: a cubic yard, a cube three feet on each side, 27 cubic feet, the volume the supplier sells, the truck carries and the calculator reports. This page says what that volume covers at the common thicknesses, how many yards of concrete a slab, a footing or a pier takes, how the yard is bought and what the ticket says, and why a pour is never ordered at exactly the calculated yards. The concrete calculator on this site works any shape into yards; the concrete pad calculator and the price calculator work a pad and its delivered price.
The volume: 27 cubic feet, and what it covers
A cubic yard is 3 feet by 3 feet by 3 feet, 27 cubic feet. Spread at 4 inches, a third of a foot, it covers 81 square feet; at 6 inches, half a foot, 54 square feet; at 8 inches, about 40. So a yard is a 9 by 9 foot patch at 4 inches or a 6 by 9 patch at 6 inches, which is a useful picture when the calculator says 6 yards: the 18 by 18 pad on this site at 6 inches is six of those 6 by 9 patches. The construction standard's concrete subpart, linked below, governs the work of placing it; the unit itself is simple geometry.
How many yards of concrete a shape takes
Length times width times thickness, all in feet, divided by 27, is the yards for a slab; the same product for a footing's length, width and depth; the area of a circle times the depth for a pier. The 18 by 18 pad at 6 inches is 18 times 18 times 0.5, 162 cubic feet, 6 yards. A 100 foot footing 2 feet wide and 1 foot deep is 200 cubic feet, 7.4 yards. A pier 2 feet across and 4 feet deep is about 12.6 cubic feet, under half a yard. The concrete calculator on this site works each shape and adds them for a foundation with all three, and the page on how to calculate concrete yards works the slab in detail.
How the yard is bought: the ticket, the mix and the increment
Concrete is ordered by the yard for a stated mix and strength, and the ticket that comes with the truck states the yards loaded, the mix, the water added and the time batched. Suppliers sell in increments, commonly the quarter yard, and have a minimum load below which a short-load charge applies. A yard is also what the calculator's waste is added to: a pour is ordered at the calculated yards plus a stated allowance for the subgrade and the forms, so 6 yards is ordered as 6.6 and rounded to the supplier's increment. The requirements for cast-in-place concrete, linked below, are what the crew works to once the yard is on site.
What a yard weighs, and why the ticket says
A yard of concrete is heavy, and how heavy depends on the mix's aggregate and water, which is why the weight per yard is stated on the supplier's mix design rather than assumed: a lightweight mix and a standard mix are different loads on the same truck. A contractor planning a pump, a crane bucket or a wheelbarrow route reads the weight per yard from the ticket or the mix sheet. The concrete price calculator on this site works what the yard costs delivered; this page's job is what the yard is, and the answer is 27 cubic feet of whatever mix was ordered, sold by the ticket.
Questions people ask about what is a yard of concrete
How big is a yard of concrete?
A cube three feet on each side, 27 cubic feet. At 4 inches thick it covers 81 square feet, at 6 inches 54, at 8 inches about 40.
How many yards of concrete do I need for a slab?
Length times width times thickness in feet, divided by 27, plus a waste allowance. An 18 by 18 slab at 6 inches is 6 yards, ordered as 6.6 and rounded to the supplier's increment.
How is concrete sold?
By the cubic yard for a stated mix and strength, in the supplier's increment, with a minimum load and a short-load charge below it. The ticket states the yards loaded, the mix, the water and the time batched.