How to calculate concrete yards is the first sum on any pour, and the one most often done in the truck's favor: the yard is a cubic yard, 27 cubic feet, and the slab is measured in feet and inches, so the thickness has to be turned into feet before it is multiplied. Length times width times thickness, all in feet, gives cubic feet; divide by 27 and that is yards of concrete before waste. A yard of concrete is that cube, delivered by the supplier at a price per yard, and the contractor who orders short pays for a second truck and a cold joint. This page works the sum on the hub's own example, an 18 by 18 foot slab at 6 inches, says how waste is added and how bags compare, and the free concrete calculator on this site does the arithmetic from your own dimensions and your supplier's price, with no account.
The formula for concrete slab volume, in yards
Cubic yards equals length in feet times width in feet times thickness in feet, divided by 27. The thickness is the step people miss: 6 inches is 0.5 feet, 4 inches is 0.333 feet, 5 inches is 0.4167 feet. On the worked example, 18 feet by 18 feet is 324 square feet; times 0.5 feet is 162 cubic feet; divided by 27 is 6.0 cubic yards. The same slab at 4 inches is 108 cubic feet, 4.0 yards. A footing is worked the same way as a long thin slab: 60 feet by 1.5 feet by 1 foot is 90 cubic feet, 3.33 yards. Write the three dimensions in feet first and the division by 27 does the rest; that is the whole of how to calculate concrete yards.
Waste, and why the order is not the formula's answer
The formula gives the geometry; the order needs an allowance for an uneven subgrade, over-excavation at the edges, the concrete that stays in the chute and the truck, and the spill at the forms. The hub's calculator takes that as a percentage the contractor sets, 10% by default: 6.0 yards becomes 6.6, and the supplier rounds to the quarter or half yard they sell, so the order is 6.75 or 7. On a tight pour with a graded and compacted base, 5% may do; on a footing dug by hand, 15% is not unusual. The point is that the allowance is yours and written down, so the next pour is ordered from the same rule. The rules on concrete work at 29 CFR 1926 Subpart Q, linked below, cover the formwork and the placing; they do not tell you the waste, and neither does the supplier.
How many bags of concrete in a yard, and when bags make sense
A bag of premixed concrete is sold by weight and yields a stated volume on the bag, so the count per yard is 27 cubic feet divided by the yield per bag. At a yield of 0.6 cubic feet per bag, that is 45 bags to a yard; at 0.45 cubic feet, 60 bags. On the worked slab, 6.6 yards of bagged mix is between 300 and 400 bags, which is the arithmetic that says why a slab that size comes on a truck. Bags make sense for a post hole, a small pad or a repair where the truck's minimum load and delivery fee would cost more than the bags; the calculator on this site shows the yards, and the bag count follows from the yield printed on the bag you can buy.
From yards to the cost of the pour
Once the yards are known the pour is priced from the supplier's quote per yard, which the calculator takes as an input rather than guessing: 6.6 yards at $150 a yard is $990 of concrete before the short-load fee, the pump if one is needed, and the finishing. That figure goes into the estimate as a line the owner can see, and later into the schedule of values as the slab's value for the draw. The American Concrete Institute and the Portland Cement Association publish the specifications the mix is ordered against; the arithmetic that turns the slab into an order and a price is the calculator's, from your own dimensions and your own supplier's price.
Questions people ask about how to calculate concrete yards
What is a yard of concrete?
A cubic yard: 27 cubic feet, the unit the ready-mix supplier sells and prices by. An 18 by 18 foot slab at 6 inches is 162 cubic feet, 6 cubic yards, before the waste allowance.
How do you figure out yards of concrete for a slab?
Length times width times thickness, all in feet, divided by 27. Turn the thickness into feet first (6 inches is 0.5 feet), then add your own waste allowance, 10% on the hub's example, and round up to what the supplier sells.
How many bags of concrete are in a yard?
27 cubic feet divided by the yield printed on the bag: 45 bags at 0.6 cubic feet per bag, 60 at 0.45. A slab of several yards is truck work; bags suit post holes, small pads and repairs.