Concrete Calculator
How many bags for a slab or post holes — includes 10% extra.
4" is standard for patios and walkways
How many bags for a slab or post holes — includes 10% extra.
4" is standard for patios and walkways
How many bags of concrete a slab or a set of post holes actually takes — including the extra you should order, because running short mid-pour is a genuine disaster.
A slab's volume is length × width × thickness, converted to cubic feet and then to cubic yards by dividing by 27. Post holes are cylinders — π × radius² × depth — with the volume of the post itself subtracted, since the post displaces concrete. Ten percent is added to whatever you order, and the total is converted into bags using the yield printed on the bag (an 80-pound bag makes about 0.6 cubic feet).
Example
A 10 × 10 foot slab, 4 inches thick, is 33 cubic feet — about 1.4 cubic yards with waste, or roughly 60 eighty-pound bags.
About 60 eighty-pound bags for a 4-inch-thick slab, including a 10 percent overage. At that quantity it is worth pricing a ready-mix delivery.
About 45. An 80-pound bag yields roughly 0.6 cubic feet, and there are 27 cubic feet in a cubic yard.
Concrete for anything that takes a load or catches wind — gates and fence posts especially. Gravel drains better and is fine for light-duty posts, but it does not hold nearly as well.