Work out exactly how much concrete a job needs before you load the truck. Choose a slab, footing, Sonotube column, post hole, set of stairs, or curb and gutter, type the dimensions the way you would on a tape measure, and the calculator returns cubic yards, cubic feet, and cubic metres along with the number of 40, 50, 60, and 80 lb bags — rounded up, with your waste allowance built in. Post holes subtract the post itself, so you do not overbuy.
Pouring a slab? Plan the base first with the gravel calculator. Setting posts for a new fence? The fence calculator counts posts, rails, and pickets, and the unit converter handles any other conversion. Everything runs in your browser.
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Standard concrete mix yields about 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb bag, 0.375 per 50 lb bag, and 0.30 per 40 lb bag (Quikrete and Sakrete published yields). That works out to 45 bags of 80 lb mix, 60 bags of 60 lb mix, 72 bags of 50 lb mix, or 90 bags of 40 lb mix per cubic yard. The calculator always rounds bags up, because a partial bag still has to be bought, and it applies your waste allowance before dividing so the count covers spillage and uneven ground.
How thick should a concrete slab be?
Four inches is the usual thickness for sidewalks, patios, shed pads, and other slabs that carry foot traffic. Garage floors and slabs that see car traffic are commonly poured at five inches, and driveways that carry trucks, RVs, or heavy equipment at six inches, often with rebar or wire mesh. Thicker is not automatically better — the subgrade matters as much as the slab, so compact a gravel base under it. Always follow local code and any engineer's plans for structural slabs, footings, and anything that supports a building.
Should I use bags or order ready-mix concrete?
Bags make sense for post holes, small pads, steps, and repairs — anything under roughly one cubic yard. Past that point you are lifting and mixing 45 or more 80 lb bags per yard, which is several tons of material and hours of mixing, and batches that set at different times leave cold joints. A ready-mix truck delivers a consistent, stronger mix in one pour. Most plants have a one-yard minimum and charge a short-load fee for small orders, so enter both prices and the fee to see which is cheaper for your job.
How much concrete do I need for a fence post?
A common rule of thumb (Quikrete's fence-post guidance) is a hole about three times the post width and one-third to one-half of the post's above-ground height deep — deeper still where the ground freezes, so the footing sits below the frost line. The calculator takes the hole as a cylinder and subtracts the volume of the post that sits in it. Remember that a 4×4 post actually measures 3½ inches and a 6×6 measures 5½, so use the actual size. A 10-inch hole 3 feet deep around a 4×4 needs about 1.4 cubic feet, or three 80 lb bags.
How much extra concrete should I order?
Add 5 to 10 percent. Forms bow outward under the weight of wet concrete, subgrades are never perfectly flat, some mix sticks to the wheelbarrow and mixer, and running short mid-pour is far worse than having a little left over. Use the low end for small, well-formed pours like Sonotubes and the high end for slabs over rough ground or excavated footings. Running short on a ready-mix order means paying for a second delivery, so contractors typically round up to the next quarter yard as well.