Concrete Calculator: How Much Concrete Do I Need?
Enter your slab, footing, or post-hole dimensions for an instant estimate in cubic yards and the number of 60 lb or 80 lb bags to buy. Handles rectangular pours and round columns, with a waste margin built in — no sign-up, no guesswork.
Concrete Calculator
Enter your dimensions and the calculator works out the volume in cubic yards and the number of 60 lb and 80 lb bags you'll need — with a waste margin built in.
How to use this concrete calculator
Pick what you're pouring. For a slab, footing, or wall, enter the length and width in feet and the thickness in inches (4 inches is standard for a slab). For a column or post hole, switch to the round option and enter the diameter in inches, the depth in feet, and how many you're pouring. The calculator returns cubic yards plus 60 lb and 80 lb bag counts.
Bags vs. ready-mix delivery
The big decision is bagged concrete you mix yourself versus ready-mix delivered by truck. Under about 1 cubic yard, bags are practical and cheaper for small jobs. Past a cubic yard — that's already around 45 bags of 80 lb mix — hand-mixing becomes a serious workout and ready-mix usually wins on cost, quality, and your weekend. The calculator tells you which side of that line you're on.
How many bags per cubic yard?
A cubic yard is 27 cubic feet. Because an 80 lb bag makes about 0.6 cubic feet, it takes roughly 45 bags to fill a cubic yard. A 60 lb bag makes about 0.45 cubic feet, so you'd need about 60 bags. Those numbers add up fast, which is exactly why the bags-vs-truck crossover matters.
Concrete for post holes and columns
Setting fence posts or deck footings? Use the round option. The volume of each hole is pi × radius² × depth, and the post itself displaces some concrete, so the estimate is slightly conservative (in your favor). For fence lines, count your posts and enter the total — fast-setting bagged mix is usually the right choice for holes.
Why a waste margin matters with concrete
Concrete is unforgiving: once the truck leaves or your bags are mixed, you can't easily add more without a visible joint or a weak spot. Subgrade is never perfectly flat, holes get over-dug, and some mix is lost to the wheelbarrow and the ground. A 5–10% over-order is standard practice — and already included in your result.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick is about 33 cubic feet, or roughly 1.2 cubic yards. With a 10% waste margin that is about 1.4 cubic yards — enough that ready-mix delivery is usually more economical than bags.
How many bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so it takes roughly 45 bags per cubic yard. A 60 lb bag yields about 0.45 cubic feet, or about 60 bags per cubic yard. The calculator gives you both bag counts.
How much concrete do I need per fence post hole?
Choose the round option and enter the hole diameter and depth. A common 4-inch post in a 10-inch-wide hole dug 3 feet deep needs roughly 1.6 cubic feet of concrete each — about 3 of the 80 lb fast-setting bags per hole. Multiply by the number of posts.
Should I buy bags or order ready-mix?
Under about 1 cubic yard, bagged concrete you mix yourself is practical. Above 1 cubic yard, mixing dozens of bags by hand is hard work, so pricing out ready-mix delivery usually wins on both cost and quality. The calculator flags the crossover point.
How much extra concrete should I order?
Add about 5–10%. Subgrade is never perfectly level, holes get over-dug, and some mix is always lost to spillage. Running short in the middle of a pour means a cold joint or a weak slab, so a small over-order is cheap insurance. The calculator includes this margin.
How do I calculate concrete for a round column or footing?
Switch the calculator to the round option and enter the diameter in inches and the depth in feet. It uses the volume of a cylinder (pi × radius² × height) and multiplies by how many you are pouring.
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