Quick answer: An 80 lb bag of concrete yields about 0.6 cubic feet, so it takes 45 bags to fill one cubic yard. To find how much you need, multiply length Γ width Γ thickness in feet and divide by 27 for cubic yards.
Calculate bags, cubic yards, and cost for slabs, sonotube piers, and footings. Works with Quikrete, Sakrete, 60lb and 80lb bags.
This calculator converts your slab dimensions into cubic feet, then into bags, using the manufacturer's published yield per bag:
Volume (cu ft) = Length (ft) Γ Width (ft) Γ (Depth (in) Γ· 12)
Cubic Yards = Volume (cu ft) Γ· 27
Bags Needed = CEIL[ (Volume Γ· Yield per Bag) Γ (1 + Waste %) ]Yield per bag: 0.60 cu ft for an 80lb bag, 0.45 cu ft for a 60lb bag (per Quikrete/Sakrete published specs).
The most common mistake DIYers make when mixing concrete is adding extra water to make it "easier to pour." Concrete is a precise chemical recipe. The bag is engineered to require exactly 3 to 4 quarts of water per 80lb bag to reach its maximum compressive strength (usually 4,000 PSI).
What happens if you add extra water? Every extra quart of water you add drops the strength of the concrete by hundreds of PSI. It also causes "shrinkage cracking" as the excess water evaporates, and it leads to "spalling" (the surface flaking and peeling off) after just one winter freeze-thaw cycle.
Walking down the concrete aisle, you'll see two main sizes. Here is the exact math to help you choose.
If you pour concrete directly onto dirt, it will crack. Period. Dirt holds water, and when that water freezes in the winter, it expands and pushes the concrete upward (frost heave). When it thaws, the concrete drops back down, snapping in half.
The Fix: You must excavate 6 inches down, and put 2 to 4 inches of compacted gravel base (like "crusher run" or 3/4" minus stone) at the bottom. The gravel allows water to drain away from the bottom of the slab, keeping the ground underneath stable year-round.
Every bag estimate on this page already assumes a 5% buffer for spillage, over-excavation, and uneven subgrade β the same margin a working contractor builds into a material order. Raise it to 10–15% if your site is sloped, your forms aren't perfectly square, or you're a first-time pour.
This calculator is built for rectangular slabs and pads. If you're pouring a round footing, sonotube, fence post, or deck pier, the math is different β cylinders use Ο Γ radiusΒ² Γ depth, not length Γ width Γ depth. Use our dedicated Concrete Footing & Sonotube Calculator to get the exact bag count for round holes instead of guessing.
Concrete sets fast, so getting your order right the first time is critical. Here is how to measure the most common concrete projects.
Yes. Many people search for a βcement calculatorβ when they actually need concrete. Cement is the powder binder, while concrete is the finished mix of cement, sand, and aggregate. This concrete (cement) calculator tells you how many bags or cubic yards of ready-mix concrete you need for slabs, footings, and piers.
For a standard patio, walkway, or shed foundation, 4 inches is the industry standard. If you are pouring a driveway that will hold cars, it must be at least 5 to 6 inches thick, and ideally reinforced with rebar or wire mesh.
For a simple shed pad or small patio, rebar is usually optional if you have a solid gravel base and control joints (cut lines). For a driveway or a slab over unstable soil, you absolutely need #3 or #4 rebar spaced 18 inches apart in a grid.
Concrete sets enough to walk on in 24-48 hours. It reaches about 70% of its full strength in 7 days. However, it takes a full 28 days to reach its maximum structural strength. Do not park heavy vehicles or build heavy structures on it for at least a week.
Never pour concrete while it is actively raining. Rainwater will ruin the surface finish and weaken the top layer. If rain is forecast within 4 hours of pouring, cover the slab with heavy plastic sheeting immediately after you finish troweling.
You can't stop it completely, but you can control it. 1) Use a solid gravel base. 2) Don't add extra water. 3) Keep it moist (spray with water) for 3 days after pouring so it cures slowly. 4) Cut "control joints" with an angle grinder every 4-5 feet to force the concrete to crack in a straight, hidden line.
Ready-mix delivered concrete typically runs $130-$180 per cubic yard depending on your region and order size, plus a delivery minimum (often 3-5 yards). For small jobs under 1 cubic yard, bagged concrete from a home center is usually cheaper overall once you factor in the ready-mix delivery minimum.
Use our Concrete Footing & Sonotube Calculator β it uses the correct cylinder formula for fence posts, deck piers, and sonotube forms instead of the rectangular math used here.
New to estimating concrete? These guides walk through the whole process:
Every constant this calculator uses and where it comes from, so you can check it rather than take our word for it.
Local codes amend the model IRC and adopt different cycles — your building department governs. Manufacturers revise yields; where your product's data sheet disagrees with a default here, use the product's number. See our full methodology.