Concrete Calculator
This concrete calculator works out how much concrete you need for a slab, footing, or round column. Enter the dimensions and you'll get the volume in cubic yards and cubic feet, the number of 40, 60, and 80 lb bags, and the amount with a waste allowance. It's built for ordering ready-mix or buying bags, so you won't come up short on pour day.
- Cubic yards & feet
- Bags of 40/60/80 lb
- Slab, footing, column
- 10% waste added
- Live as you type
Last updated June 17, 2026 1 cubic yard = 27 cubic feet Reviewed by the Calcowa team
Enter dimensions above 0.
Estimates for planning. Order a little extra for spillage and uneven ground.
10 × 10 × (4 ÷ 12) = 33.3 ft³ ÷ 27 = 1.23 yd³
How much concrete do I need?
Concrete is measured by volume, so you'll need the length, width, and thickness of the pour. Multiply them together in feet to get cubic feet, then divide by 27, since one cubic yard is 27 cubic feet and yards are how ready-mix is ordered. The only catch is thickness, which is usually given in inches, so you'll divide it by 12 first. A 10 by 10 foot slab at 4 inches works out to 1.23 cubic yards. It's built to do that chain for any slab, footing, or column.
Calculating a slab, step by step
Here's how the calculator handles a 10 by 10 foot slab, 4 inches thick:
- 1
Convert the thickness to feet4 inches ÷ 12 = 0.333 ft. Length and width are already in feet.
- 2
Multiply for cubic feet10 × 10 × 0.333 = 33.3 cubic feet of concrete.
- 3
Divide by 27 for cubic yards33.3 ÷ 27 = 1.23 cubic yards, the amount you order.
- 4
Add waste and pick bagsWith 10% waste that's about 1.36 yards, or roughly 56 bags of 80 lb mix.
How many bags of concrete?
Each bag of pre-mixed concrete yields a set volume once water is added: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Divide your total cubic feet by the yield to get the bag count, so you'll see all three sizes above. Bags suit small jobs like a few post holes or a single footing. Once you're past about a cubic yard, ordering ready-mix from a truck is usually cheaper and far less work than mixing dozens of bags by hand.
Slab thickness and waste
Most slabs are 4 inches thick, which suits patios, walkways, and shed floors. A driveway holding cars wants 4 inches as well, going to 5 or 6 if heavy vehicles park on it. Whatever the thickness, you'll want a little extra, because subgrade dips, spillage, and edges all use more than the clean calculation suggests. Five to ten percent is the usual cushion, and the result above shows the figure with 10% added so you don't run short. For the area of an odd-shaped pour, the area calculator helps, and the volume calculator covers other shapes.
Frequently asked questions
How much does a yard of concrete weigh?
A cubic yard of concrete weighs about 4,050 pounds, just over 2 tons. That's roughly 150 pounds per cubic foot. It's worth knowing for trailer loads and for checking that the ground and forms can hold the pour.
Work out the volume in cubic feet, then divide by 27 to get cubic yards, which is how concrete is ordered. For a slab, that's length × width × thickness, with the thickness in feet. A 10 by 10 foot slab at 4 inches thick is 33.3 cubic feet, or about 1.23 cubic yards. This concrete calculator does the math and adds a waste allowance, so you'll order enough.
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 since a cubic yard is 27 cubic feet. So a 12 by 12 foot patio at 4 inches (0.333 ft) thick is 48 cubic feet, which is 1.78 cubic yards. Always round up and add a little for spillage and uneven ground. The tool above handles the division and rounding, so you'll get cubic yards without the arithmetic.
It depends on the bag size, since each yields a set volume: an 80 lb bag makes about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. Divide your total cubic feet by the bag yield to get the count. For more than about a cubic yard, you'll usually find ready-mix delivery cheaper than bags. The result above lists all three bag sizes.
Four inches is standard for patios, sidewalks, and shed floors. Driveways for cars usually want 4 inches too, and 5 to 6 inches if heavy trucks will park on them. Footings and structural slabs go deeper. Set the thickness above to match your project, and you'll see the volume update as you change it.
Yes, order a little more than the exact figure. Uneven subgrade, spillage, and slab edges all eat into the total, so most pros add 5 to 10%. Running short mid-pour is a real problem, since a cold joint forms while you wait for more. The calculator shows the amount with a 10% allowance built in, alongside the exact figure.
One cubic yard covers about 81 square feet at 4 inches thick, or 65 square feet at 5 inches. So a typical truckload of 8 to 10 yards covers a large driveway or a small foundation. To go the other way, this tool turns your slab dimensions straight into yards, so you don't have to work back from coverage.
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