Concrete Slab Calculator
A concrete slab calculator finds the volume of a flat rectangular pour by multiplying length by width by thickness, then converting to cubic yards. Enter your slab dimensions below to get the cubic yards, the number of bags, and an estimated cost, with an optional waste allowance.
Thickness is entered in inches; everything else in feet by default.
- 80 lb bags
- 0
- 60 lb bags
- 0
- Estimated cost
- $0.00
Enter your dimensions to see the result.
Estimate only. Material yields and prices vary by supplier and region; confirm before ordering.
How Do You Calculate Concrete for a Slab?
To calculate concrete for a slab, multiply the length by the width to find the area, then multiply by the slab thickness to get the volume. Work in feet, so convert the thickness from inches by dividing by 12, then divide the final cubic feet by 27 to get cubic yards. A slab 12 feet by 12 feet at 4 inches thick is 12 times 12 times 0.333, which is about 48 cubic feet, or 1.78 cubic yards. The calculator above runs this instantly and adds your waste buffer.
What Thickness Should a Concrete Slab Be?
Slab thickness depends on the load. Four inches is the common standard for patios, walkways and shed bases, while driveways and areas that carry vehicles are usually poured at 5 to 6 inches. Thicker slabs use proportionally more concrete, so the thickness you choose has a direct effect on the cubic yards and cost. Set the exact thickness in the calculator to size your order correctly.
How Many Bags of Concrete for a Slab?
For small slabs you can use bags instead of ready-mix. Since an 80 pound bag yields about 0.6 cubic feet, divide your slab volume in cubic feet by 0.6 to get the bag count. A small 4 by 4 foot slab at 4 inches thick is about 5.3 cubic feet, which is roughly 9 bags. Above about half a cubic yard, bagged concrete becomes impractical and ready-mix is cheaper and easier, which is the crossover the calculator helps you see.
Frequently asked questions
- How much concrete do I need for a 12x12 slab?
- A 12 by 12 foot slab at 4 inches thick needs about 1.78 cubic yards, or roughly 1.96 cubic yards with a 10 percent waste allowance. At 6 inches thick it needs about 2.67 cubic yards.
- How thick should a concrete slab be?
- Four inches is standard for patios and walkways, while driveways and vehicle areas are usually 5 to 6 inches. Thicker slabs need more concrete, so set the exact thickness before ordering.
- How many bags of concrete for a 10x10 slab?
- A 10 by 10 foot slab at 4 inches thick is about 33 cubic feet, which is roughly 56 of the 80 pound bags. At that size ready-mix is usually more practical than bags.