Concrete Calculator
Enter the size and thickness of your slab, footing or post holes to get cubic yards, cubic metres and the number of bags to buy, with extra for waste.
- Cubic yards & m³
- 40–80 lb bags
- Slabs & post holes
- Waste and cost
Slab, footing or post-hole sizes
Enter feet (12), feet and inches (12' 6" or 12 6) or inches (18in).
For post holes or round columns, choose Circle, enter the diameter, set the depth to the hole depth, and the quantity to the number of holes.
1.36 cubic yards
Including 10% extra for spillage and uneven ground.
- Cubic feet
- 36.67
- Cubic metres
- 1.04 m³
- 40 lb bags (0.3 cu ft each)
- 123
- 50 lb bags (0.375 cu ft each)
- 98
- 60 lb bags (0.45 cu ft each)
- 82
- 80 lb bags (0.6 cu ft each)
- 62
Over about 1 cubic yard, ready-mix delivery is usually cheaper and easier than mixing 45+ bags. Many suppliers have a minimum order.
What is a concrete calculator?
A concrete calculator estimates the volume of concrete for a project from its dimensions. Concrete is ordered by volume: cubic yards in the US and cubic metres elsewhere, or by the bag for small jobs. This calculator handles rectangular slabs and footings, circular post holes and pads, and triangular pieces, adds a safety margin for waste, and converts the result into bag counts using the yields printed on common bag sizes.
How to use Concrete Calculator
- Choose feet and inches or metres.
- Enter each section: a rectangle for a slab or footing, a circle for a post hole or round pad. Set a quantity for identical pieces, such as 8 fence posts.
- Enter the thickness or depth, and keep 5–10% extra for waste.
- Read the cubic yards and bag counts. Add your supplier’s price per yard for a cost estimate.
How much concrete do I need?
- Volume = length × width × thickness. Convert the thickness to feet first: 4 inches = 0.333 ft.
- Cubic yards = cubic feet ÷ 27. A 10 × 10 ft slab at 4 inches is 33.3 cu ft = 1.23 cu yd.
- Common bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 50 lb ≈ 0.375 cu ft, 40 lb ≈ 0.30 cu ft. Check the bag you buy.
- Typical thicknesses: 4 inches for patios and walkways, 5–6 inches for driveways that carry cars, more for heavy loads.
Why use this concrete calculator
- Bags or ready-mixSee both the cubic yards to order and how many 40, 50, 60 or 80 lb bags that equals.
- Several areas at onceAdd rectangles, circles and triangles, set quantities and subtract cut-outs to match real, irregular spaces.
- Feet, inches or metresType 12' 6" or 12 6, switch to metres any time, and see results in both systems.
- Waste includedAdd a margin for spillage, uneven subgrade and form movement so you do not run short mid-pour.
Common projects
- Patios and shed bases. A 10 × 12 ft slab at 4 inches thick.
- Fence and deck posts. Round holes of a given diameter and depth, times the number of posts.
- Footings. Long, narrow strips under walls or steps.
- Driveways and paths. Several rectangles combined into one order.
Planning a bigger project? See all home and DIY calculators, or work out costs with the percentage calculator and discount calculator.
Example: a 10 × 10 ft patio
A 10 × 10 ft patio slab at 4 inches thick needs 33.3 cubic feet. With 10% extra that is 36.7 cu ft, or 1.36 cubic yards, which is 62 bags of 80 lb concrete. At that size, a ready-mix delivery is usually easier than mixing 62 bags by hand.
What to expect from the result
Estimates assume a flat, even base and exact dimensions. Rebar, mesh and gravel base are not included. Structural work such as foundations and retaining walls should follow local codes and an engineer’s specification.
Concrete Calculator questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet to get cubic feet, divide by 27 for cubic yards, then add 5–10% for waste. The calculator does this for any number of slabs, footings or post holes.
How many bags of concrete are in a cubic yard?
About 45 bags of 80 lb concrete, 60 bags of 60 lb or 90 bags of 40 lb, based on typical yields of 0.60, 0.45 and 0.30 cubic feet per bag.
How much concrete do I need for a post hole?
Choose Circle, enter the hole diameter and the depth, and set the quantity to the number of holes. For example, eight 10-inch holes 3 ft deep need about 13.1 cu ft, or 22 bags of 80 lb concrete without extra.
How thick should a concrete slab be?
Four inches is typical for patios, walkways and shed bases; 5–6 inches for driveways. Follow local codes and your project’s specification.
Should I order extra concrete?
Yes. Most contractors add 5–10% to cover spillage, uneven ground and form flex. Running short during a pour causes weak joints.