Concrete Calculator
Find how much concrete a slab, footing or round column needs. The result is given in cubic yards for ready-mix orders and in bags for small pours.
Result
- Cubic feet
- 33.33 cu ft
- Cubic meters
- 0.944 m³
- 80 lb bags
- 56 bags
- 60 lb bags
- 75 bags
- 40 lb bags
- 112 bags
- Approximate weight
- 5,000 lb
Bag counts use typical yields of 0.60, 0.45 and 0.30 cubic feet for 80, 60 and 40 lb bags of pre-mixed concrete. Weight assumes 150 lb per cubic foot.
Formula
- Slab: volume = length × width × thickness
- Column: volume = π × (diameter ÷ 2)² × height
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ yield per bag (rounded up)
How the Concrete Calculator works
Concrete is ordered by volume. Convert every dimension to feet, multiply to get cubic feet, and divide by 27 to get cubic yards. A 4 inch slab is 4 ÷ 12 = 0.333 ft thick.
Ready-mix suppliers usually sell by the cubic yard, often in quarter-yard steps. For small jobs, pre-mixed bags are easier. An 80 lb bag makes about 0.60 cubic feet of concrete, so one cubic yard takes 45 of them.
Ordering 5 to 10 percent extra covers uneven subgrade, spillage and form deflection. Enter that in the waste field.
Worked example
A 10 ft × 10 ft slab that is 4 in thick needs 10 × 10 × 0.3333 = 33.33 cu ft, which is 33.33 ÷ 27 = 1.23 cubic yards. In 80 lb bags that is 33.33 ÷ 0.6 = 55.6, so 56 bags.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. One 80 lb bag yields roughly 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags.
How thick should a concrete slab be?
Four inches is common for patios, walkways and shed floors. Driveways are usually 4 to 6 inches. Local codes and soil conditions decide the final figure, so check before you pour.
How do I calculate concrete for a round post hole?
Choose the round column option, enter the hole diameter and depth, and set the quantity to the number of holes. Subtract nothing for the post if you want a safe estimate.
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