Concrete Calculator
Work out how much concrete a slab, a footing, a round column, a tube or a set of solid stairs needs. Enter the sizes in feet and inches or in meters. The calculator shows the volume in cubic yards, cubic feet and cubic meters, and how many 60 lb, 80 lb and pre-mix bags that takes, rounded up to whole bags.
How to use the Concrete Calculator
- Enter your numbers in the Concrete Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The concrete volume formulas
Slab and footing: L × W × thickness · · · Column: π × r² × h · · · Tube: π × (R² − r²) × h · · · Stairs: W × run × rise × n(n + 1) / 2 + W × landing × rise × n
In imperial mode, lengths, widths, diameters and heights are in feet, and the slab thickness, footing depth and stair rise, run and landing are in inches, which the calculator divides by 12. In metric mode every size is in meters. A cubic yard is 27 cubic feet. The calculator counts 0.45 cubic feet of concrete per 60 lb bag and 0.60 per 80 lb or pre-mix bag and rounds each count up. It adds nothing for waste. The note under the result suggests ordering 10% extra.
Worked example
A 20 ft × 12 ft slab, 4 inches thick: 20 × 12 × (4/12) = 80 cubic feet, which the calculator shows as 2.963 cubic yards and 2.265 cubic meters. That is 178 bags of 60 lb or 134 bags of 80 lb. With 10% extra, 2.963 × 1.1 = 3.26 cubic yards. For stairs 4 ft wide with 4 steps, a 7 inch rise and an 11 inch run: 4 × 11/12 × 7/12 × (4 × 5 / 2) = 21.389 cubic feet, shown as 0.792 cubic yards and 0.606 cubic meters. That is 48 bags of 60 lb or 36 bags of 80 lb.
Frequently asked questions
How do I order concrete?
Ready-mix concrete is sold by the cubic yard in the US and by the cubic meter elsewhere. Take the matching figure from the calculator, add your waste allowance and ask the supplier how they round an order.
How much should I order over my calculation?
The calculator gives the exact volume of the shape you enter and adds nothing for waste. The note under the result suggests 10% extra for spillage. If the ground under a slab is uneven, measure the depth in several places and enter the average.
How many bags of concrete do I need?
Divide the volume in cubic feet by what one bag yields and round up. The calculator uses 0.45 cubic feet for a 60 lb bag and 0.60 for an 80 lb bag, the yields on Quikrete's data sheet for its Concrete Mix. The 80 cubic foot slab above takes 178 bags of 60 lb or 134 of 80 lb. Check the yield printed on the bag you buy, because mixes differ.
How do I work out concrete for stairs?
Click Stairs and enter the width, the number of steps and the rise and run of one step. The calculator treats each step as a block from the ground up to its tread and adds them. Enter a landing depth for a platform beyond the top step: 36 inches on the stairs in the example adds 4 × 3 × 7/12 × 4 = 28 cubic feet, for 49.389 in all. It counts the stairs as solid, so a stair poured over fill or on a sloped slab takes less. For a rise and run that meet the building code, try the stair calculator first.