Work out how much concrete you need for a slab, footing, column or set of stairs, in cubic yards, cubic feet and bags of premixed concrete, with an allowance for waste and an optional cost estimate.
Concrete Calculation Guidelines
Common Concrete Mixes:
- 80 lb bag: Covers 0.6 cubic feet
- 60 lb bag: Covers 0.45 cubic feet
- 1 cubic yard: Equals 27 cubic feet
Standard Thickness:
- Walkways: 4 inches
- Driveways: 4-6 inches
- Garage floors: 6 inches
- Footings: Typically 12-18 inches deep
Important Tips:
- Always order 5-10% extra concrete for waste and spillage
- Ready-mix concrete is sold by cubic yard
- Bags are better for small projects; ready-mix for large
- Consider reinforcement (rebar or mesh) for structural projects
- Concrete must be poured within 90 minutes of mixing
What this concrete calculator does
- Slab: length, width and thickness (in inches) to volume, with a waste factor of 5%, 10% or 15%.
- Footing: length, width and depth in feet, for one or several footings.
- Column: circular, square or rectangular columns, from height and cross-section size.
- Stairs: solid concrete steps from width, rise, run and number of steps.
How it is calculated
Volume in cubic feet is length × width × thickness (converted to feet) × quantity. A waste factor is added, and cubic yards are cubic feet ÷ 27. Bags are counted from the yield of a premixed bag: an 80 lb bag makes about 0.60 cubic feet and a 60 lb bag about 0.45 cubic feet, as listed by Quikrete. The number of bags is rounded up. If you enter a price per cubic yard, the cost is cubic yards × price.
Worked examples: a 10 ft × 10 ft slab, 4 inches thick, with 10% waste is 36.7 cubic feet, or 1.36 cubic yards. That is about 62 bags of 80 lb mix or 82 bags of 60 lb mix, and $203.70 at $150 per cubic yard. Four round columns, 12 inches across and 8 feet high, need 0.93 cubic yards (42 bags of 80 lb). A set of four solid steps 4 ft wide with a 7 inch rise and 11 inch run, with 10% waste, is 0.87 cubic yards.
The stairs tab treats each step as a solid block from the ground up, so step 1 is one rise high, step 2 is two rises high, and so on. It does not include a landing or footing.
Things to keep in mind
Dimensions are in feet and inches (US units). The Column tab has no waste setting, so add your own allowance. Bag yields differ slightly by product, so check the label of the mix you buy. For larger pours, ready-mix delivery is usually ordered by the cubic yard. These are planning estimates only. Measure your own project carefully, check product labels and local building codes, and confirm quantities with your supplier or a qualified contractor before you buy.
Frequently asked questions
How many 80 lb bags make a cubic yard?
A cubic yard is 27 cubic feet, so at 0.60 cubic feet per bag it takes about 45 bags of 80 lb mix.
Why add a waste factor?
Uneven ground, spillage and over-excavation all use extra concrete. The calculator lets you choose 5%, 10% or 15%.
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