Calculation details
Planning estimate only. Verify dimensions, waste, coverage, product specifications, and local requirements before purchasing materials.
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What does this calculator estimate?
A concrete calculator estimates the volume of concrete needed for a slab, footing, or other pour. Enter the length, width, depth, and waste percentage to get cubic yards.
- Cubic yards = (L × W × depth ft) ÷ 27
- Depth in inches ÷ 12 = feet
- Add 10% waste for uneven ground and spillage
How concrete volume is calculated
Concrete is ordered by the cubic yard. The volume of a slab is length × width × thickness, converted from cubic feet to yards (27 cu ft per yard). A waste factor covers uneven subgrade, spillage, and over-excavation.
Limitations to watch for
The estimate assumes a uniform rectangular pour. Footings, columns, and curved edges need different math. Ready-mix is delivered in whole cubic yards — you pay for the full yard. Slump, forms, and reinforcement are not included.
How to use it in practice
Measure the pour in feet (convert inches to feet by ÷12). Add 10% waste so you don't run short mid-pour — a short pour is far worse than a little extra. Round up to the nearest quarter or half yard for ordering.
['Enter the length and width of the pour.', 'Enter the depth in inches.', 'Enter waste %; the tool computes cubic yards to order.']
How this calculator works
Formula
Volume = length × width × (depth in inches ÷ 12) = cubic feet; cubic yards = cubic feet ÷ 27. Order volume = cubic yards × (1 + waste%).
Worked example
A 10 × 10 ft slab at 4 in thick: 100 sq ft × 0.333 ft = 33.33 cu ft ≈ 1.235 cu yd. With 10% waste: ≈ 1.36 cu yd to order.
Assumptions to verify
- The pour is a rectangular slab.
- Depth is uniform.
- Waste covers subgrade variation and spillage.
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length × width × thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. A 10×10 ft slab at 4 in is about 1.24 cu yd.
How many cubic feet in a cubic yard?
27 (3×3×3 ft). Ready-mix concrete is sold by the cubic yard.
How much concrete is in a bag?
A 60 lb bag yields about 0.45 cu ft; an 80 lb bag about 0.6 cu ft. For a 1.36 cu yd slab you'd need roughly 80+ of the 60 lb bags.
Why add waste?
Subgrade is rarely perfectly level, forms can flex, and concrete spills. 10% waste prevents running short — a cold joint mid-pour is a much bigger problem than a little extra concrete.
How do I convert inches to feet?
Divide by 12: 4 in = 0.333 ft, 6 in = 0.5 ft.
Does this include rebar or wire mesh?
No — reinforcement is planned separately based on the load and code.
How much does a cubic yard of concrete weigh?
About 4,000 lb (2 tons) for standard concrete — plan access and formwork accordingly.
Cite this tool
BoringToolsKit. “Concrete Calculator.” boringtoolskit.com/concrete-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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