Concrete Calculator
Calculate concrete volume in cubic yards and cubic feet. Find how many 60 lb or 80 lb bags you need.
Concrete Volume Formulas
Slab: Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft). Convert inches to feet by dividing by 12. Then divide by 27 for cubic yards.
Cylinder: Volume (cu ft) = π × (Diameter ÷ 2)² × Height (ft). Used for piers, sonotubes, and round footings.
Always add 5–10% extra to account for spillage, uneven subgrade, and waste — especially for larger pours.
Common Concrete Slab Reference
| Project | Thickness | Cu Yd / 100 sq ft |
|---|---|---|
| Walkway / path | 4 in | 1.23 |
| Patio | 4–6 in | 1.23–1.85 |
| Driveway (residential) | 4 in | 1.23 |
| Garage floor | 6 in | 1.85 |
| Foundation / structural | 8–12 in | 2.47–3.70 |
Frequently Asked Questions
How thick should a concrete slab be?
Standard concrete slab thicknesses: 4 inches for driveways and walkways, 4–6 inches for patios, 6 inches for garage floors and areas subject to vehicle traffic, and 8–12 inches for structural slabs and foundations. Thicker slabs are stronger but use significantly more concrete.
How many cubic yards of concrete do I need per bag?
A standard 60 lb bag of concrete mix yields approximately 0.45 cubic feet, or 1/60 of a cubic yard. An 80 lb bag yields about 0.60 cubic feet. To find how many bags you need, divide total cubic feet by the yield per bag. This calculator does that automatically.
When should I order ready-mix instead of mixing bags?
For projects requiring more than about 1 cubic yard (27 cubic feet), ordering ready-mix concrete from a truck is usually more cost-effective and less labor-intensive than hand-mixing bags. Bagged concrete is practical for small repairs, fence posts, and projects under half a yard.
What is a cubic yard of concrete in pounds?
One cubic yard of concrete weighs approximately 4,000 lbs (about 2 tons). This is useful to know for structural load calculations and for estimating how many wheelbarrow loads (typically 3–4 cubic feet each) you'll need to move concrete on a job site.
How do I calculate concrete for cylindrical piers or posts?
Use the cylinder formula: Volume = π × r² × height. Where r is the radius (half the diameter) in feet and height is in feet. Multiply by 27 to convert to cubic yards. This calculator handles cylindrical shapes directly — just enter diameter and depth.