DIY Concrete Mix Calculator
Calculate bags of cement, sand, and gravel for any concrete project.
Covers slabs, footings, posts, and steps in both cubic feet and cubic yards.
Concrete volume calculation determines the exact amount of material needed to fill a given space — slab, footing, post hole, or step — converting dimensions into cubic yards or cubic meters and then into pre-mixed bag counts.
Core formulas: Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft) Volume (cubic yards) = Volume (cubic feet) ÷ 27 Volume (cubic meters) = Length (m) × Width (m) × Depth (m)
For cylindrical post holes: Volume = π × radius² × depth (in consistent units)
Pre-mixed bag coverage (approximate):
| Bag Size | Yield | Best for |
|---|---|---|
| 40 lb | 0.011 cu yd / 0.30 cu ft | Small patches |
| 60 lb | 0.017 cu yd / 0.45 cu ft | Standard projects |
| 80 lb | 0.022 cu yd / 0.60 cu ft | Larger slabs |
Always add 10% waste buffer when calculating bags: bags needed × 1.1.
Standard concrete mix ratios (by volume):
- General purpose: 1:2:3 (cement : sand : gravel)
- High strength: 1:1.5:2.5
- Driveway/heavy load: 1:2:2 (richer in cement)
- Water:cement ratio: 0.45–0.55 for most applications (less water = stronger concrete)
Worked example — garden shed slab: Slab: 12 ft × 10 ft × 4 inches thick (4/12 = 0.333 ft) Volume = 12 × 10 × 0.333 = 40 cubic feet Cubic yards = 40 ÷ 27 = 1.48 cubic yards With 10% buffer: 1.63 cubic yards
80 lb bags needed = 1.63 ÷ 0.022 = 74 bags (80 lb each) Alternatively: order 1.75 yards of ready-mix concrete from a supplier for this size — it’s typically more economical than bagged concrete for slabs over 1 cubic yard.
Rule of thumb: Ready-mix delivery costs $120–$180/yard but saves enormous time and labor on any project larger than 1 cubic yard.