Concrete Calculator
Enter your dimensions and get cubic yards instantly — plus cubic meters, 80 lb bag counts, waste allowance and optional cost. Choose the pour shape below; the formula updates with it.
How to calculate concrete
Concrete is ordered by the cubic yard (27 ft³). The core formula for any rectangular pour is length × width × thickness, with thickness converted to feet by dividing inches by 12. Then divide by 27.
Worked example — 20×20 slab at 5": 20 × 20 × (5 ÷ 12) = 166.67 ft³ → 166.67 ÷ 27 = 6.17 yd³. With a 10% waste factor order about 6.8 yd³.
Common slab volumes
| Slab size | 4" thick | 5" thick | 6" thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
| 24 × 24 ft (2-car pad) | 7.11 yd³ | 8.89 yd³ | 10.67 yd³ |
| 30 × 30 ft | 11.11 yd³ | 13.89 yd³ | 16.67 yd³ |
Bagged mix yields
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
Frequently asked questions
- How much waste should I add?
- 5–10% is standard for formed residential work; use 10% or more for uneven subgrade or hand-mixed pours. Ready-mix short loads are expensive, so rounding up protects you.
- Should I order ready-mix or bags?
- Under roughly 1 yard, bags are usually practical. Above 1–2 yards, ready-mix saves hours of mixing. The calculator flags pours under ¼ yard automatically.
- How deep should a patio or driveway slab be?
- Walkway/patio slabs are commonly 4"; driveways 5–6" with reinforcement. This tool estimates volume only — follow your local code and structural drawings for thickness design.