Concrete Calculator – How Much Do You Need?
Calculate how much concrete you need in cubic yards or bags for your project.
How to Calculate Concrete Volume
Concrete is sold by the cubic yard (US) or cubic meter (metric). For a rectangular slab: Volume = Length × Width × Thickness. Make sure all units match before calculating.
Example: a 10 ft × 12 ft patio, 4 inches thick: Volume = 10 × 12 × (4/12) = 40 cubic feet = 40/27 = 1.48 cubic yards. Always order 5–10% extra for waste, spillage, and uneven subgrade.
Concrete Mix Ratios
Standard concrete mixes by cement:sand:gravel ratio:
- Standard (1:2:4): General purpose, driveways, patios
- Strong (1:1.5:3): Foundations, footings
- Extra Strong (1:1:2): Heavy loads, industrial floors
For small projects (under 0.5 cubic yards), pre-mix bags are most practical. One 80 lb bag of Quikrete yields approximately 0.6 cubic feet. For a project requiring 1 cubic yard (27 cubic feet), you need approximately 45 bags.
When to Use Ready-Mix vs Bags
Ready-mix concrete (delivered by truck) is economical for projects over ~1 cubic yard. Typical minimum delivery is 1 yard; most trucks hold 10 yards. Cost: $125–$170 per cubic yard plus delivery fee.
Bagged concrete makes sense for small repairs, fence posts, and projects under 0.5 cubic yards. Working time with bagged concrete in warm weather is 20–30 minutes — plan and work fast.
Frequently Asked Questions
How thick should a concrete slab be?
Patios and walkways: 4 inches. Driveways: 4–6 inches (6 for heavy vehicles). Garage floors: 4–6 inches. Footings: typically 8–12 inches or deeper below frost line (check local building codes).
How long does concrete take to cure?
Initial set: 24–48 hours (do not walk on it). Structural strength (~70%): 7 days. Full cure: 28 days. Concrete continues gaining strength for years. Keep it moist for the first 7 days (curing) for maximum strength.
Do I need rebar or wire mesh in my slab?
For basic patios and walkways: wire mesh (6×6 10/10 WWF) is usually sufficient. For driveways and areas with heavy loads: #3 or #4 rebar on 18-inch grid. Footings and structural elements: rebar per engineering specs. Check local codes.