Concrete Slab Calculator
Cubic yards, bags, and ready-mix for slabs & footings
Enter your project dimensions
Standard slab: 4 in · Footings: 8–12 in
Cost (optional — enter your local price)
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Formula & explanation
Length and width are in feet and depth in inches (divided by 12 to get feet). Cubic yards come from dividing by 27 (one cubic yard = 27 cubic feet). Bag counts use typical yields: a 40 lb bag ≈ 0.30 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft. The waste allowance is added before dividing, so you order extra material, not extra volume.
Example calculation
24 × 24 ft patio, 4 in thick, 10% waste, 80 lb bags:
Volume = 24 × 24 × (4÷12) = 192 cu ft. With 10% waste = 211.2 cu ft = 7.82 cu yd = 5.99 m³. 80 lb bags: 211.2 ÷ 0.60 = 352 bags. Ready-mix: 1 truckload.
Buying & project-planning tips
- For slabs over 4 inches thick or load-bearing pads, a ready-mix delivery is usually cheaper than bagged concrete.
- Order 5–10% extra — running short mid-pour causes cold joints and weak spots.
- A standard mixer truck holds about 10 cubic yards; a short load fee often applies under 3 yards.
- Add rebar or fiber mesh per your local code for driveways and structural slabs.
- Concrete needs to cure properly — cover with plastic or mist for 7 days for full strength.
Frequently asked questions
How many cubic yards in a concrete truck?+
A standard ready-mix truck holds about 9–10 cubic yards. Deliveries under ~3 yards often carry a short-load fee, making bagged concrete more economical for small pours.
How thick should a concrete slab be?+
Patios and walkways: 4 inches. Driveways: 4–6 inches. Footings: 8–12 inches. Load-bearing slabs may require engineering — check local code.
Should I use bagged or ready-mix concrete?+
Under about 1 cubic yard, bags are cost-effective. Over 2 yards, ready-mix is usually cheaper and far less labor, even with delivery fees.
Does this calculator account for rebar?+
No — it calculates concrete volume only. Rebar displaces a small amount of concrete but is typically ignored in volume estimates. Follow your engineer's rebar schedule.