Concrete Calculator
Calculate cubic yards and bag counts for any concrete project — slabs, footings, columns, and walls. Includes a 10% waste factor.
Project Details
How to Calculate Concrete Volume
Concrete volume is calculated in cubic feet and then converted to cubic yards (divide by 27). The basic formula for a rectangular slab is: Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = Cubic Yards. Always add a 10% waste factor for spills, uneven subgrade, and form overfill.
Ready-Mix vs. Bagged Concrete: When to Use Each
Use ready-mix (truck delivery) for any job over about 0.5 cubic yards (roughly 27 bags). Ready-mix costs $140–$180 per cubic yard delivered and is more consistent in quality. Use bagged concrete for small jobs — post holes, repair patches, and footings under 10 linear feet.
- 60-lb bag: 0.45 cubic feet → ~60 bags per cubic yard
- 80-lb bag: 0.60 cubic feet → ~45 bags per cubic yard
- Ready-mix: $140–$180/yd³ delivered, minimum load of 1 yd³ typically
Concrete Thickness by Application
- Walkways / sidewalks: 4 inches minimum
- Residential patios: 4 inches
- Driveways (passenger cars): 5 inches
- Driveways (heavy vehicles): 6 inches
- Garage floors: 4–6 inches with wire mesh or rebar
- Foundation footings: Width = 2× wall thickness, Depth = local frost depth
How many bags of concrete do I need for a 10×10 slab at 4 inches thick?
A 10×10×0.333 ft slab = 33.3 cubic feet = 1.23 cubic yards (add 10% waste = 1.35 yd³). That's about 81 × 60-lb bags or 61 × 80-lb bags. At this volume, ready-mix concrete is often more cost-effective.
What PSI rating do I need?
Driveways: 4,000 PSI. Sidewalks/patios: 3,000–4,000 PSI. Footings: 3,000 PSI minimum. Structural slabs: 4,000–5,000 PSI. For freeze-thaw climates, ask for air-entrained mix when ordering ready-mix.
How long does concrete take to cure?
Concrete reaches 70% strength in 7 days and 99% strength in 28 days. You can typically walk on it after 24–48 hours, drive on it after 7 days, and load it fully after 28 days.