Calculate how much concrete you need for slabs, footings, columns, and steps. Results in cubic yards (for ready-mix delivery) and 60 lb / 80 lb bags (for DIY projects). Includes cost estimate and 10% waste factor. Used for driveways, patios, sidewalks, and foundations. Free, instant, no sign-up.
Slabs, footings & columns — cubic yards & bags
Concrete volume is calculated in cubic yards (1 cubic yard = 27 cubic feet) for ready-mix ordering, or in bags for small DIY projects. Formula for a rectangular slab: Volume (cubic yards) = (Length × Width × Thickness) / 324, where all measurements are in feet and thickness in inches. For a 10×10 ft patio that is 4 inches thick: (10 × 10 × 4) / 324 = 1.23 cubic yards. With 10% waste = 1.36 cubic yards. In 80 lb bags: 1 cubic yard = ~45 bags, so 1.36 yards = 62 bags. At $6.50 per bag: ~$403 in material vs. ~$204 for ready-mix at $150/yard — bags cost roughly 2× more for larger projects.
For projects over 1 cubic yard (approximately 7×7 foot slab at 4 inches), ready-mix delivery is typically more cost-effective than bags. Most ready-mix companies have a minimum order of 1 cubic yard. Typical strength for residential concrete: 3,000–4,000 PSI. Driveways and garage floors: 4,000 PSI minimum. Footings and foundations: 3,000–3,500 PSI. Decorative flatwork: 4,000+ PSI. The water-to-cement ratio is the primary factor in strength — too much water weakens concrete significantly. Standard residential mix uses a 0.45–0.50 water/cement ratio.
Sidewalks: 4 inches. Patios: 4 inches. Driveways (passenger cars): 4–5 inches. Driveways (trucks/RVs): 6 inches. Garage slabs: 4–6 inches. Footings (residential): 6–8 inches minimum. The minimum thickness for residential flatwork per most building codes is 3.5 inches; however, 4 inches is standard practice. Rebar or wire mesh adds structural strength.
Use bags: under 0.5 cubic yards (small repairs, fence posts, stepping stones). Use ready-mix: over 1 cubic yard (patios, driveways, foundations). 0.5–1 cubic yard: borderline — compare bag cost + your labor vs. ready-mix delivery fee. Ready-mix minimum charge (even for small loads) is typically $150–$300. Short-load fees apply for deliveries under 5 cubic yards.
Don't pour concrete when air temps are below 40°F or above 90°F without special precautions. Cold weather slows curing and risks freezing before adequate strength develops. Hot weather causes rapid water evaporation and cracking. Concrete reaches ~70% strength in 7 days and ~99% in 28 days. Keep new concrete moist (wet burlap or plastic sheeting) for at least 7 days to ensure proper curing.
Ready-mix concrete: $120–$180 per cubic yard depending on region (highest: West Coast $160–$200, lowest: Midwest $100–$130). 80 lb bags: $5.50–$7.50 at Home Depot/Lowe's. Delivery charge for ready-mix: $75–$150. Short-load fee (under 5 yards): $50–$100 extra. Labor to pour and finish: $3–$8 per sq ft on top of material cost.