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.
Concrete is ordered by volume, and in the US that volume is quoted in cubic yards while slabs are measured in feet and inches — so the conversion is where errors enter. One cubic yard is 27 cubic feet, and thickness given in inches must be divided by 12 before multiplying. Because concrete cannot be topped up once a pour has begun to set, a waste allowance of 5–10% is standard practice to cover subgrade irregularities, spillage and slight over-excavation. Running short mid-pour produces a cold joint, which is a structural defect rather than a cosmetic one.
Cubic feet = length(ft) × width(ft) × (thickness_in ÷ 12)Cubic yards = cubic feet ÷ 27Order quantity = cubic yards × (1 + waste%)where:
Assumptions: Volume only. Reinforcement, sub-base preparation and thickened edges are separate, and ready-mix suppliers usually impose a minimum load with a surcharge for short loads.
Convert slab dimensions into a ready-mix order.
Result7.41 cubic yards of slab — order 8.25 to allow for waste
Increasing the slab to 6 inches raises the requirement to 8.89 cu yd before waste — a 20% increase in concrete cost for 20% more thickness. That is usually worth it on a driveway carrying vehicles, where 4 inches is the minimum and 5–6 is the durable choice.