This concrete calculator determines the exact cubic yards, cubic feet, and bag counts (40, 60, and 80 lb) required for slabs, footings, and post holes. It includes an instant waste buffer toggle and ready-mix versus bagged cost guidance so you can order materials with confidence.
Total Volumecu yd
Total Volumecu ft
80-lb Bags (0.60 cu ft yield)bags
60-lb Bags (0.45 cu ft yield)bags
40-lb Bags (0.30 cu ft yield)bags
Please check your inputs.
Concrete Volume and Bag Formulas
Concrete calculations convert project dimensions into cubic volume, adjust for spillage, and divide by bag yields.
Rectangular Slabs and Footings: Volume (cu ft) = Length (ft) × Width (ft) × (Thickness (in) / 12) × Quantity
Cylindrical Post Holes: Volume (cu ft) = 3.14159 × (Diameter (in) / 24)² × Depth (ft) × Quantity
Total Volume with Waste: Total (cu ft) = Base Volume (cu ft) × (1 + Waste Percentage / 100)
Choosing between ready-mix delivery and mixing bags by hand depends primarily on project size:
Under 1.5 cubic yards (under 68 eighty-pound bags): Bagged concrete is significantly cheaper. Delivered ready-mix typically incurs a short-load charge of 100 to 200 USD for orders under 4 to 5 cubic yards.
1.5 to 2.5 cubic yards: Material costs are roughly comparable (around 400 to 600 USD). Ready-mix saves extensive manual labor and guarantees continuous pouring without cold joints.
Over 2.5 cubic yards: Delivered ready-mix is almost always cheaper, faster, and provides superior compressive consistency.
Frequently asked questions
How much extra concrete should I add for waste?
A 10 percent buffer is standard for most residential slabs, patios, and footings. This covers uneven excavation, slight subbase settling, form deflection, and spillage. For smooth, rigid modular forms, 5 percent is sufficient, while rough dirt trenches require 15 percent.
How many bags of concrete make a cubic yard?
One cubic yard equals 27 cubic feet. It requires 45 bags of 80-pound concrete (0.60 cubic feet each), 60 bags of 60-pound concrete (0.45 cubic feet each), or 90 bags of 40-pound concrete (0.30 cubic feet each).
At what point is ready-mix cheaper than buying bags?
Ready-mix trucks become cost-competitive around 1.5 to 2 cubic yards. While concrete bags cost around 6 to 8 USD each, ready-mix trucks charge short-load fees on small batches. Above 2.5 cubic yards, ready-mix is cheaper and saves days of manual labor.
How deep should a concrete slab or patio be?
Standard pedestrian walkways and patio slabs should be 4 inches thick poured over 4 inches of compacted gravel. Driveways and heavy equipment pads require 5 to 6 inches of 4,000 PSI concrete reinforced with rebar or wire mesh.