Find out exactly how many cubic yards (or cubic feet) of concrete you need for your project. Works for slabs, footings, walls, columns, and circular pads.
For a rectangular slab: multiply Length × Width × Thickness (in feet) to get cubic feet, then divide by 27 to convert to cubic yards. Always order 10% extra to account for spillage and uneven subgrades.
One cubic yard fills a 4-inch slab covering approximately 81 square feet. For most residential slabs, concrete is ordered in cubic yards from a ready-mix truck (minimum order typically 1 yd³). Bags are practical only for small jobs under about 0.5 cubic yards.
Standard residential slabs are 4 inches thick. Driveways typically require 4–6 inches. Garage floors and heavy vehicle areas should be 5–6 inches. Sidewalks are usually 4 inches. Structural slabs and footings may require 8+ inches depending on load requirements and local building codes.
For projects under about 0.5 cubic yards (~14 cubic feet), bags are practical. For anything larger, ordering ready-mix concrete from a truck is faster, stronger (professional mixing), and often cheaper per yard. Minimum truck orders are typically 1 yard (~$165), with smaller quantities often available at a short-load fee.
One cubic yard = 27 cubic feet. A 60-lb bag makes approximately 0.45 cubic feet, so you need about 60 bags per cubic yard. An 80-lb bag makes about 0.60 cubic feet, so about 45 bags per cubic yard. For small projects, 80-lb bags are more economical.