How many bags or cubic yards of concrete do you need? Covers slabs, footings, and columns.
Concrete is measured in cubic yards or cubic feet. To calculate how much you need, multiply the length, width, and depth of your project area (all in feet), then divide by 27 to convert cubic feet to cubic yards. For pre-mixed bags, divide your total cubic feet by the bag's coverage (typically 0.45 cubic feet for an 80 lb bag or 0.6 cubic feet for a 60 lb bag).
The standard rule is to order 10-15% more concrete than your calculated amount. Uneven ground, forms that are not perfectly level, and spillage during pouring all contribute to needing more than the theoretical amount. Running short mid-pour is far worse than having a small amount left over.
Sidewalks and patios: 4 inches. Driveways for passenger vehicles: 4-6 inches. Driveways for heavy vehicles: 6-8 inches. Footings: typically 8-12 inches deep and 12-24 inches wide, depending on the structure. Slab foundations: 4-6 inches with thickened edges. Always check local building codes for minimum depth requirements.
One cubic yard of concrete weighs approximately 3,900 pounds. For small projects (under 1 cubic yard), pre-mixed bags are practical. For larger projects, ordering a ready-mix truck delivery is more economical and ensures consistent quality. Concrete typically sets within 24-48 hours and reaches full strength at 28 days. Avoid pouring in temperatures below 40 degrees F or above 90 degrees F without special precautions.