Work out how many cubic yards of concrete you need for a slab, footing or round column, plus the weight and number of bags.
Last updated: October 31, 2026
Volume formulas for slabs and columns
The amount of concrete is the volume of the space you're filling. Multiply the dimensions in feet, then divide by 27 to get cubic yards: • Slab, footing or path: length × width × thickness • Round column: π × (diameter ÷ 2)² × height Example: a 10 × 10 ft slab, 4 inches thick, is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. A 12-inch column 8 feet high is 3.14 × 0.5² × 8 ≈ 6.3 cubic feet. Waste: forms flex, ground is uneven and some concrete spills, so you need a little more than the exact volume. Adding 5–10% is standard. Running short mid-pour is far worse than having a little left over. Weight is based on normal concrete at about 4,000 lb per cubic yard (2,400 kg/m³). Bag counts assume an 80 lb bag of premix yields about 0.6 cubic feet, as printed on common brands; 60 lb bags yield about 0.45 cubic feet. Check the yield on your bag. For jobs over about 1 cubic yard, ordering ready-mix from a truck is usually cheaper and easier than mixing bags. For structural work, ask an engineer about the concrete strength and reinforcement.
Common questions about concrete
About 45 bags of 80 lb, or 60 bags of 60 lb. That's why ready-mix is usually the better choice for bigger jobs.
Other tools you may find useful
Long side of the slab.
Short side of the slab.
Depth of the concrete.
How many of this size?
Spills and uneven ground, usually 5–10%.