Concrete Calculator
How much concrete do you need? Cubic yards, bags, and a waste allowance for slabs, footings, columns, and steps
That is a lot of bags. For roughly a cubic yard or more, many people compare the cost of ready-mix delivery with mixing bags by hand. Ask your local supplier about minimum orders and delivery fees.
Volume = length × width × thicknessWorked Example: Concrete Slab
A slab 20 ft long, 30 ft wide, and 4 in thick: 20 × 30 × (4 ÷ 12) = 200 cubic feet. Divide by 27 to get 7.41 cubic yards. Adding 10% for waste gives about 8.15 cubic yards to order.
How Many Bags of Concrete?
Bag yield depends on bag size: a 40 lb bag makes about 0.30 cubic feet, a 50 lb bag about 0.375, a 60 lb bag about 0.45, and an 80 lb bag about 0.60. Yields vary by brand and mix, so check the number printed on the bag you buy. This calculator rounds up to whole bags.
Why Add a Waste Allowance?
Ground is rarely perfectly level, forms can bow slightly, and some concrete is spilled or left in the mixer. Ten percent is a common starting point, and running short mid-pour is worse than having a little left over. Steps assume solid concrete filling each step down to the ground and do not include landings.
Related tools: Volume Calculator and Square Footage Calculator. Estimates only. Confirm quantities with your concrete supplier or a contractor before ordering.