Slabs, patios, walkways, footings, and round pads. Get cubic yards and how many bags to buy.
For a rectangle, volume in cubic feet is length × width × thickness in inches ÷ 12. For a round pad it is π × radius squared × thickness ÷ 12. The tool adds your extra percentage for spills and uneven ground, then divides by 27 to get cubic yards. Bag counts divide the cubic feet by the yield of each bag size: about 0.30 for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb, and 0.60 for 80 lb, rounded up.
Four inches is common for walkways, patios, and shed slabs. Driveways and heavier loads often use five to six inches. Check local building code, especially for footings.
One cubic yard equals about 45 bags of 80 lb mix. For anything near a yard or more, ready-mix delivery is usually easier and often cheaper than mixing bags by hand.
Subgrade is rarely perfectly level and some concrete is lost to spills. Running short mid-pour can leave a weak seam, so ordering a little extra is cheap insurance.
No. It only calculates the concrete itself. Base material and reinforcement are separate and depend on your project and local requirements.