Work out concrete for slabs, footings, tubes and post holes - in cubic yards, and the exact count of 80, 60 and 40 lb bags.
Concrete is volume: length x width x thickness. A 10 x 10 ft slab at 4 inches thick is 33.3 cubic feet - 1.23 cubic yards. Bagged concrete is sold by yield, not weight alone: an 80 lb bag makes 0.60 cu ft, a 60 lb bag 0.45, and a 40 lb bag 0.30. That same slab takes about 56 of the 80 lb bags - which is why anything much bigger than a small slab is usually a ready-mix truck job. This calculator gives you both, so you can compare a bag count against a truck order honestly.
Always add waste. Real pours spill, forms bow and the ground under a slab is never perfectly level. 10% extra is the standard allowance, and running short mid-pour is far worse than having a bag or two left over - the calculator includes it by default.
Most calculators treat a post hole as a plain cylinder. Ours subtracts the post itself - a 4x4 post sitting in a 12-inch hole displaces a real share of the concrete, and over a fence line of 20 posts that difference is several bags you did not need to buy or mix.
You can never remove wet concrete and start again, so measure twice. If your pour is an odd shape, split it into rectangles, run each one, and add the results.
About 45 bags - a yard is 27 cu ft and an 80 lb bag yields 0.60 cu ft. For 60 lb bags it is about 60, and for 40 lb bags about 90.
4 inches suits patios, paths and shed bases. Driveways and anything carrying vehicles usually need 5-6 inches, often with reinforcement.
Roughly beyond 1-2 cubic yards. Bags win on small jobs - a few post holes, a step, a small pad - because a truck has minimum-load and delivery charges.
10% is the standard waste allowance for spills, bowed forms and uneven ground. The calculator adds it by default; set it to 0 only for exact, contained forms.
More for the project: Mulch Calculator, Tile Calculator, Volume Converter and all Home Building Calculators.