Enter your slab, footing or column size and get the cubic yards, cubic feet and number of bags of concrete to buy.
Concrete is measured by volume in cubic yards. For a slab or footing, multiply length × width × thickness (all in feet, so divide the thickness in inches by 12) to get cubic feet, then divide by 27 for cubic yards. For a round column or post hole, the volume is π × radius² × height. This calculator does both.
Bagged concrete mix yields roughly: an 80 lb bag ≈ 0.60 cubic feet, a 60 lb bag ≈ 0.45 cubic feet, and a 40 lb bag ≈ 0.30 cubic feet (these are the yields printed on standard mixes). Since a cubic yard is 27 cubic feet, that's about 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags.
Bags are fine for posts, small pads and repairs. Once you're past about one cubic yard — roughly 45 of the 80 lb bags — hand-mixing gets exhausting, and ordering a ready-mix truck is usually cheaper and much easier. Most suppliers have a minimum load, so ask.
This is a planning aid. Bag yields are the standard figures for common concrete mixes — check your bag, since yields vary a little by product. Always order about 10% extra: subgrade is never perfectly flat and some concrete is always lost to spillage.