Concrete & Masonry calculators
Slabs, bags, and cubic yards of concrete.
Use the concrete calculator when you are pouring a slab, patio, sidewalk, footing, or round pad and will buy ready-mix or bagged mix. Enter length and width in feet and thickness in inches. The result is cubic yards plus a bag count from mix yield, not from the weight printed on the sack. If you only need the volume of a hole, dumpster, or dirt pile, use the cubic yard calculator instead and leave concrete off until you actually order mix.
US ready-mix plants sell by the cubic yard (27 cubic feet). Bagged mix is sold as 40, 60, or 80 lb sacks. The number that matters is the yield on the bag: about 0.60 cubic feet for an 80 lb sack and 0.45 for 60 lb. Divide cubic feet by that yield and round up. Ready-mix usually costs less once you pass about one cubic yard after waste, and it arrives as a truck, not a stack of bags.
Common mistakes: typing 4 feet when you meant 4 inches, which inflates the load twelvefold; counting bags from the weight of cured concrete instead of the bag yield (0.60 cu ft per 80 lb bag); skipping 5 to 10 percent extra for over-excavation and spillage; treating a 10 × 10 ft patio at 4 inches as about a yard. That slab is about 1.23 cubic yards before waste. Confirm thickness with a tape before you call the plant.