Concrete Calculator: How Much Concrete Do You Need?

One cubic yard is 27 cubic feet and an 80 lb bag yields about 0.6 cubic feet, so it takes about 45 bags with no waste. Because job-site ground is rarely perfectly level, many people buy 5–10% more, roughly 48–50 bags, to avoid running short mid-pour.

A 10 ft × 10 ft slab at 4 inches deep equals 33.3 cubic feet, or about 1.23 cubic yards with no waste. That is roughly 56 bags of 80 lb mix or 75 bags of 60 lb mix. With a 10% waste buffer it becomes about 1.36 cubic yards, or 62 bags of 80 lb mix.

For pours over about 1 cubic yard (roughly 40–45 bags), a ready-mix truck is almost always faster and cheaper per yard than bagging, once you factor in labor. Smaller pours — post holes, small patches — are usually more practical with bags.

Ready-mix concrete often runs roughly $150–$200 or more per cubic yard, depending on region and mix design. Deliveries under about 4–5 cubic yards commonly carry a short-load fee, often $75–$150 or more. Ask your local supplier for a current quote.

10% is the standard default for most flatwork. Use 5% for very controlled, form-boarded pours, and 15–20% for uneven ground, hand-dug footings, or irregular shapes where over-excavation is likely.

No — reinforcement steel displaces a negligible amount of volume and is not subtracted here. The waste factor built into each calculator already covers this along with minor sub-base and form variance.