Concrete Calculator

How it works

Concrete for slabs and footings is sold by the cubic yard (yd³). One cubic yard equals 27 cubic feet, enough to pour a slab roughly 3 ft × 3 ft × 3 ft. For a typical 4-inch residential slab, one cubic yard covers about 81 ft² of surface area.

To calculate, multiply your length × width × thickness — keeping units consistent. Length and width in feet, thickness usually in inches (then divide by 12 to convert to feet). Multiply the three to get cubic feet, then divide by 27 to get cubic yards. This calculator does the conversion automatically.

For small projects, concrete is sold in pre-mix bags. A 60-pound bag yields about 0.45 ft³ of concrete; an 80-pound bag yields about 0.6 ft³. We round up the bag count because partial bags are useless. For projects over 2 yd³, getting a ready-mix truck is usually cheaper and definitely easier than mixing bags by hand.

Always order extra. Add 5–10% to your calculation to cover spillage, uneven subgrade, edge over-pour, and the risk of running short — a partial truckload is much more expensive than a full one. For irregular shapes (footings with steps, curved patios), add 10–15%.

Other considerations. Standard residential mix is 3,000-4,000 PSI; structural elements may need 5,000+. Air-entrained concrete is required in freeze-thaw climates to prevent surface spalling. Reinforcement (rebar, mesh) is separate. Don't forget the gravel/sand base under the slab — usually 4 inches minimum, compacted.

Setting time. Concrete is workable for about 60-90 minutes after mixing. Initial set is 4-8 hours; full design strength (28-day strength) is reached at 28 days, though most of the strength is gained in the first 7 days. Plan finishing work accordingly.