How much concrete do I need?

Multiply length × width × thickness to get the volume, then convert to cubic yards (divide cubic feet by 27). For example, a 10 ft × 10 ft slab at 4 inches thick is about 1.23 cubic yards — roughly 1.36 yards once you add 10% for waste. This calculator does the math from your own dimensions.

Cubic yards = (length in feet × width in feet × thickness in feet) ÷ 27. Convert thickness from inches to feet by dividing by 12. Always order a little extra — about 10% — to cover spillage, uneven subgrade, and over-excavation.

A cubic yard is 27 cubic feet. An 80lb bag of pre-mixed concrete yields about 0.6 cubic feet, so a cubic yard takes roughly 45 bags; a 60lb bag yields about 0.45 cubic feet, or about 60 bags per yard. Bags are practical for small pours — for anything over about a cubic yard, ready-mix delivery is usually cheaper.

Add about 10%. Subgrade is rarely perfectly level, forms flex, and some concrete is always lost to spillage — running short mid-pour is far more expensive than ordering a little extra. This calculator adds 10% to the “with waste” figure automatically.