Concrete Calculator

For a rectangular slab, patio, or sidewalk, multiply Length × Width × Thickness, all converted to the same unit (feet). Length and width are usually measured in feet, but thickness is almost always measured in inches since most slabs are only a few inches thick — so divide the thickness in inches by 12 before multiplying. For example, a 10 ft × 10 ft slab poured 4 inches thick is 10 × 10 × (4/12) = 33.3 cubic feet of concrete.

Ready-mix concrete trucks in the US measure and sell concrete by the cubic yard, not the cubic foot, because a cubic yard (27 cubic feet) is a more practical unit for the volumes typical construction projects require. To convert, divide your total cubic feet by 27. Most concrete suppliers also enforce minimum order sizes (often 1 yard or more), so it helps to know your cubic yardage before calling for a delivery quote.

Bag count depends on bag size and yield: a 60 lb bag yields about 0.45 cubic feet of mixed concrete, while an 80 lb bag yields about 0.6 cubic feet. Divide your total volume in cubic feet by the yield per bag and round up. For example, 33.3 cubic feet requires 75 bags at 60 lb each, or 56 bags at 80 lb each — the 80 lb bags require fewer bags but are heavier to lift and mix.

For a solid cylindrical column or footing, use Volume = π × radius² × height, where radius is half the diameter. For a hollow tube form footing (a cylinder with a center void), subtract the inner cylinder's volume from the outer one: Volume = π × height × (outer radius² − inner radius²). Both formulas return cubic feet when diameter and height are entered in feet, which you can then convert to cubic yards or bags.

Yes — always add a waste and spillage buffer of roughly 5–10% on top of your calculated volume. Uneven subgrade, form leakage, over-excavation, and minor measurement errors almost always mean a project uses more concrete than the pure geometric calculation predicts. Running short mid-pour is far more costly and disruptive than having a little extra, so most contractors round up their order to the nearest half or full cubic yard after adding this buffer.

Cubic yards are the standard unit for ordering ready-mix concrete in the United States, while cubic meters are standard in most other countries. One cubic yard equals about 0.7646 cubic meters, or equivalently one cubic foot equals about 0.0283 cubic meters. If you're ordering concrete internationally or working from metric project plans, converting your cubic-foot volume directly to cubic meters (multiply by 0.0283168) avoids confusion with suppliers.