About this calculator
Concrete is sold by the cubic yard (yd³) in ready-mix or by the bag for small jobs. Volume = length × width × depth in consistent units. This calculator converts feet/inches to cubic yards and recommends ready-mix vs bag-mixing based on volume.
Common depths
- 2" — barely structural, fine for decorative overlays.
- 4" — standard for patios, walkways, garage floors. Almost always with rebar or wire mesh.
- 5–6" — driveways, heavy load areas. Always reinforced.
- 8–12" — footings, foundation walls. Engineered designs only.
Ready-mix vs bag math
A 80-lb bag of concrete mix yields about 0.6 cubic feet. A 4×4×4" slab needs 0.05 yd³ or 1.3 cubic feet, about 2 bags. A 10×10×4" slab needs 1.2 yd³ or 32 cubic feet, about 53 bags — at that point, ready-mix delivery is the rational choice even with the short-load fee.
Why waste matters
Excavations are never perfectly to depth — some areas dig out an extra half-inch. Spillage during placement is normal. Order 10–15% over your calculated volume. Coming up short means an extra trip and another minimum-load fee from the concrete plant ($300–500), so the math always favors ordering extra.
Ready-mix logistics
Most plants have a 1-yard minimum for ready-mix delivery, often with a "short load" surcharge ($100–300) for orders under 4 yards. Truck holds 8–11 yards. Discharge time is limited (45–60 minutes) before the load starts to set — have everything (forms, rebar, helpers) ready before the truck arrives. Plan for cleanup of the truck chute area too.