Concrete Calculator
Volume from length, width, and thickness (feet and inches). Converts to cubic yards.
Slab dimensions
Volume: cubic yards
≈ cubic feet · About bags (80 lb premix, rough estimate)
Why concrete is measured in cubic yards
Ready-mix concrete is sold and delivered by the cubic yard, so this calculator converts your slab's length × width × thickness (in cubic feet) into cubic yards by dividing by 27 (since one cubic yard equals 3×3×3 = 27 cubic feet). The bag estimate offers a rough alternative for smaller DIY pours using premixed bags instead of a truck delivery.
Frequently asked questions
Should I order extra concrete beyond the calculated volume? Yes — most contractors add 5–10% extra to account for uneven subgrade, spillage, and minor form deflection, since running short mid-pour is a much bigger problem than having a small amount left over.
Why does thickness use both feet and inches? Slab thickness is almost always specified in inches (like a standard 4-inch slab) while length and width are typically in feet — splitting the input this way avoids needing to convert inches to a decimal fraction of a foot yourself.
When would I use bags instead of a ready-mix truck delivery? Bagged premix is practical for small pours (like a fence post footing or a small patch), while anything approaching a full cubic yard or more is generally far more economical and easier to place via truck delivery.