How Much Concrete Do I Need?

Concrete is sold and poured by volume, not by area, so the first step in any pour — a slab, a footing, a set of fence posts — is converting your dimensions into cubic yards. The math is simple once you know the steps, and getting it right the first time avoids the two worst outcomes: running short mid-pour or paying for a truck’s worth of leftover concrete.

Length times width times thickness

Concrete volume starts with three measurements: length, width, and thickness, all in feet. Thickness is usually given in inches, so convert it first by dividing by 12 — a standard 4-inch slab is 0.333 feet thick. Multiply length by width by thickness to get cubic feet. A 10-foot by 10-foot slab at 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet. Run your own numbers through the concrete slab calculator to skip the manual conversion and get cubic yards directly.

Converting cubic feet to cubic yards

Ready-mix concrete is priced and delivered by the cubic yard, and one cubic yard equals 27 cubic feet. Divide your cubic-feet total by 27 to get the figure you’ll actually order: 33.3 ÷ 27 ≈ 1.23 cubic yards. For footings, which are typically long, narrow, and shallow, the same formula applies — just be careful measuring depth accurately, since a shallow footing is more sensitive to small errors. The concrete footing calculator handles the footing-specific version of this math, including linear-footage inputs.

Bagged concrete: how many bags per cubic yard

For smaller jobs — a fence post base, a small patch, a single step — bagged concrete mix is usually more practical than ordering a truck. Bag yield depends on bag size: an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45 cubic feet, and a 40 lb bag about 0.3 cubic feet. That works out to roughly 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix per cubic yard. The concrete bags calculator converts your cubic-foot or cubic-yard total into a bag count for whichever bag size you’re buying.

When to switch to ready-mix

Bagged concrete makes sense up to roughly 1 cubic yard — beyond that, the labor of mixing dozens of bags by hand (or with a small mixer) usually costs more in time than a ready-mix delivery costs in money. Most ready-mix suppliers also have a delivery minimum, often around 1 cubic yard, so a project right at that threshold is worth pricing both ways before deciding.

Add a waste margin

Order 5–10% more concrete than your calculated volume. Subgrade isn’t perfectly flat, forms can bow slightly under the weight of wet concrete, and spillage during placement is unavoidable. Running short mid-pour is far more costly than a small surplus — a cold joint where an unfinished pour has to wait for a second batch is a real structural weak point, not just a cosmetic one.