How Much Concrete Do I Need for a Slab?
Quick answer: multiply the slab's length × width × thickness (in feet) for cubic feet, then divide by 27 for cubic yards. A 10 × 10-foot slab at 4 inches is about 1.23 cubic yards, or roughly 1.4 with a 10% overage. The concrete slab calculator gives you both cubic yards and bag counts; here's how it works.
The math
Concrete volume is just length times width times thickness — the only catch is that thickness is in inches while the footprint is in feet, so divide the thickness by 12 first:
Cubic feet = Length(ft) × Width(ft) × (Thickness(in) ÷ 12) Cubic yards = Cubic feet ÷ 27
For a 10 × 10 slab at 4 inches: 10 × 10 × (4 ÷ 12) = 33.3 ft³ ÷ 27 = 1.23 cubic yards.
How thick should the slab be?
- 4 inches — patios, shed floors, walkways
- 4–6 inches — driveways
- 6 inches or more — heavy vehicles or loads
Anything structural, and most slabs over 4 inches, want rebar or welded wire mesh. The thickness drives the concrete volume, so get it right before you order.
Bags or ready-mix?
The concrete slab calculator shows both, because the right answer flips with size:
- Bags make sense up to about half a cubic yard — roughly a 3×3 slab at 4 inches.
- Ready-mix by the cubic yard wins past that. A 10×10 slab is over 60 eighty-pound bags — a brutal day of mixing — versus one short truck delivery.
Don't forget the base and the overage
Most slabs sit on 4 inches of compacted gravel that spreads load and drains, which prevents cracking — size it with the gravel calculator. And always order about 10% extra concrete: you can't pause a pour to mix more, and the subgrade is never perfectly flat.
Pouring footings under the slab edge too? The concrete footing calculator sizes those.
Sources
- QUIKRETE Concrete Mix (No. 1101) product data — bag yields used for the bag counts
- American Cement Association — slab thickness, base, and ready-mix practice
Frequently asked questions
How many cubic yards of concrete for a 10x10 slab?+
About 1.23 cubic yards for a 10×10-foot slab at 4 inches thick, or roughly 1.4 cubic yards with a 10% overage. Enter your slab dimensions in the concrete slab calculator for an exact figure.
How many bags of concrete for a slab?+
An 80-lb bag yields about 0.6 cubic feet, so a 10×10 slab at 4 inches (≈33 ft³) needs about 62 eighty-pound bags with waste. Past about half a cubic yard, ready-mix is cheaper than bags.
How thick should a concrete slab be?+
Four inches is standard for a patio, shed floor, or walkway; 4–6 inches for a driveway; and 6 inches or more for heavy loads. Thicker and structural slabs usually need rebar or wire mesh.
How much concrete for a 12x12 slab?+
About 1.78 cubic yards at 4 inches thick — 12 × 12 × (4 ÷ 12) = 48 cubic feet ÷ 27 — or roughly 2 cubic yards with a 10% overage. At 6 inches it is about 2.67 cubic yards before overage.
How much does a cubic yard of concrete cover?+
About 81 square feet at 4 inches thick, 65 square feet at 5 inches, and 54 square feet at 6 inches. The same volume spreads over less area as the slab gets thicker, so settle the thickness before you order.
Do I need gravel under a concrete slab?+
Yes — a 4-inch compacted gravel base spreads the load, drains water away, and helps prevent cracking and heaving. Size it with the gravel calculator and compact it before you pour; a slab on soft, uneven ground fails early.
How much extra concrete should I order?+
About 10%. You cannot pause a pour to mix more, the subgrade is never perfectly flat, and forms bow slightly. For the 1.23-cubic-yard example slab, order about 1.4 cubic yards, or round up to the truck's next increment.