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.