Concrete Slab: How Many Bags (or Yards) You Need

Work out the volume

Volume = length × width × thickness, with thickness converted from inches to feet. A 10 × 10 slab at 4 inches is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or about 1.3 cubic yards (there are 27 cubic feet in a yard).

Try it Concrete Calculator → Work out cubic yards of concrete and how many 60 or 80 lb bags you need for a slab, with a waste allowance and rough cost.

Bags vs. ready-mix

An 80 lb bag yields about 0.60 cubic feet; a 60 lb bag about 0.45. That 33.3 cu ft slab needs roughly 56 eighty-pound bags — which is a lot of mixing. As a rule, once you pass about half a cubic yard, price out ready-mix delivery.

• Add 5–10% for spillage and an uneven subgrade.
• Compact and dampen the base, and set forms dead level — concrete won't fix a bad base.
• For footings or structural work, check local code for depth and rebar.

How thick should the slab be?

4 inches is standard for patios and walkways; 5–6 inches for driveways or anything bearing vehicles. Thickness drives volume directly, so measure it honestly.