Concrete Calculator — cubic yards and bags for a slab

How it works

Concrete is ordered by the cubic yard (for ready-mix trucks) or bought in bags (for small pours). Both start from the slab volume:

cubic yards = (length × width × thickness) ÷ 27

with all three dimensions in feet. Thickness is entered in inches — a 4-inch slab — and converted to feet (÷12). Dividing the cubic-foot volume by 27 gives cubic yards, since one cubic yard is 27 cubic feet.

For bags, divide the volume in cubic feet by the yield of one bag and round up. An 80-pound bag of concrete mix yields about 0.6 cubic feet; a 60-pound bag about 0.45. The bag yield is an input here, so set it to match the product you are buying.

Always order a little extra — about 5–10% — to allow for spillage, uneven subgrade and over-excavation. Running short mid-pour means a cold joint.

Worked example

A 10 × 10 ft patio, 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet = 1.23 cubic yards. At 0.6 ft³ per bag that is ⌈33.3 ÷ 0.6⌉ = 56 bags — a lot, which is why anything this size is usually ready-mix. A 40 × 12 ft driveway, 4 inches thick, is 160 cubic feet = 5.93 cubic yards (about 267 bags, clearly a truck job).

Frequently asked questions

How much concrete do I need for a slab?

Multiply length × width × thickness (all in feet) and divide by 27 for cubic yards. For thickness in inches, divide by 12 first. A 10-by-10-foot slab 4 inches thick needs about 1.23 cubic yards.

How many bags of concrete are in a cubic yard?

It depends on the bag. One cubic yard is 27 cubic feet, so at 0.6 ft³ per 80-pound bag that is 45 bags per cubic yard; at 0.45 ft³ per 60-pound bag, 60 bags. Set the bag yield to match your product.

When should I use bags versus ready-mix?

Bags are practical for small jobs — a few post holes, a small pad, repairs — up to roughly half a cubic yard. Beyond that the bag count (and mixing effort) climbs fast, so ready-mix delivery by the cubic yard is cheaper and easier.

How much extra concrete should I order?

Add about 5–10%. Subgrades are never perfectly flat, forms bow slightly, and some concrete is always lost to spillage. For ready-mix, round up to the next quarter yard; running out during a pour creates a weak cold joint.

What thickness should a slab be?

Common thicknesses are 4 inches for patios, walkways and shed floors, and 5–6 inches for driveways and areas bearing vehicles. Thicker slabs and those bearing heavy loads usually need reinforcement; check local building requirements for structural work.

Does this include reinforcement or a gravel base?

No — this calculates the concrete volume only. A slab typically sits on a compacted gravel base (calculate that separately) and may need rebar or wire mesh. Those add to the project but do not change the concrete volume.