Concrete Calculator

How it is calculated

Cubic Yards = (Length [ft] × Width [ft] × Thickness [in] ÷ 12) ÷ 27; Add 10% for subgrade variance

Worked example: Pouring a 12 ft × 12 ft Patio Slab 4 Inches Deep

12 ft length, 12 ft width, 4 inch slab thickness, 10% waste buffer.

1. Convert thickness to feet: 4 inches ÷ 12 = 0.333 feet.
2. Multiply dimensions for cubic feet: 12 × 12 × 0.333 = 48 cubic feet.
3. Add 10% waste: 48 × 1.10 = 52.8 cubic feet.
4. Convert to cubic yards: 52.8 ÷ 27 = 1.96 cubic yards (round up to 2.0 yds for ready-mix truck dispatch).
5. Calculate 80lb bags: 52.8 ÷ 0.60 = 88 bags.

1.96 Cubic Yards (or 88 bags of 80lb pre-mix).

Frequently asked questions

How many cubic feet are in a cubic yard of concrete?

There are exactly 27 cubic feet in one cubic yard (3 ft × 3 ft × 3 ft).

When should I call a ready-mix truck instead of mixing bags?

Pours requiring more than 1 to 1.5 cubic yards (over 45-60 bags) are generally much faster, cheaper, and structurally more consistent when ordered from a ready-mix truck.

How many 80 lb bags do I need for one cubic yard?

One 80 lb bag yields about 0.60 cubic feet, so one cubic yard (27 cubic feet) needs 45 bags before any waste margin. The calculator applies the same yield to your slab volume.

How many 60 lb bags do I need for one cubic yard?

One 60 lb bag yields about 0.45 cubic feet, so one cubic yard (27 cubic feet) needs 60 bags before any waste margin.

How thick should a driveway slab be?

The calculator defaults to 4 inches, which is standard for patios and sidewalks. For a driveway carrying heavy vehicles, use 6 inches instead — thickness changes the volume directly, so always enter the planned depth.

Why does the calculator add a waste margin?

Uneven subgrades, spillage, and form deflection mean pours almost always need 8% to 12% more concrete than the theoretical volume. The calculator offers 5%, 10%, and 15% margins, with 10% as the recommended standard.