How to Estimate Concrete for a Slab

Getting your concrete quantity right before ordering can save you hundreds of dollars and prevent a mid-pour scramble. Here's the exact formula and the factors that affect how much you actually need.

The Basic Formula

Concrete volume is calculated in cubic yards (cy). The formula is:
Volume (cy) = (Length × Width × Thickness) ÷ 27

All dimensions in feet. You divide by 27 because there are 27 cubic feet in 1 cubic yard.

Example: A 10 ft × 20 ft patio, 4 inches thick (0.333 ft):
10 × 20 × 0.333 = 66.6 cubic feet ÷ 27 = 2.47 cubic yards

Use our Concrete Calculator to skip the math and get the result instantly.

Add a Waste Factor

Always order 5–10% more than your calculated volume. Concrete is lost to:

For a simple rectangular slab, 5–8% extra is usually enough. For complex shapes, forms with angles, or rough subgrade, budget 10%.

Bags vs. Ready-Mix

Bagged concrete (80 lb bags): Best for small jobs under ~1 cubic yard. One 80 lb bag yields about 0.60 cubic feet, so you need roughly 45 bags per cubic yard. Labor-intensive but no minimum order.

Ready-mix truck: Best for anything over 1 cubic yard. Most suppliers have a minimum order of 1 yard, with a short-load surcharge for orders under ~3–4 yards. If you're right on the edge, it's often worth bumping to the minimum to avoid a fee.

Common Slab Thicknesses

If in doubt, go thicker — it's much cheaper to pour 5 inches than to tear out and repour a cracked 3.5-inch slab later.