How do I calculate concrete for a slab?

Multiply the length (in feet) by the width (in feet) by the thickness (in feet). Convert inches to feet by dividing by 12. The result is cubic feet. Divide by 27 to get cubic yards.

How many cubic yards of concrete do I need?

Calculate the volume in cubic feet (Length × Width × Thickness in feet), then divide by 27. For example, a 12 × 10 ft slab at 4 inches thick requires about 1.48 cubic yards.

How do I convert cubic feet to cubic yards?

Divide the number of cubic feet by 27. There are 27 cubic feet in one cubic yard (3 × 3 × 3 = 27).

Should I account for extra concrete?

Yes. Most contractors add a small buffer — typically 5–10% — to account for spillage, uneven subgrade, and minor measurement errors. It is better to have a little extra than to run short during a pour.

What measurements do I need before ordering concrete?

You need the length, width, and thickness of the area you are filling. For irregular shapes, break the area into rectangles or circles and calculate each section separately.

Guide

How Much Concrete Do I Need? A Simple Concrete Volume Guide

Whether you are pouring a patio slab, a driveway footing, or a fence post base, figuring out how much concrete to order comes down to one thing: volume. This guide walks through the formula, a worked example, and the common mistakes to avoid so you order the right amount the first time.

Key Takeaways

Concrete volume is calculated as Length × Width × Thickness.
Always convert inches to feet before multiplying for slab thickness.
Divide cubic feet by 27 to get cubic yards — the unit most suppliers use.
Order a little extra to account for spillage, uneven subgrade, and waste.
Use the NOVORY Concrete Calculator for an instant, accurate estimate.

Calculate Your Concrete

How to Calculate Concrete Volume

To figure out how much concrete you need, calculate the volume of the area you are filling. For a rectangular slab, that means multiplying the three dimensions together. The key is making sure all your measurements are in the same unit before you multiply.

1Measure the length and width of the area in feet.
2Determine the thickness (depth) of the slab in inches, then convert to feet by dividing by 12.
3Multiply Length × Width × Thickness (in feet) to get cubic feet.
4Divide the result by 27 to convert cubic feet to cubic yards.
5Add a waste allowance if your project requires it.

The Concrete Volume Formula

For a rectangular slab, the formula is straightforward:

Volume (cu ft) = Length (ft) × Width (ft) × Thickness (ft)

Worked Example: 12 ft × 10 ft Slab at 4 Inches Thick

Let us say you are pouring a slab that is 12 feet long, 10 feet wide, and 4 inches thick. Here is how the math works step by step.

Step 1

Measure the slab: 12 ft long × 10 ft wide, 4 inches thick.

Step 2

Convert thickness to feet: 4 ÷ 12 = 0.333 ft.

Step 3

Calculate cubic feet: 12 × 10 × 0.333 ≈ 40 cubic feet.

Step 4

Convert to cubic yards: 40 ÷ 27 ≈ 1.48 cubic yards.

Common Mistakes

Forgetting to convert thickness from inches to feet before multiplying — this is the most common error and overstates volume by 12×.
Measuring in mixed units (e.g., feet for length, inches for width) without converting first.
Not accounting for subgrade irregularities — if the ground is uneven, you may need more concrete than the math suggests.
Ordering exactly the calculated amount with no buffer. A small surplus is safer than running short mid-pour.
Confusing cubic feet with cubic yards when placing an order with a supplier.

Practical Tips

Round up to the nearest quarter yard when ordering — suppliers typically sell in quarter-yard increments.
If your project has irregular shapes, break it into rectangles and calculate each section separately, then add the volumes.
For post holes, use the cylinder formula: π × radius² × depth, then convert to cubic yards.
Check with your supplier about minimum load sizes and delivery fees before placing an order.
Use the NOVORY Concrete Calculator to double-check your manual calculations.