Concrete Slab Calculator – Estimate Cubic Yards & Cost Instantly

How to Calculate How Many Cubic Yards You Need for a Concrete Slab

When planning a concrete project for your metal building—whether it’s a driveway, shed foundation, or backyard patio—knowing how much concrete you need is crucial. Overestimating can waste money, and underestimating could leave you scrambling to mix more in the middle of a pour.

Fortunately, calculating how many cubic yards of concrete you’ll need for a slab is straightforward with the right steps.

What Is a Cubic Yard of Concrete?

A cubic yard is a measure of volume that’s equal to:

3 feet x 3 feet x 3 feet = 27 cubic feet

Concrete is typically sold by the cubic yard in the U.S., so all volume calculations for slabs must be converted into cubic yards.

Step-by-Step Guide: Calculating Cubic Yards for a Concrete Slab

To calculate how many cubic yards of concrete you need for your slab, you’ll follow this formula:

Let’s break it down step by step.

1. Measure Your Slab Dimensions

You’ll need three dimensions:

• Length (in feet)

• Width (in feet)

• Thickness (in inches)

📏 Example: You want a 10-foot by 20-foot slab that is 4 inches thick.

2. Convert the Thickness from Inches to Feet

Since the formula requires uniform units, convert inches into feet:

3. Multiply Length × Width × Thickness

Now calculate the volume in cubic feet:

4. Convert Cubic Feet to Cubic Yards

Now divide the cubic feet by 27 (since 1 cubic yard = 27 cubic feet):

🧱 You’ll need approximately 2.44 cubic yards of concrete.

5. Add a Margin for Waste

Always add 10% extra to account for spillage, over-excavation, or mismeasurement:

🔎 Final order: Round up to 2.7 or 3.0 cubic yards to be safe.

Quick Reference Table

Concrete Volume Formula Recap

Here’s the universal Concrete Slab Calculator formula you can plug into a calculator:

Tips to Avoid Over/Under Buying

• Double-check measurements in multiple places.

• Use stakes and string lines to verify the slab perimeter.

• Always round up to the nearest 0.25 cubic yards.

• Order from suppliers who allow returns or partial credits (just in case).

How Many Bags of Concrete Is That?

If you’re mixing concrete by hand using 80-lb bags:

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• Each 80-lb bag yields about 0.6 cubic feet

• So divide your cubic feet total by 0.6

🔢 Example: 66 cubic feet ÷ 0.6 = 110 bags (for our 10×20 slab)

This method is labor-intensive, so ready-mix is better for large pours.

Conclusion

Calculating concrete for a slab doesn’t have to be complicated. All you need are three measurements—length, width, and thickness—and a simple Concrete Slab Calculator formula to convert that into cubic yards. Always round up, add a waste margin, and consider ordering a little extra to avoid last-minute shortages. Concrete Slab Calculator