Concrete Volume & Yardage Calculator
Estimate how much concrete you need for slabs, footings, columns, and stairs. Calculate cubic yards and the number of bags required.
Pro Tip: Always order slightly more than you think you'll need. A 10% wastage factor is standard to account for spillage, subgrade variations, and form deflection.
How to Calculate Concrete Needs
Calculating the amount of concrete needed for a project is essential to avoid waste and ensure you have enough material to finish the job. The basic formula for volume is:
However, most concrete is sold by the Cubic Yard. Since measurements are often taken in feet or inches, you must convert the final volume. There are 27 cubic feet in one cubic yard.
For circular projects like holes or columns, the formula is:
Concrete Bags vs. Cubic Yards
If you are using pre-mixed bags from a hardware store, use this table to understand how many bags make up a cubic yard.
| Bag Size | Yield (Cubic Feet) | Bags per Cubic Yard |
|---|---|---|
| 80 lb Bag | 0.60 cu ft | 45 Bags |
| 60 lb Bag | 0.45 cu ft | 60 Bags |
| 40 lb Bag | 0.30 cu ft | 90 Bags |
How Much Extra Concrete Should I Order?
In engineering and construction, it is standard practice to add a 5% to 10% wastage factor to your order. This accounts for several real-world variables:
- Subgrade variations: If the ground isn't perfectly level, some areas will be deeper than measured.
- Form deflection: The weight of wet concrete can push out wooden forms slightly.
- Spillage: Small amounts of concrete are often lost during the pouring and leveling process.
- Evaporation: In very hot conditions, a tiny amount of volume can be lost to moisture evaporation.
Frequently Asked Questions
It takes exactly 45 bags of 80 lb pre-mixed concrete to equal one cubic yard. Each 80 lb bag yields approximately 0.6 cubic feet of wet concrete.
For a standard 4-inch thick 10x10 slab, you need approximately 1.23 cubic yards of concrete. Including a 10% wastage factor, you should order or mix 1.35 cubic yards (about 61 bags of 80 lb concrete).
Cement is an ingredient in concrete. Concrete is a mixture of aggregates (sand and gravel), water, and cement. Cement acts as the "glue" that binds the aggregates together.
For walkways and patios, 4 inches is standard. For driveways or slabs that will support heavy vehicles, 5 to 6 inches is recommended with rebar reinforcement.