*Bag estimates are based on typical yield of 0.6 ft³ per 80lb bag and 0.45 ft³ per 60lb bag. Always check manufacturer specifications.
How to Calculate Concrete for Your Project
Whether you are pouring a simple backyard patio, a driveway, or footings for a new deck, determining the exact amount of concrete required is crucial. Ordering too little can result in a structural seam or a "cold joint" that weakens your slab, while ordering too much is a waste of money and resources.
The Basic Formula
Concrete is measured by volume, typically in Cubic Yards. To find this volume, you must measure the length, width, and thickness of your pour area.
The calculation generally follows these steps:
- Calculate Cubic Feet: Multiply Length (ft) × Width (ft) × Depth (ft). Note: Since depth is usually measured in inches, divide the inches by 12 to convert to feet first.
- Convert to Cubic Yards: There are 27 cubic feet in one cubic yard. Divide your total cubic feet by 27.
Why Include a Waste Margin?
Professional contractors rarely order the exact mathematical volume. Several factors contribute to volume loss during a pour:
- Uneven Subgrade: If the ground isn't perfectly flat, the concrete will fill the deeper pockets, increasing the required volume.
- Spillage: Some concrete is inevitably lost during the transfer from the truck or mixer to the form.
- Form Bowing: Wooden forms may bow slightly outward under the heavy pressure of wet concrete, slightly increasing the width of the slab.
We recommend adding a 5% to 10% safety margin to your order to account for these variables.
Bags vs. Ready-Mix Truck
When should you buy bags, and when should you call a truck?
Use Bags (60lb or 80lb): For small projects requiring less than 1 cubic yard (approx. 45-60 bags). This is ideal for setting fence posts, small walkways, or landing pads.
Order a Truck: For any project requiring more than 1 cubic yard. Mixing 60+ bags of concrete by hand is labor-intensive and makes it difficult to achieve a consistent finish across the entire slab before it begins to cure.