How to Calculate Concrete
To determine how much concrete you need, measure the dimensions of the area to be filled. For a rectangular slab, you will need the length, width, and thickness.
Formula for Rectangular Slab:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
Volume (cubic yards) = Volume (cubic feet) ÷ 27
Example Calculation
A rectangular patio is 12 ft long, 10 ft wide, and 4 inches thick.
- Thickness in feet: 4 / 12 = 0.333 ft
- Volume in cubic feet: 12 × 10 × 0.333 = 40 cubic ft
- Cubic yards: 40 / 27 = 1.48 cubic yards
To account for variations in grading or spillage, you would normally add a 10% waste factor, bringing the total order recommendation up to roughly 1.63 cubic yards.
How Much Should I Buy?
You should almost always round up your final calculation and add a 5% to 10% waste factor. Concrete requirements can vary due to uneven subgrade, forms bowing, and material spillage.
Frequently Asked Questions
How much concrete do I need for a slab?
Measure the length, width, and depth of the slab in feet, multiply them together to get cubic feet, and divide by 27 to get cubic yards. You can easily find this out using our concrete calculator above.
How many bags of concrete do I need?
This depends on the bag size. An 80 lb bag of concrete mix typically yields about 0.60 cubic feet. Divide your total cubic feet by 0.60 to find the number of 80 lb bags required. Always check the manufacturer's yield on the bag.
How many cubic yards are in a cubic foot?
There are 27 cubic feet in one cubic yard. So, 1 cubic foot equals 1/27 (or about 0.037) cubic yards.
Should I add extra concrete for waste?
Yes. A 10% waste factor is standard for most residential flatwork to account for uneven subgrades and spillage during the pour. A sudden shortage during a pour can cause a cold joint and compromise the structural integrity.
How do I calculate concrete for footings?
Footings are calculated the same way as slabs: Length × Width × Depth. If your footing is a continuous trench, measure the total linear length of the trench, the width of the trench, and its depth.