Estimate the concrete volume and bags needed for a slab.
Calculated instantly in your browser.
How do you calculate how much concrete you need for a slab?
Volume = length × width × thickness, where an 80 lb bag yields about 0.6 ft³ and a 60 lb bag about 0.45 ft³. For example, a 10 ft × 10 ft slab, 4 inches thick, needs about 1.23 cubic yards of concrete. Add roughly 5–10% extra for spillage, over-excavation and uneven ground; bag yields are approximate.
Understanding your result
Add roughly 5–10% extra for spillage, over-excavation and uneven ground. Bag yields are approximate.
Formula and method
Volume = length × width × thickness. An 80 lb bag yields about 0.6 ft³ and a 60 lb bag about 0.45 ft³.
Worked example
A 10 ft × 10 ft slab, 4 inches thick, needs about 1.23 cubic yards of concrete.
How to use this tool
- Choose your units.
- Enter the length, width and thickness.
- Press Calculate.
About the Concrete Calculator
The Concrete Calculator estimates how much concrete a slab needs, in cubic yards and cubic metres, and how many pre-mixed bags that is.
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Frequently asked questions
How many bags of concrete do I need?
It depends on the volume and bag size. The calculator estimates both 60 lb and 80 lb bag counts for your slab.