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.
Assumptions and limitations
This estimates a regular slab from the dimensions you enter, and bag yields are approximate. Real needs vary with over-excavation, uneven ground, spillage and the exact product, so add a waste allowance and round up. It does not size reinforcement, footings or ready-mix loads; confirm large orders with your supplier.
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.
Who should use this tool
DIYers and tradespeople pouring a slab, footing or small pad who want a quick volume and bag count before ordering. Handy for garden projects, shed bases and paths, and for checking a supplier's quote against a rough figure of your own.
Benefits
- Estimates volume in cubic yards and cubic metres
- Converts the volume into a pre-mixed bag count
- Helps you order the right amount before you start
- Free and browser-based, usable right on site
Practical use cases
- Sizing a slab for a shed or garden base
- Working out bags for a small path or pad
- Checking a ready-mix quote against your own figure
- Budgeting concrete before a trip to the merchant
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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.
Should I order bags or ready-mix?
Bags suit small jobs where you can mix by hand or in a small mixer, while larger pours are usually cheaper and faster with ready-mix delivered by truck. As a rough guide, once you are into cubic yards rather than a handful of bags, ready-mix is worth pricing up.
How much extra concrete should I allow?
Adding roughly 5 to 10 percent covers spillage, over-excavation and uneven ground, and a little more helps on rough or sloping sites. Concrete cannot be topped up once it starts to set, so it is safer to order slightly over than to fall short mid-pour.