How to calculate concrete for a slab
Concrete is ordered by volume. Multiply length × width × thickness, keeping every measurement in the same unit, then add a margin for waste.
cubic yards = L (ft) × W (ft) × (thickness in ÷ 12) ÷ 27
cubic metres = L (m) × W (m) × (thickness cm ÷ 100)
Worked example
A 10 × 10 ft patio, 4 inches thick: 10 × 10 × 0.333 = 33.3 cu ft, which is 1.23 cubic yards. Add 10 percent and you need about 1.36 cubic yards, or 62 bags of 80 lb premix.
How thick should the slab be?
| Use | Typical thickness |
|---|---|
| Garden path, patio, shed base | 4 in (100 mm) |
| Hot tub pad, heavier equipment | 4 to 6 in (100 to 150 mm) |
| Driveway for cars | 5 to 6 in (125 to 150 mm) |
These are common starting points. Building codes, soil conditions and reinforcement change the answer, so check local rules and get engineering advice for anything structural, such as house foundations.
Why add extra for waste?
The ground under a slab is never perfectly flat, forms bow slightly when they fill, and some concrete stays in the mixer or wheelbarrow. Most contractors add 5 to 10 percent. Running short is the expensive mistake: a second delivery costs time, and a joint between two pours is weaker than one continuous slab.
Bags or ready-mix truck?
Bagged premix suits small jobs such as fence posts, steps and pads under about one cubic yard. Beyond that, mixing by hand becomes heavy work and it is hard to keep the pour continuous. Ready-mix suppliers deliver larger volumes, but many have a minimum order or charge extra for small loads, so ask before you book.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Frequently asked questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. That takes 45 bags of 80 lb premix, 60 bags of 60 lb, or 90 bags of 40 lb.
How do I convert cubic yards to cubic metres?
One cubic yard is 0.765 cubic metres, and one cubic metre is 1.31 cubic yards.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab is 1.23 cubic yards before waste, or about 1.36 cubic yards with 10 percent extra.
Does this work for round holes and footings?
This calculator is for rectangular slabs. For a square footing, enter its length, width and depth. For round post holes, multiply 3.14 × radius² × depth.
Last updated November 1, 2026.