How this is calculated
Concrete volume is length × width × depth, with all three dimensions converted to the same unit before multiplying. Depth is usually the smallest of the three, so it's easy to enter it in the wrong unit — this calculator keeps depth in inches (or centimetres) specifically because that's how depth is specified on a real set of plans.
Ready-mix suppliers and bagged concrete are both sold in fixed volume increments, so this calculator adds a waste allowance on top of the raw volume before recommending an order quantity — pouring exactly the calculated volume with no allowance for spillage, an uneven subgrade, or over-excavation almost always comes up short.
Formula & methodology
Volume = Length × Width × Depth. Cubic yards = Volume (ft³) ÷ 27. Recommended order = Volume × (1 + waste %).
Worked example
A 10 ft × 10 ft patio slab, 4 in thick
- 1.Convert depth to feet: 4 in ÷ 12 = 0.33 ft
- 2.Volume = 10 ft × 10 ft × 0.33 ft = 33.3 ft³
- 3.Cubic yards = 33.3 ÷ 27 = 1.23 yd³
- 4.With a 10% waste allowance: 1.23 × 1.10 = 1.36 yd³ — round up to what your supplier can deliver.
Frequently asked questions
How thick should a concrete slab be?
A typical patio or walkway slab is 4 inches (10 cm) thick. Driveways for cars are usually 5–6 inches (13–15 cm), and slabs carrying heavier vehicles go thicker still. Check local building code for load-bearing requirements before pouring.
Should I order ready-mix or use bagged concrete?
Ready-mix delivery is almost always cheaper and less labor once you're above roughly 1 cubic yard (0.75 m³). Below that, bagged concrete mixed on site is usually more practical — this calculator shows both.
Why does the recommended quantity include extra on top of the calculated volume?
The waste allowance covers an uneven subgrade, spillage, and minor over-excavation. Most contractors add 5–10%; irregular or hand-dug forms often need closer to 10–15%.
Does this include rebar or a gravel sub-base?
No — this calculator estimates concrete volume only. Sub-base gravel is estimated separately with the Gravel Calculator, and rebar/mesh requirements depend on your local code and the slab's use.
Last updated 2026-07-01. Formulas are planning methods — confirm with a supplier before you order.