Concrete Calculator
How much concrete for a slab, path or round post hole.
Good to know. Running out one bag short is a very bad afternoon. Order a little extra.
Approximate. Real-world values vary.
How it works
In plain words
Volume is length × width × thickness for a slab, or π × radius² × depth for a round hole. Convert everything to the same unit first (thickness is usually in cm or inches), then add the waste allowance.
The formula
slab: V = length × width × thickness round: V = π × (diameter ÷ 2)² × depth × count order = V × (1 + waste ÷ 100)
Worked example
A 4 × 3 m slab, 10 cm thick: 4 × 3 × 0.1 = 1.2 m³. With 10% extra, order 1.32 m³ (about 1.73 cubic yards).
Sources
- Quikrete and Sakrete bag labels What a bag fills: 0.60 cubic feet for 80 lb, 0.45 for 60 lb.
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B Exact conversion factors for US customary units.
Sources checked September 29, 2026. 5 automated tests check its answers on every change.
About the concrete calculator
Concrete is ordered by volume, in cubic metres or cubic yards, so the job is to turn your measurements into a volume. The most common mistake is mixing units: the length and width are usually in metres or feet, but the thickness is in centimetres or inches. A 10 cm slab is 0.1 m thick. The calculator handles the conversion.
Always order a little more than the exact volume. Ground is rarely perfectly level, formwork bows slightly, and some concrete is lost in the mixer, the barrow or the pump. 5 to 10% extra is typical, more for uneven ground. Running short in the middle of a pour is far worse than having a little left over.
Typical thicknesses: garden paths and shed bases around 7.5 to 10 cm (3 to 4 inches), patios and driveways for cars around 10 to 15 cm (4 to 6 inches). Heavier loads need thicker slabs, reinforcement and a proper sub-base. For anything structural, follow your engineer’s or building control’s specification.
For small jobs, bagged concrete is easier. In the US an 80 lb bag makes about 0.6 cubic feet and a 60 lb bag 0.45, so a cubic yard (27 cubic feet) takes 45 bags of 80 lb: past a yard or so, ready-mix from a truck is usually cheaper and far less work. Bag sizes and yields elsewhere vary by product, so check the bag. Ready-mix suppliers often have a minimum order.
Questions people ask
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in the same unit. 4 m × 3 m × 0.1 m = 1.2 m³. Add 5–10% for waste.
How many cubic yards are in a cubic metre?
About 1.308. One cubic yard is about 0.765 m³.
How many 80 lb bags of concrete make a yard?
45. An 80 lb bag makes about 0.6 cubic feet, and a cubic yard is 27 cubic feet. With 60 lb bags (0.45 ft³ each), it takes 60.
How thick should a concrete slab be?
Commonly 10 cm (4 in) for patios and paths and 10–15 cm (4–6 in) for driveways. Structural slabs need an engineer’s specification.