Concrete Calculator
Enter the length, width, and depth of your slab, footing, or pad to instantly get the concrete volume in cubic metres and cubic yards, along with how many 80-lb bags of ready-mix you would need if you go the bagged route instead of ready-mix delivery.
How this calculation works
Concrete volume is simply length times width times depth: Volume = L × W × D. All three dimensions must be in the same unit (metres here), which is why depth — usually given in centimetres — needs to be converted to metres first (10 cm = 0.10 m).
The cubic-metre result is converted to cubic yards using the standard factor 1 m³ = 1.30795 yd³, since ready-mix trucks and many US suppliers quote and sell concrete by the cubic yard.
For the bag count, one 80-lb bag of dry ready-mix concrete yields about 0.017 m³ (roughly 0.6 ft³) once mixed with water. Dividing your total volume by 0.017 and rounding up gives the minimum number of full bags — buy a few extra to cover spillage and an uneven subgrade.
Worked example
Typical slab depths
Depth depends on what the slab needs to carry. Use these as a starting point, then confirm against your local building code:
| Project | Typical depth | Notes |
|---|---|---|
| Garden path / stepping pad | 5–7.5 cm (2–3 in) | Foot traffic only |
| Patio | 10 cm (4 in) | Standard residential patio |
| Sidewalk | 10 cm (4 in) | Reinforce with wire mesh in freeze-thaw climates |
| Driveway (cars) | 10–15 cm (4–6 in) | Add rebar or mesh for durability |
| Driveway (trucks/RVs) | 15–20 cm (6–8 in) | Heavier reinforcement recommended |
| Shed / equipment pad | 10–15 cm (4–6 in) | Check equipment weight rating |
Ready-mix vs. bagged concrete: the break-even point
Ready-mix trucks are batched to order, but most suppliers charge a minimum-load fee once you're under about 1 cubic yard (roughly 0.75–1 m³), and a 'short load' surcharge on top of that for small deliveries. Bagged concrete has no delivery minimum and lets you mix exactly what you need, but the material cost per cubic metre is meaningfully higher and hand-mixing 100+ bags is a serious physical job.
As a rule of thumb: under about 1 m³, buy bags. Between 1–2 m³, compare local bag pricing against the short-load fee. Above 2 m³, ready-mix delivery almost always wins on both cost and labor.
Steps to pour a slab
- Mark out and excavate the area to the required depth plus a gravel sub-base (typically 10 cm of compacted gravel).
- Build and level wooden or metal forms around the perimeter.
- Add reinforcement (wire mesh or rebar) for anything wider than a garden path.
- Order or mix concrete using this calculator's volume, plus 5–10% extra.
- Pour, screed level with the forms, then float and finish the surface.
- Cure for at least 7 days (28 days for full strength), keeping the surface moist for the first few days in hot or dry weather.