How it’s worked out
Worked out by this calculator, before any extra, with bags of ready-to-mix making about 11 litres each (a typical 25 kg bag):
Slab or footing: length × width × depth Round hole: π × (diameter ÷ 2)² × depth, less the post
| Job | Concrete | Bags |
|---|---|---|
| Patio slab 3 × 3 m, 10 cm deep | 0.9 m³ | 82 |
| Shed base 2.4 × 1.8 m, 10 cm | 0.432 m³ | 40 |
| Strip footing 10 m × 45 cm, 25 cm deep | 1.125 m³ | 103 |
| 4 post holes, 30 cm across, 60 cm deep, 10 cm posts | 0.146 m³ | 14 |
In imperial, a 10 × 10 ft slab 4 in deep is 1.23 cubic yards, or 56 bags making 0.6 ft³ each (a typical 80 lb bag).
Bags or a delivery?
Bags suit small jobs such as fence posts. For more than about half a cubic metre, ready-mixed concrete delivered by truck is usually cheaper and easier. Order a little over: running short mid-pour leaves a weak joint.
Mixing your own
A common general-purpose mix is 1 part cement, 2 parts sand and 3 parts gravel by volume, with just enough water to make it workable. Foundations and structural work should follow the engineer’s or local building rules’ specification.
How to read the result
- Concrete: the volume with the extra you chose.
- Bags: when you type what one bag makes.
- The other unit: cubic yards or cubic metres, for ordering a delivery.
Common mistakes
- Typing the depth in metres rather than centimetres.
- Ordering exactly the volume: running short mid-pour leaves a weak joint.
- Forgetting to take the posts out of post holes.
How to use it
- Choose the shape: slab, footing, post holes or round column.
- Type the sizes, and how many holes or columns.
- Choose the extra (5–10% is usual) and, for bags, what one bag makes.
Questions
How much concrete for a 3 × 3 m slab?
0.9 m³ at 10 cm deep, before extra: about 82 bags of 25 kg.
How many bags of concrete make a cubic metre?
About 90 bags of 25 kg ready-to-mix, at roughly 11 litres a bag. Bag yields vary, so check yours.
Should I add extra?
Yes, usually 5–10%, for spills, uneven ground and the formwork bowing slightly.
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