Concrete volume is length × width × thickness. A slab of 3 × 3 m, 10 cm thick, needs 0.9 m³; with 10% extra for waste and uneven ground, order 0.99 m³ (1.29 yd³). Enter your sizes in meters, centimeters, feet or inches – each field has its own unit – and the calculator gives cubic meters, cubic yards, cubic feet, the number of premixed bags and the weight of the concrete.
Typical slabs
| Slab | Volume with 10% extra | Cubic yards | 80 lb bags |
|---|---|---|---|
| 3 × 3 m, 10 cm thick | 0.99 m³ | 1.29 | 59 |
| 4 × 4 m, 10 cm thick | 1.76 m³ | 2.30 | 104 |
| 10 × 10 ft, 4 in thick | 1.04 m³ | 1.36 | 62 |
| 12 × 12 ft, 4 in thick | 1.50 m³ | 1.96 | 88 |
| 20 × 20 ft, 4 in thick | 4.15 m³ | 5.43 | 245 |
The last row shows why bags stop making sense for larger jobs: 245 bags weigh nearly 9 tonnes (about 20,000 lb) and take days to mix by hand.
Bags of premixed concrete
| Bag | Yield (about) | Bags per m³ | Bags per yd³ |
|---|---|---|---|
| 80 lb | 0.60 ft³ (17 L) | 59 | 45 |
| 60 lb | 0.45 ft³ (12.7 L) | 79 | 60 |
| 40 lb | 0.30 ft³ (8.5 L) | 118 | 90 |
| 25 kg | about 12 L | 84 | 64 |
| 20 kg | about 10 L | 100 | 77 |
Yields are printed on the bag and vary a little by brand and mix; the US figures are those of standard concrete mix. Choose the bag you are buying in the calculator.
Thickness guide
| Use | Common thickness |
|---|---|
| Garden path, shed base | 7.5–10 cm (3–4 in) |
| Patio, garage floor | 10–15 cm (4–6 in) |
| Driveway for cars | 10–15 cm (4–6 in) |
| Footings | As designed; often 20–30 cm (8–12 in) or more |
Local building codes and soil conditions decide what is required. For foundations, columns and anything that carries a load, follow an engineer’s design.
Post holes and columns
For a round hole or form, the calculator uses π × radius² × depth. A 30 cm (12 in) hole 1 m (3.3 ft) deep holds about 0.071 m³, or 2.5 ft³ – just over four 80 lb bags, so buy five. Enter how many holes you have and it multiplies for you.
Ordering ready-mix
Ready-mix suppliers sell by the cubic meter or cubic yard and often charge extra for small loads. Round your total up to the supplier’s increment, and remember that running short in the middle of a pour is much worse than having a little left over.
On a building site with poor reception, the calculator still works once zet4u is saved on your phone (how it works offline).
Frequently asked questions
How do I calculate concrete for a slab?
Multiply length × width × thickness, all in meters (or feet), then add 5–10% for waste. A 3 × 3 m slab 10 cm thick is 3 × 3 × 0.1 = 0.9 m³, or 0.99 m³ with 10% extra.
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. With 80 lb bags yielding about 0.6 ft³ each, that is 45 bags; with 60 lb bags, 60; with 40 lb bags, 90.
How do I calculate concrete for a post hole?
Use the volume of a cylinder: π × radius² × depth. A hole 30 cm wide and 1 m deep holds 0.071 m³; subtract the post's own volume if you want to be exact.
Should I use bags or ready-mix?
For small jobs (under about 1 m³ or 1.3 yd³), bags are practical. Beyond that, ready-mix delivered by truck is usually cheaper, more consistent and much less work.
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