Quick answer
Multiply length × width × thickness (all in metres) to get cubic metres, then add about 10% for waste and uneven ground. A 10 × 10 foot slab, 4 inches thick, needs about 0.94 cubic metres (1.23 cubic yards), or roughly 1.04 m³ with 10% extra. That's about 71 bags of 30 kg premix, and at that size ready-mix delivery is usually easier.
The calculator above handles metric or imperial, square slabs or round post holes, and several of the same pour at once. Here's how the numbers work, so you can check them.
How to calculate concrete
- Convert everything to one unit. Thickness is usually in centimetres or inches, so convert it to metres (10 cm = 0.10 m) or feet (4 inches = 0.333 ft).
- Slab or pad: length × width × thickness = volume.
- Round hole or column: 3.1416 × radius × radius × depth = volume. The radius is half the diameter.
- Add extra. 5% to 10% is common for spillage, forms that bow, and uneven ground.
- Convert to what you're buying. Ready-mix is sold by the cubic metre in Canada. One cubic yard is 0.7646 cubic metres.
How many bags of concrete?
| Bag | Yield per bag | Bags per cubic metre | Bags per cubic yard |
|---|---|---|---|
| 30 kg premix | 0.52 cubic feet (0.0147 m³) | about 68 | about 52 |
| 25 kg premix | 0.43 cubic feet (0.0122 m³) | about 83 | about 63 |
Yields are from the Canadian maker of QUIKRETE Concrete Mix. Other brands are close but check the bag. The calculator always rounds up to whole bags.
Common jobs, worked out
| Job | Concrete | 30 kg bags (no extra) | 30 kg bags (+10%) |
|---|---|---|---|
| 10 × 10 ft slab, 4 inches thick | 0.94 m³ (1.23 yd³) | 65 | 71 |
| 3 × 3 m pad, 10 cm thick | 0.90 m³ (1.18 yd³) | 62 | 68 |
| One post hole, 30 cm wide × 1 m deep | 0.071 m³ | 5 | 6 |
For a fence, put in one post hole and set "How many" to the number of posts. Remember the post itself takes up some of the hole, so the true amount is a little less.
Safety: Wet concrete is caustic and can burn skin. Wear gloves, eye protection, long sleeves and boots, and wash skin that touches it right away. Before digging post holes, request a locate through BC 1 Call so you don't hit a buried gas or power line.
Bags or ready-mix?
Bagged premix is handy for post holes, small pads and repairs. Once a job needs dozens of bags, weigh the cost and the labour of mixing against a ready-mix delivery. Ask local suppliers about their minimum order and short-load fees before you book.
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick, about 65 bags of 30 kg premix, or 71 with 10% extra. At 6 inches thick, about 97 bags, or 106 with extra.
How many 30 kg bags make a cubic metre?
About 68, using a yield of 0.52 cubic feet (0.0147 m³) per bag.
How many bags of concrete per fence post?
A 30 cm wide, 1 m deep hole holds about 0.071 m³, which is about 5 bags of 30 kg premix before subtracting the post. A narrower or shallower hole takes less. Enter your own size above.
How do I convert cubic yards to cubic metres?
Multiply cubic yards by 0.7646. One cubic metre is about 1.31 cubic yards.
How much extra concrete should I order?
5% to 10% is typical. Use more if the ground is uneven or the forms aren't square.
Official sources
An estimate for general information, not financial advice. Rates change, so check the official sources above before you rely on a number. Spotted something out of date? Tell us.