How to calculate concrete
- Measure the job. Length and width in metres, thickness or depth in millimetres. For round holes and piers, measure the diameter.
- Convert millimetres to metres. Divide by 1,000: a 100 mm slab is 0.1 m thick.
- Multiply to get cubic metres. Length × width × thickness gives m³. For a round hole use π × radius² × depth.
- Add waste. Multiply by 1.05 to 1.10 for spillage, uneven ground and formwork that bows.
- Convert to bags or order readymix. About 107 bags of 20kg premix make 1 m³. Above 2–3 m³ a readymix truck is usually easier.
Worked example: a 6 × 3 m shed slab, 100 mm thick
- Volume: 6 × 3 × 0.1 = 1.80 m³. Plus 5% for waste: 1.89 m³.
- 20kg bags at about 0.0094 m³ each: 202 bags, about 4,040 kg to carry.
- Readymix: under the usual 3 m³ minimum, so expect to pay for 3 m³ unless you share the load.
Concrete formulas by shape
All measurements in metres; divide millimetres by 1,000 first. Results are in cubic metres.
- Slab
- length × width × thickness
- Round slab
- π × (diameter ÷ 2)² × thickness
- Continuous footing
- total length × width × depth
- Pad footing
- length × width × depth × number of footings
- Monolithic slab
- length × width × thickness + perimeter band × (edge depth − thickness), where perimeter band = length × width − (length − 2 × edge width) × (width − 2 × edge width)
- Bell footing
- π × r² × pier height + π × bell height ÷ 3 × (R² + R × r + r²), with r = pier radius and R = bell radius
- Wall
- length × height × thickness
- Column / pier
- π × (diameter ÷ 2)² × height
- Post hole
- (hole area − post area) × depth; round hole π × (diameter ÷ 2)², square hole width²
- Tube / ring
- π × [(outer ÷ 2)² − (inner ÷ 2)²] × height
- Curb & gutter
- [curb depth × (curb height + flag thickness) + gutter width × flag thickness] × length
- Steps (solid, n steps)
- width × rise × run × (1 + 2 + … + n). With a landing, the landing is the top step: width × rise × [run × (1 + … + (n − 1)) + landing depth × n]
1 m³ = 1,000 litres.
How many 20kg bags of concrete per cubic metre
| Bag size | Yield per bag | Bags per m³ |
|---|---|---|
| 20kg | 9.4 litres | 107 |
Frequently asked questions
How many 20kg bags of concrete make 1m³?
About 107. Australian Builders states around 100 bags of its 20kg mix per m³ (10 litres a bag); we count 9.4 litres, what a bag really gives once mixed, so you don't run short. Some brands yield even less: Cement Australia's coverage figure works out at about 0.0084 m³ a bag, closer to 120 bags per m³. If your bag says less, raise the waste percentage.
How much concrete do I need for a 6m x 3m slab?
At 100 mm thick it is 1.8 m³, or 1.89 m³ with 5% extra: 202 bags of 20kg premix, which is why most people order readymix for a slab this size.
Is there a minimum order for readymix concrete?
Usually yes. Boral charges loads under 3 m³ as if the truck carried 3 m³ (NSW Metro, from 30 September 2026), and other suppliers set similar minimums. For a small job, compare that charge with the cost of bags.
How thick should a concrete slab be?
Paths and patios are commonly 75–100 mm, shed and garage slabs 100 mm or more with reo mesh. Your engineer or the slab design for your building has the final word.
How much does a cubic metre of concrete weigh?
About 2,400 kg, so 1 m³ of concrete is roughly 2.4 tonnes. 107 bags of 20kg premix is over 2 tonnes to carry.
Can I enter feet and inches?
Yes. Type them with their marks, like 10' 6" or 6ft, and the calculator converts them. Results stay in cubic metres and kilograms.
How we estimate prices
We don't show Australian prices yet. Open "Use your own prices" in the calculator, enter what your hardware store or readymix plant charges and every cost appears with your numbers.
We will only show our own estimates once they are checked against real merchant prices, with the source and date.