How to calculate concrete
- Measure the job. Length and width in metres, thickness or depth in millimetres. For round holes, 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 and uneven ground.
- Convert to bags or order readymix. We count about 0.0117 m³ for a 25kg bag of Dricon CivilCrete (Dricon states 0.012), so 1 m³ takes about 86 bags.
Worked example: a 3 × 3 m shed slab, 100 mm thick
- Volume: 3 × 3 × 0.1 = 0.90 m³. Plus 5% for waste: 0.95 m³.
- 25kg bags at 0.0117 m³ each: 80.7, so 81 bags (2,025 kg to carry).
- Readymix: about 1 m³, worth a phone call to compare with the bags.
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 25kg bags of concrete per cubic metre
| Bag size | Yield per bag | Bags per m³ |
|---|---|---|
| 25kg | 11.7 litres | 86 |
Frequently asked questions
How many 25kg bags of concrete make 1m³?
About 86. Dricon's data sheet gives 0.012 m³ for a 25kg bag of CivilCrete as a guide; we count 0.0117 m³, what a bag really gives once mixed: 1 ÷ 0.0117 = 85.4, rounded up to 86 bags, about 2.15 tonnes to carry.
How much concrete do I need for a 3m x 3m slab?
At 100 mm thick it is 0.9 m³, or 0.95 m³ with 5% extra: 81 bags of 25kg premix. Most people order readymix for a slab this size.
Is there a minimum order for readymix?
Suppliers charge a small-load fee on small deliveries, but they don't publish one threshold, so ask your plant when you book. The calculator rounds your order to 0.2 m³ steps.
How much does a cubic metre of concrete weigh?
About 2,400 kg, so 1 m³ is roughly 2.4 tonnes on your trailer or driveway.
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 New Zealand prices yet. Open "Use your own prices" in the calculator, enter what your merchant 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.