How to calculate concrete
- Work out the volume. Length × width × thickness in metres, plus 5–10% extra.
- Choose the mix. 1 : 2 : 3 (cement : sand : aggregate) for paths and driveways, 1 : 3 : 3 for footings, 1 : 1.5 : 3 for high strength.
- Count the cement. Volume × bags per m³: 1.89 m³ of a 1 : 2 : 3 mix needs 1.89 × 16 = 30.2, so 31 bags of 20kg.
- Measure by the bucket. For each bag of cement, 2 buckets of sand and 3 of aggregate, with the same bucket. Add water a little at a time.
Worked example: a 6 × 3 m path slab, 100 mm thick, 1 : 2 : 3
- Volume: 6 × 3 × 0.1 = 1.80 m³. Plus 5%: 1.89 m³.
- Cement: 1.89 × 16 bags = 30.2, so 31 bags of 20kg.
- Sand: 1,452 kg. Aggregate: 2,177 kg. Water: about 302 litres.
Concrete mix per cubic metre
| Mix (cement : sand : aggregate) | 20kg cement bags | Sand (kg) | Aggregate (kg) | Water (L) |
|---|---|---|---|---|
| High strength 1 : 1.5 : 3 | 17 | 630 | 1,260 | 170 |
| Paths and driveways 1 : 2 : 3 | 16 | 768 | 1,152 | 160 |
| Footings 1 : 3 : 3 | 13 | 1,005 | 1,005 | 130 |
Bags per m³ from Cement Australia's GP cement data sheet. Sand and aggregate from 2,400 kg per m³ of fresh concrete, split by the volume parts; water at half the cement weight.
20kg bags of cement by volume
| Volume | High strength 1 : 1.5 : 3 | Paths and driveways 1 : 2 : 3 | Footings 1 : 3 : 3 |
|---|---|---|---|
| 0.25 m³ | 5 | 4 | 4 |
| 0.50 m³ | 9 | 8 | 7 |
| 1.00 m³ | 17 | 16 | 13 |
| 1.50 m³ | 26 | 24 | 20 |
| 2.00 m³ | 34 | 32 | 26 |
| 3.00 m³ | 51 | 48 | 39 |
No waste included: add 5–10% to the volume.
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.
Frequently asked questions
What is the best concrete mix ratio?
For paths, driveways and slabs Cement Australia gives 1 part cement, 2 parts sand and 3 parts aggregate by volume. Footings can use 1 : 3 : 3; for higher strength and water tightness 1 : 1.5 : 3.
How many 20kg bags of cement in a cubic metre of concrete?
16 for a 1 : 2 : 3 mix, 13 for footings (1 : 3 : 3) and 17 for high-strength concrete (1 : 1.5 : 3), from Cement Australia's General Purpose cement data sheet.
How much water do I add?
Cement Australia doesn't give a figure. As a guide we use half the cement weight, about 10 litres per 20kg bag; add it a little at a time until the mix is workable, since extra water makes weaker concrete.
Is it cheaper than 20kg bags of premix?
For bigger jobs, usually: 1 m³ takes about 107 bags of premix, against 16 bags of cement plus sand and aggregate. For a few post holes premix is simpler. Enter your prices in the calculator to compare.
About the prices
Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.