How to calculate concrete
- Work out the volume. In cubic metres: length × width × thickness, all in metres.
- Add 5–10% extra. So you don't run out halfway through the pour.
- Divide by the bag yield. We count about 0.0094 m³ (9.4 litres) for a 20kg bag.
- Round up. You can't buy part of a bag: 100.9 bags means 101.
Worked example: 12 fence posts, 300 mm holes 600 mm deep
- One hole: π × 0.15² × 0.6 − 0.1 × 0.1 × 0.6 = 0.0364 m³.
- Twelve holes plus 5%: 0.459 m³.
- 20kg bags: 49.
20kg bags for common volumes
| Concrete | 20kg bags | Weight |
|---|---|---|
| 0.1 m³ | 11 | 220 kg |
| 0.25 m³ | 27 | 540 kg |
| 0.5 m³ | 54 | 1,080 kg |
| 1 m³ | 107 | 2,140 kg |
| 1.5 m³ | 161 | 3,220 kg |
| 2 m³ | 214 | 4,280 kg |
| 3 m³ | 321 | 6,420 kg |
No waste included: add 5–10% to your volume first. 20kg bags at about 0.0094 m³ each.
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
How much concrete does a 20kg bag make?
We count 9.4 litres (0.0094 m³). Australian Builders states 10 litres for its 20kg mix; a bag gives a little less once mixed. Some brands make less still (Cement Australia's figure is about 8.4 litres), so check the bag.
How many 20kg bags of concrete per post?
Usually 3 to 4 for a fence post: a 300 mm hole 600 mm deep around a 100 × 100 mm post takes about 0.036 m³, so 4 bags. See the post hole calculator for your size.
How many 20kg bags can I mix by hand?
Physically, as many as you can lift, but 107 bags is over 2 tonnes. Past 2–3 m³ most people hire a mixer or order readymix.
How many 20kg bags are in a cubic metre?
About 107 bags at 0.0094 m³ each. See the table below for common volumes.
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.