How to calculate concrete
- Work out the volume. In cubic feet: length × width × thickness, all in feet.
- Add 5–10% extra. So you don't run out halfway through the pour.
- Divide by the bag yield. A 30 kg bag makes about ½ cu ft (0.014 m³).
- Round up. You can't buy part of a bag: 70.3 bags means 71.
Worked example: 10 fence posts, 10 in holes 3 ft deep, 4×4 posts
- One hole: π × (10 ÷ 12 ÷ 2)² × 3 − (3.5 ÷ 12)² × 3 = 1.38 cu ft.
- Ten holes plus 5%: 14.5 cu ft, or 0.411 m³.
- 30 kg bags at 0.014 m³ each: 30, about 900 kg to carry.
30 kg bags for common volumes
| Concrete | m³ | 30 kg bags | Weight |
|---|---|---|---|
| ½ cu ft | 0.014 | 2 | 60 kg |
| 1 cu ft | 0.028 | 3 | 90 kg |
| 5 cu ft | 0.142 | 11 | 330 kg |
| 10 cu ft | 0.283 | 21 | 630 kg |
| ½ cu yd | 0.382 | 28 | 840 kg |
| 1 cu yd | 0.765 | 55 | 1,650 kg |
| 1 m³ | 1.000 | 72 | 2,160 kg |
| 2 cu yd | 1.529 | 110 | 3,300 kg |
No waste included: add 5–10% to your volume first. 30 kg bags at 0.014 m³ each.
Concrete formulas by shape
All measurements in feet; divide inches by 12 first. Results are in cubic feet.
- 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]
Cubic yards = cubic feet ÷ 27. Cubic metres = cubic feet ÷ 35.31.
Frequently asked questions
How much concrete does a 30 kg bag make?
About 0.014 m³, which is ½ cubic foot, for Sakrete and Quikrete concrete mix.
How many 30 kg bags per fence post?
Usually 1 to 2: a 10 inch hole 2 ft deep around a 4×4 post takes about 0.92 cu ft, so 2 bags.
How many 30 kg bags in a cubic metre?
About 72 bags (1 ÷ 0.014 = 71.4, rounded up). A cubic yard takes about 55.
Bags or ready-mix?
Past about 1 cubic yard (55 bags, 1.65 tonnes) mixing by hand gets hard. Compare a ready-mix delivery, including any small-load charge, with the bags at your store.
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.