How to calculate concrete
- Measure the form. Length and width in feet, thickness in inches. For sonotubes and post holes, measure the diameter.
- Convert inches to feet. Divide by 12: a 4 inch slab is 0.333 ft thick.
- Multiply to get cubic feet. Length × width × thickness. For a round form use π × radius² × height.
- Add waste. Multiply by 1.05 to 1.10 for spillage and uneven ground.
- Convert. Divide cubic feet by 27 for cubic yards or by 35.31 for cubic metres. A 30 kg bag makes about ½ cu ft (0.014 m³).
Worked example: a 10 × 12 ft patio, 4 inches thick
- Volume: 10 × 12 × (4 ÷ 12) = 40.0 cu ft. Plus 5%: 42.0 cu ft.
- That is 1.56 cubic yards (÷ 27), or 1.19 m³ (÷ 35.31).
- 30 kg bags at 0.014 m³ each: 85 bags. Ready-mix: 1.25 m³, under the usual 3 m³ minimum.
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.
How many 30 kg bags of concrete per cubic metre
| Bag size | Yield per bag | Bags per m³ |
|---|---|---|
| 30kg | 14 litres (½ cu ft) | 72 |
Frequently asked questions
How many 30 kg bags of concrete are in a cubic yard?
A 30 kg bag makes about 0.014 m³ (½ cu ft), so a cubic yard (0.76 m³) takes about 55 bags and a cubic metre about 72.
How much concrete do I need for a 10x10 slab?
At 4 inches thick with 5% extra: about 1.30 cubic yards, or 0.99 m³. That is 71 bags of 30 kg, so most people order ready-mix.
Do ready-mix plants in Canada sell by the yard or the metre?
Most quote and deliver in cubic metres. The calculator shows both: the volume in yards and metres, and the ready-mix order in metres. 1 cubic yard = 0.765 m³.
Is there a minimum order for ready-mix?
Usually. Small loads, often under about 3 m³, carry an extra charge: the federal standing offer for Ontario, for example, prices an underload for orders under 3 cubic metres. Ask your plant before you book.
How deep do footings and sonotubes need to go?
Below the frost line, which in much of Canada is about 4 ft (1.2 m) or more. Your municipality and the building code set the depth for your area.
Can I enter metric measurements?
Yes. Type the unit with the number, like 3.2 m or 100 mm, and the calculator converts it.
How we estimate prices
We don't show Canadian prices yet. Open "Use your own prices" in the calculator, enter what your store or ready-mix 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.