Why do I need geometry to order concrete?
Concrete is sold by volume, and a slab is a box: length times width times thickness. Get the volume right and add about ten percent for spills and uneven ground, and you won't run short halfway through a pour. The same math sizes pools, raised beds, and gravel bases.
- Concrete to order
- 1.32 m³
- In cubic yards
- 1.73 yd³
- 80 lb bags (if mixing yourself)
- 78
Challenge: Size a slab that needs about one cubic yard.
More settings
Play
Change the size and thickness. The number to order updates as you go.
Challenge: Size a slab that needs about one cubic yard. The box under the picture turns green when you get it.
Stuck? Pick one of the examples from the “Try an example” menu, or press “New example.”
Understand
A slab is a rectangular box, and a box's volume is
The trap is units. Thickness is usually given in centimeters or inches while length is in meters or feet, so convert it first. Volume units are cubed: a cubic yard is cubic feet.
Use
Every input has a unit menu, so you can type values in the units you already have. Results follow your units.
Show the work
- Thickness in meters
12\ \text{cm} = 0.12\ \text{m} - Volume
V = 5 \times 2 \times 0.12 = 1.2\ \mathrm{m^3} - Plus extra
1.2 \times 1.1 = 1.32\ \mathrm{m^3} = 1.726\ \mathrm{yd^3}
Export
Enter length, width, and thickness in any units. Set the extra in "More settings." Results are in cubic meters, cubic yards, and 80 lb bags.
- For round columns, use height instead.
- For pools, the same volume in liters is .
For learning and estimation. Verify with applicable codes, standards, and a qualified professional before using in design, construction, or safety-critical work.
Cheat card
| Symbol | Meaning | Unit |
|---|---|---|
| length and width | m or ft | |
| thickness | cm or in |
- Convert the thickness to the same unit as length first (4 in = 1/3 ft).
- An 80 lb bag makes about 0.6 ft³. A cubic yard is 45 bags.
- Order a little extra. Running short leaves a cold joint.
Where it’s used
- Civil & Construction
Contractors estimate concrete, asphalt, and fill by volume on every job. - Trades
Concrete finishers order by the cubic yard or cubic meter from the ready-mix plant. - Home Projects
Homeowners size shed pads, patios, and post holes before buying bags.
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Questions people ask
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, 10 × 10 × 1/3 ft = 33.3 ft³ = 1.23 yd³. With 10% extra, order about 1.35 yd³.
How many bags of concrete make a cubic yard?
About 45 bags of 80 lb, or 60 bags of 60 lb.
Why add extra?
Forms bow, the ground isn't perfectly flat, and some spills. Ten percent covers most small jobs.