Concrete calculator
Pick the shape, enter the measurements, get the volume to order.
Your inputs
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Example loaded. You can change any value.
There is no figure yet: nothing is calculated until you ask for it.
- Enter your measurements.
- Check the units and the assumptions.
- Press Calculate, or load the example.
Rather see a worked case? Press “Load example” and change any value afterwards.
Next step
A compacted gravel sub-base goes under the slab, over the same area.
Where each number comes from Conversion constants are exact; densities and coverage rates come from cited sources; everything else is an assumption you can change. Methodology · Sources and updates
How to use it
- Pick the shape: slab, footing, wall or round column.
- Enter the measurements for that shape; leave the others empty.
- If you are pouring several identical elements, say how many.
- The result comes out in cubic yards, cubic feet and cubic metres.
How it is calculated
The whole calculation is elementary geometry. The only real decision is picking the right shape:
The 5 % waste covers what stays in the mixer or truck, the unevenness of the ground under a slab, and the concrete that spills. On an excavation in soil, the bottom is never as flat as the drawing: that is where the extra goes.
The answer is given in cubic yards, cubic feet and cubic metres because ready-mix is ordered in cubic yards in the United States and cubic metres in Europe, and plants usually have a minimum order.
Worked example
A garage slab 16 x 10 ft, 5 in thick, with 5 % waste:
- Volume: 16 x 10 x 0.417 = 66.7 cubic feet
- With waste: 66.7 x 1.05 = 70 cubic feet
- That is 2.59 cubic yards, or 1.98 m³
Ordering from a plant, round up to 3 cubic yards: few plants sell fractions that fine, and running short mid-pour ruins the slab.
Good to know
- This is volume, not structure. Knowing how much concrete fits in a form says nothing about whether the element will hold. Reinforcement and sections are an engineer's job.
- One pour, one slab. If the concrete arrives in two loads hours apart, you get a cold joint. Better to over-order than to stop halfway.
- Bags or truck. Below about half a cubic yard, bags usually win; above that, a truck is almost always cheaper per yard.
- Ground eats concrete. Over loose gravel or uneven soil, raise the waste to 10 %.
Frequently asked questions
How much concrete do I need for a slab?
Multiply length by width by thickness. A 16 x 10 ft slab, 5 in thick, is 66.7 cubic feet, which with 5 % waste becomes 70 cubic feet, or 2.59 cubic yards.
How many bags of concrete make a cubic yard?
It depends on the yield printed on the bag. An 80 lb bag typically yields about 0.6 cubic feet, so a cubic yard takes around 45 bags. The calculator asks for the yield instead of assuming it.
How much waste should I allow for concrete?
5 % on a prepared surface with clean formwork. 10 % if you are pouring onto natural ground or gravel, because the bottom is never flat and the concrete fills those hollows.
Can I calculate several columns at once?
Yes. Enter the dimensions of one and set how many identical elements there are. The result shows both the unit volume and the total.
Can I use this for structural design?
No. This works out how much concrete fits, not whether the element is strong enough. Structural sizing and reinforcement need a qualified engineer.