Concrete Volume Calculator
How much concrete you actually need — cubic yards, bags and cost — for slabs, footings, round columns, walls and stairs, with waste allowance and a rebar estimate.
How Much Concrete Do You Need?
The real question is never just volume — it is how many bags, how many cubic yards, and what it costs. This calculator takes the dimensions of a slab, footing, round column, wall or set of stairs and answers all three at once, with a waste allowance built in and an optional rebar estimate.
The Shapes and Their Formulas
| Shape | Volume |
|---|---|
| Slab / footing | length × width × thickness |
| Round column / pier | π × radius² × height |
| Wall | length × height × thickness |
| Stairs | width × run × rise × n(n+1)/2 |
Everything is converted internally and reported in cubic yards, cubic feet and cubic meters so the number fits whatever you are ordering.
Bags or Ready-Mix?
Small jobs get bagged; big pours get a truck. As a rule of thumb a cubic yard is 27 cubic feet, so it takes about 45 of the 80 lb bags (0.6 cu ft each) to make one cubic yard — which is why anything past roughly half a yard is usually cheaper and easier as ready-mix.
| Bag size | Yield | Bags per cu yd |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Always Order a Little Extra
Subgrade is never perfectly flat and some concrete is always lost to spillage and over-excavation, so a 5 to 10 percent waste allowance is standard — the calculator adds it for you. Running short mid-pour and waiting on a second batch creates a cold joint, which is far worse than buying one extra bag.
Frequently Asked Questions
How many 80 lb bags are in a cubic yard?
About 45, since a cubic yard is 27 cubic feet and each 80 lb bag yields roughly 0.6 cubic feet.
How thick should a slab be?
Four inches is typical for patios and walkways; five to six inches for driveways and anything carrying vehicles. Thickness drives the volume directly.
Does the rebar estimate include laps?
No — it is a simple grid total (bars each way at your spacing). Add roughly 10 to 15 percent for laps, bends and waste when ordering.
Related calculators
- Concrete Slab Calculator — the DIY slab-focused version.
- DIY Concrete Calculator — for patios, posts and landscaping.
- Material Weight Calculator — weights of other materials and stock.
- Beam Load Calculator — loads on the structure above.
- Bearing Capacity Calculator — coming soon to the Civil shelf.