Same arithmetic as every other bulk material, with one difference that matters: concrete arrives on a truck at a fixed time and cannot be topped up later. Running short is a different kind of problem here.
Cubic yards equals length in feet times width in feet times depth in inches, divided by 324.
A slab 20 by 12 feet at 4 inches: 20 times 12 times 4 is 960, divided by 324 is 2.96 cubic yards.
For a standard 4 inch slab, divide the square footage by 81. A 240 square foot slab is 240 divided by 81, which is 2.96 cubic yards. Same answer, one operation, and 4 inches covers most residential slab work.
Four inches is the long-standing residential standard for patios, walkways and driveways carrying cars and light SUVs, over a properly compacted granular base.
Five to six inches is common for garage floors and heavier duty driveways, and many contractors in colder climates recommend five inches on a driveway for extra margin against frost movement.
These are typical figures rather than a specification for your site. Actual depth depends on subgrade, climate, expected loads and local code, and it is worth confirming rather than assuming. What matters for measuring is that the depth is decided before you calculate, because it scales the entire order.
With mulch or gravel, running short means another delivery next week. With ready mix, it means a partly poured slab going off while you wait, and a cold joint through the middle of it.
The standard advice is to order around 10 percent more than the calculation, and the reasons are specific rather than generic caution.
The subgrade is not flat. A base that looks level has hollows, and every hollow takes concrete that your calculation assumed was solid ground. On a large pour this is the single biggest source of shortfall.
Spillage and waste in handling. Concrete left in the truck, in the barrow, on the ground.
Forms move. Formwork under the pressure of wet concrete bows outward slightly, and the finished slab is fractionally larger than the form dimensions you measured.
A standard mixer truck carries somewhere in the range of 8 to 10 cubic yards, so most residential pours are well inside one load.
The complication is at the small end. Suppliers will typically deliver a minimum of one to two cubic yards, but many prefer orders of four or more, and short loads usually attract an additional charge that can be a significant fraction of the material cost on a small job.
That changes the economics of ordering generously. On a two yard pour, adding half a yard costs relatively little. Coming up half a yard short may mean a second short-load delivery costing more than the concrete itself, if the supplier can even get a truck to you in time.
Below about half a cubic yard, bagged concrete mixed on site is usually the more sensible route, and that is a different calculation based on bag yield.
Split irregular slabs into rectangles and add. The splitting methods work the same here.
For a circular pad, area is pi times the radius squared, then multiply by depth and divide as usual.
Footings and trenches are long thin boxes: length times width times depth, in consistent units. A trench 40 feet long, 18 inches wide and 24 inches deep is 40 times 1.5 times 2, which is 120 cubic feet, divided by 27 is 4.4 cubic yards.
Where a slab sits on footings or has a thickened edge, calculate the slab and the thickened sections separately and add. The thickened edge around a slab perimeter is easy to overlook and it is a meaningful volume on anything sizeable.
A typical single car driveway or a modest patio lands somewhere around 2 to 5 cubic yards. A double garage floor is nearer 8 to 10.
If your figure has come out at 30 yards for a patio, check the depth conversion first, because using 4 instead of 0.333 for a four inch depth is the error that produces exactly that kind of result. The general concrete measuring method is here, and the cubic yard conversion in full is here.
Not sure where to begin? The tools we point most people at first are gathered in our main picks.