Measure the two numbers, then decide the third
Two of the three figures are measurements and the third is a decision. Length and width you take with a tape. Thickness gets chosen for what the slab has to carry, and it is the one that moves the volume the most.
Standard flatwork is 4 inches. A garage floor goes to 5. A foundation is poured at 8. Stepping from 4 inches to 5 is 25 percent more concrete before anything else about the job changes, which is why thickness is worth settling before you run the numbers rather than after somebody has already quoted you.
For an odd shape, break it into rectangles, work each one and add them up. A curved patio edge or a clipped corner is close enough if you square it off and take the slightly larger figure, because you are ordering with a waste allowance on top of it anyway.
Feet, then cubic feet, then divide by 27
The arithmetic is short. Get all three numbers into feet, multiply them together, divide by 27. The only step people trip on is the thickness, because it arrives in inches and has to go in as a fraction of a foot.
- 4 inches is 0.33 of a foot
- 5 inches is 0.42 of a foot
- 6 inches is 0.5 of a foot
- 8 inches is 0.67 of a foot
The same tape measure, two very different volumes
Take a 20 by 20 patio. That is 400 square feet. At 4 inches it is 400 times 0.33, or 133 cubic feet, and 133 divided by 27 is 4.9 cubic yards.
Pour that identical 400 square feet at 8 inches instead and it is 268 cubic feet, or 9.9 cubic yards. Twice the concrete off the same tape measure, which is the reason a volume figure quoted without a thickness beside it does not tell anybody anything.
So when you compare your number against somebody else's, check that both were figured at the same thickness before you decide one of them is wrong.
Add the 10 percent before you call anybody
Order 10 percent over the calculated volume. The 4.9 yard patio above becomes 5.4.
That allowance is not padding. Forms flex outward under the weight of wet concrete, the subgrade is never perfectly flat no matter how well it was graded, and some concrete always stays behind in the chute or the line. Ordering the exact calculated figure is how a crew ends up a wheelbarrow short.
Running short is the expensive version of this mistake, because what you get is a cold joint through the middle of a slab that was meant to be one piece, and there is no taking it back once the concrete has stiffened. Running a little long is the cheap version. The extra goes to the washout spot that was agreed on before the pour started.
Where your number lands against a load
Ready mix comes in 10 cubic yard loads and a job rounds up to the next full one. Our page on by the yard versus by the job works through what that does to a pour sitting near the boundary. For your own planning, the useful thing is simply knowing which side of it you are on.
Two other things ride on the yardage. A pour that runs about 8 cubic yards or more is usually a line pump job in our book, along with any foundation and any site where the truck cannot get near the forms. And the volume tells you how much crew has to be standing in the forms when the concrete arrives, because a slab is placed and finished at the rate people can work, not at the rate a truck can empty.
The yardage is not the price
Cubic yards tell you what to order. They do not tell you what the job costs, because the concrete is one line inside a finished slab that also takes layout, excavation, a compacted stone base, forms, reinforcement, placement, finishing and joints cut on schedule.
We price by the installed square foot for exactly that reason. The 400 square foot patio above sits in the patio band of $9 to $14 per square foot installed, which plans out at $3,600 to $5,600. Our cost page carries the rest of the bands by project type, and our page on by the yard versus by the job explains why the two units look so different on paper.
Use cubic yards to sanity check a truck count. Use installed square feet when you are holding two crews' pages side by side.