How to Figure Concrete Yardage

Why yardage is measured in cubic yards

Concrete fills a three-dimensional space, so it is measured by volume rather than by area or by weight. The unit the trade settled on is the cubic yard, which is a cube three feet on every side. Three times three times three is 27, so one cubic yard holds 27 cubic feet of concrete. That 27 is the number doing all the work in the formula above, and it is the single step most people forget. Everything else is just measuring.

Suppliers load trucks by the yard, price by the yard, and talk in yards, so learning to turn a slab into a yard number lets you order concrete, compare two quotes, and sanity-check an estimate without taking anyone's word for it.

The concrete yardage formula, step by step

Take the formula apart and it is three moves plus a division.

1. Measure length and width in feet. Run a tape along the two sides of the pour and write both down in feet. If a side is 12 feet 6 inches, call it 12.5 feet.

2. Turn the thickness into feet. Thickness almost always gets talked about in inches, and the formula wants feet, so divide the inch figure by 12. A 4-inch slab is 4 ÷ 12 = 0.33 feet. A 6-inch slab is 6 ÷ 12 = 0.5 feet. This is the conversion people skip, and skipping it makes the answer twelve times too big.

3. Multiply the three numbers together. Length times width times thickness gives you the volume in cubic feet.

4. Divide by 27. That turns cubic feet into cubic yards, which is what you order.

Written as one line for a slab, figuring concrete yardage looks like this:

cubic yards = (length ft x width ft x thickness in ÷ 12) ÷ 27

A worked example

Say you are pouring a patio that is 14 feet long, 12 feet wide, and 4 inches thick.

• Thickness in feet: 4 ÷ 12 = 0.33
• Volume in cubic feet: 14 x 12 x 0.33 = 55.4
• Cubic yards: 55.4 ÷ 27 = 2.05

So the patio needs a little over 2 cubic yards of concrete before you add anything for waste. Notice how small the yard number is compared to the square footage. A 168-square-foot patio only takes about 2 yards, which is why yardage, not area, is the number that matters when you order.

Figuring yardage from square footage

If you already know the area of a slab, there is a shortcut the ready-mix industry has used for years. At a standard 4-inch thickness, one cubic yard covers about 81 square feet, so you can divide your square footage by 81 to get the yardage. ConcreteNetwork.com calls 81 the "magic number" for exactly this reason (Concrete Network).

For the patio above, 168 square feet ÷ 81 = 2.07 yards, which matches the long-form answer. The shortcut only holds at 4 inches, though. Change the thickness and the coverage per yard changes with it, which is the next thing to get right.

Thickness changes everything

Two slabs with the same footprint can need very different amounts of concrete if one is thicker. A 6-inch slab needs half again as much concrete as a 4-inch slab of the same size, because you are pouring 50 percent more depth. This table shows how much a single cubic yard covers as the pour gets thicker, which is the fastest way to feel the effect.

The practical takeaway: always confirm the thickness before you figure yardage, and never assume 4 inches. Driveways that carry vehicles are often poured at 5 or 6 inches, and that extra inch or two moves the yard count more than people expect.

Always add a waste factor

Never order the exact calculated number. This is not a rounding habit, it is standard practice, and every ready-mix source says the same thing. Concrete Network's guidance is blunt: "never order the exact amount of needed concrete." Inch Calculator recommends "ordering 10 to 15 percent additional material to account for spillage or forms that aren't exactly to dimension" (Inch Calculator).

The reason is that the real world eats concrete. Subgrade is never perfectly flat, so low spots swallow more than the math predicts. Forms bow out under the weight of a full load. Some concrete sticks in the chute, some spills off the wheelbarrow, and the last bit clings to the drum. Add 10 percent for a normal slab on decent ground, and closer to 15 percent if the subgrade is rough or the forms are hand-built.

Contractors in the trade back this up. On the Garage Journal forum, the rule members repeat is to "order 10 percent more than you need or than what you calculate the form will hold," because coming up short creates low spots and cold joints that are far worse than having a little extra (Garage Journal). The same thread is worth reading for what to do with leftover concrete, since a wheelbarrow of extra can become a stepping stone or a trash-can pad instead of going to waste.

For the 2.05-yard patio, adding 10 percent brings you to 2.26 yards, and you would ask the supplier for 2.25 or 2.5 depending on how they sell partial yards.

Why coming up short is the expensive mistake

There is an asymmetry worth understanding. A little extra concrete costs you a few dollars a yard. Coming up short costs you a lot more. If the first truck empties before the pour is finished and you wait on a second, the concrete already down starts to set, and the seam where old meets new becomes a cold joint. A cold joint is a plane of weakness that can crack and let water in. On top of that, a small emergency delivery carries a short-load fee that can rival the cost of the concrete itself. A bit of waste is cheap insurance against both.

Short loads and the cost of small pours

Yardage does not just tell you how much concrete to order, it tells you how you will be charged for it. Ready-mix suppliers price a full truck at around 10 yards, and anything well under that picks up a short-load fee, often 40 to 60 dollars per yard below the minimum. That means a 2-yard patio can carry fees a 10-yard driveway never sees. Figuring your yardage accurately lets you see this coming and decide whether to combine pours, size the job up to a fuller load, or use bagged concrete instead. Our cost-per-yard guide breaks down every fee that lands on the invoice.

Quick reference: common slab sizes

If your project matches one of these, here is the yardage at a 4-inch thickness, before the waste cushion. Add 10 percent to any of these and round up before you order.

Figuring yardage for shapes that are not slabs

Not every pour is a rectangle. Footings, round columns, sonotubes, curbs, and stairs each need the volume worked out a little differently before you divide by 27. A round column, for example, uses π x radius² x height ÷ 27 instead of length times width. Because these are easy to get wrong by hand, they have their own walkthrough in concrete yardage by shape, which covers each one with a worked example.

How to check a contractor's yardage

One of the most useful reasons to know this math is to check someone else's number. When a contractor quotes yardage, you can measure the job yourself, run the formula, add a sensible waste factor, and see whether the two numbers land close. If a quote is far above your figure with no explanation, ask what the extra covers. It might be a thicker pour, a deeper footing, or a genuine allowance for bad subgrade, all of which are fair. But it might also be padding. Being able to figure yardage turns you from a spectator into someone who can ask the right question.

Skip the arithmetic

If you would rather not run the numbers by hand, the concrete yardage calculator does every step for you. Enter length, width, and thickness and it returns the cubic yards to order, folds in a waste factor, and gives you a bag count if you are mixing by hand instead. It also handles the awkward shapes, so footings, columns, and stairs come out right without you touching π. For a broader plan of a whole project, the how much concrete do I need guide ties yardage together with cost and delivery.