Calculating How Much Stone Dust You Need

Simple Equation for the Mathematically Challenged

Two chairs around a fire pit on a flagstone patio.
Sand or stone dust are often used to underlay patios.. 1Photodiva/Getty Images

If you ever sat through a math class in school wondering when you would ever use any math in the real world, now you have your answer. You are getting ready to build a stone walkway or similar structure, notice the instructions call for an underlayment of stone dust (or sand, which more pros now prefer) down to a certain depth, and are perplexed as to just how much to order.

Simple Formula to Tell How Much Stone Dust or Sand Is Required

Never fear: A simple equation exists to tell you just how much you will need:

(L' x W' x H') / 27 = cubic yards of stone dust needed

But to make use of this equation, you first have to measure the area in question. For example, let's say you are laying a stone patio. Wherever you have determined the patio will be, mark the area by pounding stakes in the ground and running string from stake to stake. You now have a rectangle. To make it easier to see your rectangle, mark its perimeter using a can of spray paint. Measure the length and width of the rectangle using a tape measure. Along with the figure for depth (height) that you already have from your instructions, these are the numbers that you will plug into the equation.

Multiply the length (L), in feet, by the height (H), in feet, by the width (W), in feet, and divide by 27. This will tell you how many cubic yards of sand or stone dust you need (in the construction world, most materials are measured in cubic yards).‚Äč

For example, let's say your stone patio is 20 feet long and 20 feet wide. Furthermore, let's say the instructions call for a depth of 1 inch of sand or stone dust to place under the stones. We must convert that 1 inch figure into feet; rounding it off, we come up with 0.08 (to convert inches to feet, divide the number of inches by 12). Here is how you would plug those numbers into the formula:

(20' x 20' x 0.08') / 27 = 1.19 cubic yards

What to Do After You Have Used the Equation

If your number comes out as a fraction, and it probably will, round up. Stone dust, sand, and other base materials are usually sold in 0.5 cubic yard increments. So here you would round the 1.19 cubic yards to 1.5 cubic yards of sand or stone dust. It is better to have a little extra than to run short and have to pause your project to order more.

To buy stone dust or sand, call your local stone yard. For a project of any size, you will probably want them to deliver a load of it for you. To prepare for the delivery, select a suitable storage spot in your yard and lay down a tarp there. When you have the stone yard on the phone:

  • Tell them how much you need.
  • Get a price quote, including for delivery.
  • If that figure is acceptable to you, give them a delivery address and instruct them to dump the load on the tarp.

What Decomposed Granite Is, Where It Comes From

Landscapers can lay all sorts of different stone and stone-like products down upon the ground, depending on the project. The diversity is so great that beginners are easily confused. The confusion is increased by the fact that not everyone agrees on terminology.

One way to think of it is that there are large materials such as paving stones and smaller, finer materials. Stone dust is just one example of the latter. It is an umbrella term that includes:

Decomposed granite is a naturally occurring material, not a byproduct. It results from the weathering of granite over time due to exposure to the elements and is mined in stone quarries. Both types of stone dust come in different colors. And while sand is now preferred as an underlayment for patio stones, for example, both types of stone dust are valued as paving materials for paths.

Pea gravel is another example of a smaller, finer stone material useful in paving paths. It is composed of pea-sized stones that have been rounded over time through natural weathering. The individual pebbles range in size from 1/8 inch to 3/8 inch and come in many colors.