How to Calculate a Cubic Yard of Rock

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Cubic yards are a common form of measurement used in landscaping. Are you trying to calculate a cubic yard of rock? Since materials like sand, gravel, rock and mulch fill in spaces that are wide, high and deep, it's necessary to measure the volume of the area that needs to be filled and not only the surface. Anyone who wants to purchase significant amounts of any of these materials (and other ones like compost, concrete, topsoil or fill dirt) should understand how to calculate cubic yards.


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Measuring the Space

Rock is normally sold in units of cubic yards because it is used to fill in a space rather than merely covering up a surface. Rock can be quite expensive and heavy to transport, so making a few easy calculations before buying it can save a lot of time and headaches. No one wants to waste money on extra rocks or have to head back out to buy more.


Measuring for rock is fairly straightforward and can be done with a long tape measure. To make this task easier, enlist the help of a friend to hold one end of the tape while you measure and record the length and width of the area in feet. Then, multiply these two numbers to find the square footage. For example, if a garden bed measures 30 feet long by 20 feet wide, the bed is 600 square feet.

For the depth, a general rule of thumb is to match up the depth to the size of the rocks. If the rocks are 2 inches in size, you need a depth of at least 2 inches. If you are choosing larger-sized rocks, you will need to adjust this accordingly. So, if the rocks are 3 to 8 inches in size, the depth will also have to be at least 3 to 8 inches.


Finding Cubic Yards

Cubic units are used to measure volume, just as gallons are used to measure fluid volume. Since inches, feet and yards measure length, height and width, you have to convert these dimensions to cubic units to determine the space's volume.

Now you can take that square footage measurement and multiply it by the depth. If the bed ended up measuring 600 square feet and the rocks will be 2 inches deep, you would multiply these two numbers and end up with 1,200.


Now, divide 1,200 by 324, which is the number of square feet that 1 cubic yard of material will cover to a depth of 1 inch: 1,200 ÷ 324 = 3.7 cubic yards. This easy calculation is necessary to measure the volume of that garden bed. If you only measure the area, it will not reflect the volume, and you will incorrectly estimate how much rock is needed to fill up the space.

What About Mulch?

Landscapers often suggest about 3 to 4 inches of mulch for gardens. Mulch can be sold by the truckload and delivered by a landscaper or it can be bought by the bag. Often, the landscaper can drive out to your property and estimate the amount of mulch needed. If you buy it by the bag, it is easier to order more, and it is much lighter than rocks.



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