How to Calculate River Rock

River rock comes in several sizes, such as 1 1/2-inch stones or 4-inch stones. The size of stone you choose affects how many bags or tons it takes to cover the area you're landscaping. Smaller stones work well filling in between stepping stones or as flowerbed mulch, while larger stones help with erosion control.

Knowing the size of the area you plan to fill with river rocks helps you accurately calculate how much of the rock you need. For square or rectangular areas, just multiply the length of the space by the width. For example, if your flowerbed is 10 feet long and 6 feet wide, it's 60 square feet.

For circles, measure the radius, which is the distance from the center of the circle to an outer edge. Multiply the radius by itself, then multiply that number by 3.14 to find the square footage. For example, if the radius is 5 feet, multiply 5 by 5 to get 25, then multiply that by 3.14 for 78.5 square feet.

Break down odd shapes into smaller circles or squares when possible, then add the square footages together.

How deep you plan to install the river rocks makes a difference in how much you need. This number helps you determine the number of cubic feet or cubic yards of rock required. To find this number, decide how deep you want the rocks to be, such as 2 inches. Convert the inches into feet by dividing 2 by 12 to get 0.167. Multiply the square footage by the depth, such as 60 times 0.167 for about 10 cubic feet.

When you divide the cubic feet by 27, you get the number in cubic yards. If you had 120 cubic feet, for example, divide 120 by 27 to get about 4 1/2 cubic yards. If you need the amount in tons, multiply the cubic yards by 1.35.

With smaller jobs, you probably need a few bags of river rocks to complete the projects. Read the information on the bags carefully to ensure you have enough rocks to finish the job. Bags that look the same size might not cover the same area; a bag containing 4-inch river rocks doesn't cover as much square footage or cubic yardage as one containing 2-inch rocks. Divide the cubic yards you need to cover with river rock by the number one bag of covers. For example, if you need 25 cubic feet of river rock and each bag covers 1/2 cubic foot, buy 50 bags.

The same rule applies when you purchase river rock by the ton. Ask the landscaping company how many cubic yards a ton of river rock covers at various rock sizes and purchase the number of tons necessary to cover your desired area.

If the estimate of how much you need falls between sizes by the bag or ton, buy the extra amount rather than shorting yourself. You can always make the river rock coverage deeper to use the extra rock, but you can't get the coverage you want with too little rock.