How to Calculate River Rock

Calculate how many square feet are to be covered. Measure the length of the area and the width and multiply the two numbers. For instance, a patio area that is 24 feet long by 16 feet wide would be 384 square feet. Circular areas can be calculated by measuring from the center of the circle to the edge -- the radius -- and multiplying that number by itself. A circle with a radius of 6 feet would contain 36 square feet.

Decide how deep the rock should be. Convert the inches into a decimal fraction of a foot by dividing by 12. Some common depths:

2 inches = .166 foot

4 inches = .333 foot

6 inches = .5 foot

Calculate the cubic feet by multiplying the square footage by the depth in feet. For a plot that is 24 feet long by 16 feet wide with rock 4 inches deep, multiply 384 square feet times .333 feet to get 127.872 cubic feet, which can be rounded up to 129 cubic feet.

Calculate the cubic yards by dividing the cubic feet by 27 since there are 27 cubic feet in a cubic yard: 129 cubic feet divided by 27 equals 4.78 cubic yards.

Calculate the tons by multiplying the cubic yards by 1.325 tons, the average weight of a cubic yard of river rocks: 4.78 cubic yards times 1.325 tons equals 6.33 tons, the amount of river rock needed to cover a space 24 feet long by 16 feet wide to a depth of 4 inches.