A ton of gravel can cover anywhere from 20 square feet to 240 square feet depending on depth. More rocks per square foot result in a lower coverage rate.
At a depth of 1 inch, the coverage is up to 240 square feet, and 3 inches can reduce the coverage rate to 80 square feet.
If you use a conservative estimate, calculate 70 square feet for every inch of depth and ton of material.
This is dependent on the type of job that you are purchasing gravel for. Different projects require different depths of material and should be adjusted accordingly.