To find the amount needed to fill a sandbox, determine how high you would like the sand to go, and then multiply that height by the length and width of the box to determine how much sand will be needed to fill the sandbox. As a simple rectangular prism, the volume a sandbox can hold can be calculated with the equation length times width times height.
The area of rectangle is given by multiplying its width by its length. If the measurements were taken in inches, the area has the units of inches-squared. When a rectangle is extended into three dimensions, like a sandbox, the volume of material it can contain can be calculated by multiplying the area by the height. The volume will be a result with units of inches-cubed, which can be converted into any equivalent volume units.