What is the formula for filling a trench with gravel?

The way to fill a trench with gravel depends on the purpose of the trench and how much drainage you need. For example, in wetter soils you'd need a deeper layer of gravel than in drier soil. Generally, for a trench where water or sewer (such as for a backyard pit) pipes are laid in the trench, about 2 to 3 inches of moderate sized gravel would work. If you are intending for the trench to act as drainage around or near plants, 1 to 1.5 inches of gravel may help provide adequate drainage.

When my dad and I laid sewer pipes, we used 3" in clay soil (which holds more moisture/rain). Lay the gravel first, leveling it as much as possible. Make sure to increase or decrease on sloping land, as needed. Lay and join the pipe, making sure the pipe is at a slight downward angle from any building. Continue joining pipe along the length. It is important to TEST the pipe at this point to be sure the connections are secure; run water from a hose, run water from the house, or flush a water-only toilet (no waste in toilet). Once the pipe was tested, my dad then added a layer of gravel on top of the piping, to a 1.5 inch depth. Then, shovel in topsoil and gently pack (tamp) down.