'average thickness' is correct and can be used in written English.
For example, you could say, "The average thickness of a sheet of paper is 0.1 millimeters."
The average thickness is about 1.5 miles (2.45 km).
After you have ordered your materials, excavate the path area deep enough to accommodate the average thickness of the stones and two inches of sand.
The average thickness of the ice floes measured by the team was 1.8m, a depth considered too thin to survive the next summer's ice melt season.
Nevertheless, at the end of the year, the average thickness of the ice had declined, in some spots by as much as a third.
As an example, in 1965 the average thickness of a can wall was about 0.25 millimetre (0.01 inch).
The average thickness of the terrestrial crust for both East and West Antarctica approximates that of other continents.
Average thickness of injection well.
Average thickness is close to 5 km.
The average thickness was 20 nm.
The average thickness of the membrane was 35 μm.
The films have an average thickness of 4.3 μm.
