The surface area of a sphere is A = 4π R²
The volume of a sphere is V = (4/3) ( π) (R³)
The ratio of (surface area) to (volume) is
4 π R² / (4/3) (π) (R³)
Divide top and bottom by 4π : R² / (1/3) (R³)
Multiply top and bottom by 3: 3 R² / R³
Divide top and bottom by R² : 3 / R .
For the Earth,
A / V = (3) / (3,960 miles) = 0.00076 per mile (rounded)