Convert diameter inch to linear inch?
An inch is an inch is an inch is an inch....
You cannot.A linear inch is a measure of length in 1-dimensional space while a square inch is a measure of area in 2-dimensional space. The two measure different things and, according to basic principles of dimensional analysis, any attempt at conversion from one to the other is fundamentally flawed.
The question, as stated, cannot be answered sensibly. A square inch is a measure of area, with dimensions [L2]. A linear foot is a measure of distance, with dimensions [L]. The two measure different things and elementary dimensional analysis teaches that you cannot convert between measures with different dimensions such as these without additional information.