Calculation of characteristic Impedance of parallel lines.
The following formula is used to calculate the impedance of two parallel lines.

Zo = 276/sqrt(K) x  Log( 2S/d )

where:- S is the centre to centre Spacing of the conductors in mm,
d is the diameter of the conductors in mm
and K is the relative permittivity or dielectric constant of the insulation.

For pvc insulated twin cable use 2.5 as the permittivity K.
Use 1 for Air spaced.

This page uses Java so you may need to enable ActiveX to use this calculator.
  S (spacing between wires in mm)   (mm) inch
  d (diameter of conductor in mm)  (mm) inch
  K (dielectric constant)  

Zo (Impedance)    Ohms  
  Velocity factor = 1/Sqrt(K)  
Once you hit Calculate and have a result, you can change any value and Calculate again.