W:		trace width
T:		trace thickness
H (/H1)		height of trace or dielectric above return plane
C:		distance between the differential stripline pair
er:		relative permittivity of the dielectric

Electrical Parameter Definitions

Zo:		characteristic impedance
Zc:		effective characteristic impedance including the capacitance of distributed loads
Tpd:		propagation delay
Lo:		inductance per unit length
Co:		capacitance per unit length

Acknowledgements:

The source for formulas used in this calculator (except where otherwise noted) is:
IPC-D-317A, Design Guidelines for Electronic Packaging Utilizing High-Speed Techniques, Jan 1995; Section 5, pp. 13 - 36.

THESE FORMULAS ARE APPROXIMATIONS!
They should not be used when a high degree of accuracy is required.

The transmission line calculator concept was developed by Douglas Brooks and is described in his paper: PCB Impedance Control: Formulas and Resources, Printed Circuit Design Magazine, March 1998.