http://emclab.mst.edu/pcbtlc2/index.html
PCB Trace Impedance Calculator Calculates the characteristic impedance and per-unit-length parameters
of typical printed circuit board trace geometries. Select a configuration: Microstrip Trace Embedded Microstrip Trace Stripline Trace Dual Stripline Trace               Geometric Parameter Definitions:


W:   trace width T: trace thickness H (/H1) height of trace or dielectric above return plane C: distance between the differential stripline pair εr: 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 the Design Guide for Electronic Packaging Utilizing High-Speed Techniques (4th Working Draft, IPC-2251, February 2001. THESE FORMULAS ARE APPROXIMATIONS! They should not be used when a high degree of accuracy is required. This transmission line calculator was based on a concept developed by Douglas Brooks of UltraCAD Design, Inc.