There are two IPC documents which are frequently referenced for approximate microstrip and stripline impedance formulas:
The IPC-D-317A has been superseded by IPC-2251 which can be purchased from www.ansi.org for about $110.
The IPC-D-317A formulas seem to be good for RF4 to about 5% but fall apart for other dielectric constants.
FR4 has an wildly varying dielectric constant from 4.5 to over 5.0.
For calculators using IPC-D-317A see http://www.emclab.umr.edu/pcbtlc/.
The default impedance calculations for Protel 2004 are based on IPC-D-317A.
Microstrip impedance formulas from IPC-D-317A:
Embedded microstrip impedance formulas from IPC-D-317A:
Stripline impedance formulas from IPC-D-317A:
Asymmetrical stripline impedance formulas from IPC-D-317A:
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