You can work these formulas manually, or download this great free microstrip
calculator (TX Line) from NI/AWR.
Equations are given for calculating the effective width and height prior to plugging
the dimensions prior to plugging them in to the equations for effective dielectric
constant and then for calculating the impedance.
Here is a really nice summary of many controlled impedance line equations, by
RF / Microwave PC Board Design and Layout
Be aware that there are many microstrip impedance calculators on the Internet
that give sometimes widely varying results. Be sure of which one you finally decide
to use. Most claim to use some or all of these formulas these formulas, but I have
discovered ones, for instance, that appear to mistakenly substitute 0.4 for 0.04
in the εeff formula for the W/H<1 case.