Microstrip impedance calculator

The microstrip form is the IPC-2141 approximation, valid as printed for w/h ratios of 0.1 to 3 and Er from 1 to 15; the stripline form is its symmetric twin, read against the plane-to-plane spacing. The tool computes outside those windows anyway and flags the answer instead of hiding it. What it does not model: etch undercut (the real trapezoid cross-section), solder mask over the outer layers, copper roughness, and weave variation. For boards that live or die on controlled impedance, a Hammerstad-Jensen calculation or a 2D field solver is the next step up.