Microstrip & Stripline Impedance Calculator

A PCB trace over a ground plane is a transmission line, and its characteristic impedance depends on the trace width, its height above the plane, the copper thickness and the laminate's dielectric constant. This calculator works both ways: the impedance of a given trace, or the width that gives a target such as 50 Ω single-ended — on 1.6 mm FR-4 with εr 4.3 and 1 oz copper that is about 3.07 mm. Surface microstrip uses the Hammerstad–Jensen model (1980) with its strip-thickness correction, which the authors give as accurate to better than 0.2 % for 0.01 ≤ w/h ≤ 100 and εr up to 128; it also returns the effective dielectric constant, which sets how fast signals travel. Stripline — a trace centred between two planes b apart — uses Wheeler's 1978 formula, which handles thickness and stays within 0.5 % of the exact zero-thickness result while the strip is not very wide. From the impedance and effective εr come the propagation delay (roughly 140–190 ps per inch on FR-4) and the inductance and capacitance per unit length. The limits are those of any closed-form model. The microstrip figures are quasi-static: at several gigahertz dispersion raises the effective εr and the impedance drifts. Solder mask, which covers most outer traces, typically lowers microstrip impedance by a few ohms and is not modelled; nor are an off-centre stripline, differential pairs, or the fact that FR-4's εr varies with frequency, resin content and glass weave. Fabricators hold impedance to about ±10 %, and their own field solver with the actual stack-up is the final word — this gives you the right starting geometry.