Microstrip impedance calculator
Interactive engineering and procurement calculator. Instant client-side computation with no sign-up.
Trace geometry
Results
Characteristic impedance (Z0)
60/√ε_eff · ln(8H/W + W/4H) (W/H ≤ 1)
Effective dielectric (ε_eff)
(εr+1)/2 + (εr−1)/2 · (1/√(1+12H/W) + 0.4·(1−W/H)²)
Capacitance (C)
0.67/2.54·(εr+1.41)/ln(5.98H/(0.8W+T))
Propagation delay
33.5·√ε_eff
Verdict
USB 90 Ω · HDMI 100 Ω · Ethernet 100 Ω are differential (~2× single-ended)
About the model
A microstrip is a signal trace over a continuous ground plane, separated by the substrate dielectric. Narrow traces (W/H ≤ 1) follow Z0 = 60/√ε_eff · ln(8H/W + W/4H), and wide traces (W/H ≥ 1) the 120π/(√ε_eff·(W/H + 1.393 + 0.667·ln(W/H + 1.444))) form — both with ε_eff between 1 and εr because part of the field travels through air. The capacitance and delay approximations follow IPC-2141. As a rule of thumb a 50 Ω line on 1.6 mm FR4 (εr 4.2) lands near W ≈ 1.9 × H ≈ 3 mm with 1 oz copper — but impedance shifts with the copper weight and solder mask, so check against the fabricator's impedance coupons.
For high-speed boards, designers also reach for high-speed connectors and RF connectors where the trace leaves the board.
Browse related parts: