Microstrip impedance calculator
Work out the characteristic impedance of a PCB microstrip trace from its width, height above ground and the substrate dielectric.
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.
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Frequently asked questions
How is microstrip characteristic impedance calculated?
For a trace width-to-height ratio W/H ≤ 1 the impedance is Z0 = (60/√ε_eff)·ln(8H/W + W/4H), where ε_eff = (εr+1)/2 + (εr−1)/2·(1/√(1+12H/W)) accounts for field lines partly travelling through air. Above W/H = 1 the tool stops reporting Z0 and asks you to reduce W or raise H.
What dielectric constant should I use for FR4?
The calculator defaults to εr = 4.2 for standard FR4 and offers a 4.5 preset for hi-end FR4. The effective dielectric ε_eff always sits between 1 and εr, and the tool also reports capacitance (0.67·(εr+1.41)/ln(5.98H/(0.8W+T)) pF/cm) and propagation delay (85·√ε_eff ps/cm) per the IPC-2141 approximations.
How do I design a 50 Ω or 90 Ω differential pair?
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 (35 µm). The tool also compares the result against differential targets — USB 90 Ω and HDMI/Ethernet 100 Ω pairs are designed with roughly half the single-ended impedance per line, since a loosely coupled pair lands near 2× the single-ended Z0.
Model valid for W/H ≤ 1; real impedance depends on the fabricator's stack-up and solder mask. ICBOMS provides this tool for reference only.