PCB Trace Impedance Calculator
To calculate PCB trace impedance, pick the geometry (microstrip or stripline), then enter trace width, copper thickness, dielectric height and the laminate's relative permittivity (Er). The calculator applies the IPC-2141 closed-form models to return single-ended characteristic impedance in ohms. For differential pairs, use our differential-pair calculator.
Cross-section (not to scale)
48.7 Ω
First-order estimate (IPC-2141 closed form). Sign off production stack-ups with a 2D field solver and a fab-house controlled-impedance coupon.
FAQ
- What is the difference between microstrip and stripline impedance?
- A microstrip routes on an outer layer with the dielectric and reference plane on one side only, so its field partly travels through air. A stripline is buried between two reference planes, fully embedded in dielectric. For the same width and Er, stripline gives lower impedance and tighter, more predictable control, while microstrip is faster to route and easier to probe.
- Why is 50 ohm the default single-ended target?
- 50 ohm is a practical compromise: on typical FR-4 geometries it balances conductor loss (favouring higher impedance) against power handling and dielectric loss (favouring lower impedance), and it matches the reference impedance of most connectors, test gear and SerDes I/O. Many differential pairs target 90 or 100 ohm instead, so always check the interface spec (USB, Ethernet, PCIe, HDMI) before routing.
- What Er value should I use for FR-4?
- FR-4 is a class of glass-weave laminates, not a single material, so its relative permittivity (Er) is not fixed. It typically falls around 4.2 to 4.6 at low frequency and drops slightly as frequency rises. For controlled impedance, do not assume a generic 4.5 figure: pull the Er and loss tangent from your fabricator's specific laminate datasheet at the relevant frequency, since glass-resin ratio and resin content shift the value.
- How does copper weight affect trace impedance?
- Copper weight sets the finished trace thickness (1 oz is roughly 35 micrometres, 2 oz roughly 70). Thicker copper increases the conductor cross-section coupling to the reference plane, which lowers characteristic impedance, and it also enlarges trace-edge fringing and etch undercut. Heavier copper therefore usually needs a slightly narrower or differently spaced trace to hold the same target impedance.
- How much do fabrication tolerances shift the calculated impedance?
- A calculator gives a nominal value; the board house introduces real variation. Etch width tolerance, dielectric height spread between prepreg and core, copper-weight tolerance and Er variation across a panel all stack up. Real-world controlled-impedance builds are commonly specified at plus or minus 10 percent. Always send your stack-up to the fabricator for a controlled-impedance report rather than trusting the nominal number alone.
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