Microstrip Impedance Calculator
A microstrip is a flat copper trace running over a ground plane separated by a dielectric. Its characteristic impedance controls signal integrity on high-speed PCBs, so it must be matched to the driving and receiving circuits. This calculator estimates the characteristic impedance from the trace width, the dielectric (substrate) height, and the relative permittivity of the board material using the Wheeler/Hammerstad closed-form approximation. It returns the impedance in ohms and the effective dielectric constant, which sets the propagation speed. Adjust width and height until the impedance reaches your target.
Microstrip impedance formula (Wheeler/Hammerstad)
Let u = W / H
Eeff = (Er+1)/2 + (Er-1)/2 * (1 + 12/u)^(-0.5) [for u >= 1]
If u < 1 add term: (Er-1)/2 * 0.04 * (1-u)^2
If u >= 1: Z0 = (120*pi / sqrt(Eeff)) / (u + 1.393 + 0.667*ln(u+1.444))
If u < 1: Z0 = (60/sqrt(Eeff)) * ln(8/u + u/4)
This is the standard closed-form microstrip model. Accuracy is a few percent for typical FR-4 geometries; use a field solver for controlled-impedance fabrication tolerances.
PCB impedance design context
- 50 ohms is the common single-ended target for high-speed digital traces.
- Increasing trace width lowers impedance; increasing dielectric height raises it.
- FR-4 Er is roughly 4.2 to 4.6 and drops at high frequency; use your laminate datasheet value.
- Effective Er is between 1 and Er because the field is partly in air above the trace.
- Signal speed equals the speed of light divided by the square root of effective Er.
Microstrip impedance: frequently asked questions
What model does this microstrip calculator use?
It uses the widely cited Wheeler / Hammerstad closed-form approximation, the same family of equations documented in IPC design references. It computes the effective dielectric constant from the trace width to substrate height ratio, then the characteristic impedance. It is an approximation accurate to a few percent for typical PCB geometries; a full-wave solver is needed for tight tolerances.
What is a good target impedance for PCB traces?
Single-ended high-speed signals are commonly routed at 50 ohms, and some video and RF systems use 75 ohms. The exact target is set by the interface specification. Adjust trace width and dielectric height until the calculated impedance matches your target.
What dielectric constant should I use for FR-4?
Standard FR-4 has a relative permittivity (Er) of roughly 4.2 to 4.6 at low frequencies, falling slightly at high frequencies. Because it varies by laminate and frequency, Er is a user-editable input here. Use the value from your laminate manufacturer's datasheet for accurate results.
Does trace thickness matter for impedance?
Copper thickness has a smaller effect than width and dielectric height, so the basic Wheeler form used here omits it for clarity. For 1 oz copper on typical substrates the error is small. If you need thickness correction, use a full field solver or a thickness-corrected model.
Is the calculation in metric or imperial units?
Enter trace width and dielectric height in the same unit (both mils or both millimetres). Because impedance depends only on the ratio of width to height, the result is unit-independent as long as both inputs share a unit.
Official sources
- IEEE Standards Association: electromagnetic and transmission-line standards.
- NIST: National Institute of Standards and Technology (electromagnetic metrology).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.