Microstrip Impedance Calculator
Enter board stack-up and geometry values below. This calculator uses standard closed-form microstrip equations (quasi-static model) for quick PCB design estimates.
Note: This model assumes negligible conductor thickness and homogeneous dielectric for first-pass design. Always verify with your PCB fab stack-up and a field solver for final RF work.
What is microstrip impedance?
A microstrip is a PCB transmission line formed by a copper trace on an outer layer above a reference plane. Its characteristic impedance (Z0) depends mainly on dielectric constant, substrate thickness, and trace width. Controlled impedance matters in high-speed digital signals, RF design, antennas, and matching networks.
Why this calculator is useful
During layout, engineers often need fast answers: “How wide should a 50-ohm trace be on this board?” or “What impedance does this current width produce?” This tool gives practical first-order results without opening a heavy EM simulator.
- Estimate characteristic impedance from geometry.
- Estimate effective dielectric constant (εeff).
- Estimate propagation delay and guided wavelength.
- Solve approximate width for a target impedance.
Inputs explained
1) Relative dielectric constant (εr)
This is the dielectric constant of the substrate material. FR-4 is commonly around 4.1 to 4.7 depending on resin, weave, frequency, and vendor. Use your board manufacturer’s stack-up value when possible.
2) Substrate height (h)
The vertical distance from trace to reference plane. Larger h generally increases impedance for the same width.
3) Trace width (w)
Wider traces generally lower impedance. Narrow traces increase impedance.
4) Frequency and length
Impedance in this simplified model is quasi-static, but frequency is used here to compute guided wavelength and electrical phase length.
Equations used
The calculator uses classic microstrip approximations:
- For narrow lines (
w/h ≤ 1):
Z0 = (60 / sqrt(εeff)) * ln(8h/w + 0.25w/h) - For wider lines (
w/h > 1):
Z0 = (120π) / (sqrt(εeff) * (w/h + 1.393 + 0.667 ln(w/h + 1.444)))
Effective dielectric constant is estimated with a standard closed-form expression that accounts for fringe fields in air.
Quick reference examples (FR-4 style values)
| εr | h (mm) | w (mm) | Approx Z0 (Ω) |
|---|---|---|---|
| 4.3 | 1.6 | 3.0 | ~50 |
| 4.3 | 1.6 | 1.5 | ~75 |
| 4.3 | 0.8 | 1.5 | ~50 |
Design tips for better controlled impedance
- Use the actual fabrication stack-up from your PCB manufacturer.
- Include copper thickness and soldermask effects in final simulation.
- Keep a continuous, unbroken return path under high-speed lines.
- Avoid sudden width changes and sharp corners in RF paths.
- For differential pairs, use a differential impedance calculator (not single-ended only).
Limitations and best practice
This page provides an engineering estimate, not a sign-off electromagnetic solution. Real boards include conductor thickness, roughness, soldermask, nearby copper, and frequency dispersion. For production-critical RF/high-speed channels, validate with a 2D/3D field solver and your fabricator’s impedance coupon data.