Model note: this uses standard closed-form approximations and ignores copper thickness/roughness. Great for early design, then validate in your field solver or PCB tool.
What this microstrip/stripline calculator does
This calculator helps you quickly estimate PCB transmission line behavior for two common controlled-impedance geometries: microstrip and stripline. You can either compute impedance from known dimensions, or solve for the required trace width for a target impedance (like 50 Ω).
It is designed for practical board design workflows: fast iteration, immediate feedback, and useful secondary outputs such as effective dielectric constant, velocity, and guided wavelength.
Microstrip vs. stripline
Microstrip
A microstrip is routed on an outer layer over a reference plane. Some electromagnetic field is in dielectric and some in air, so the effective dielectric constant is lower than the substrate’s nominal εr. Microstrip is widely used for RF traces, clocks, and many high-speed signals.
Stripline
A stripline is embedded between two reference planes. Most field is inside dielectric, yielding better shielding and lower radiation. In this simple model, effective dielectric constant is approximately equal to εr.
Inputs explained
- εr (dielectric constant): Material permittivity (for example FR-4 often ~4.0 to 4.7 depending on frequency and resin/glass mix).
- h (mm): For microstrip, dielectric thickness from trace to plane. For stripline, spacing between ground planes in this simplified symmetric model.
- w (mm): Conductor width.
- Target Z0 (Ω): Desired characteristic impedance when solving for width.
- Frequency (GHz): Optional; used to estimate guided wavelength and quarter-wave length.
Equations used in this page
Microstrip (closed-form approximation)
Uses common quasi-static formulas for effective permittivity and characteristic impedance with width-to-height ratio (w/h). These are widely used for first-pass estimates and usually close enough for early stackup and routing decisions.
Stripline (symmetric approximation)
Uses a simplified centered-stripline relation:
Z0 ≈ (30π / √εr) / (w/b + 0.441),
where b is plane-to-plane spacing (represented by h in this calculator).
Good design workflow
- Start from stackup values supplied by your fabricator.
- Use this calculator to get initial widths for 50 Ω single-ended or your target value.
- Route and keep reference-plane continuity clean (avoid splits/voids under traces).
- Send controlled-impedance requirements with stackup to PCB fab for final coupon tuning.
- For critical RF/high-speed channels, confirm with 2D/3D field solver and measurement.
Practical tips
- Real impedance depends on copper thickness, solder mask, etch profile, and weave effects.
- FR-4 εr varies with frequency and vendor; use realistic values from data sheets.
- Narrow traces raise impedance; wider traces lower impedance.
- For cleaner EMI behavior, stripline is often quieter than microstrip.
Limitations
This is an engineering calculator for quick estimation, not a replacement for fabrication DFM checks or electromagnetic simulation. Use it as a fast pre-layout and sanity-check tool, then finalize with your PCB manufacturer’s controlled-impedance process.