microstrip line impedance calculator

What this microstrip line impedance calculator does

This calculator estimates the characteristic impedance of a microstrip transmission line using a well-known closed-form method (Hammerstad and Jensen model). A microstrip is a PCB trace on the outer layer over a ground plane, and its impedance is mainly controlled by the dielectric constant, substrate thickness, and trace width.

In practical RF and high-speed PCB design, controlled impedance matters for signal integrity, reflections, and power transfer. Typical targets include 50 Ω (single-ended RF), 75 Ω (video/coax interfaces), and differential pairs that depend on line spacing and stack-up.

Inputs explained

1) Relative dielectric constant (εr)

This is the substrate dielectric constant. For FR-4, the value varies with resin, glass weave, and frequency. Use your board fabricator's stack-up value whenever possible.

2) Substrate height (h)

Height is the distance from the trace to its reference plane. On an outer layer microstrip, this is often the prepreg/core thickness to the nearest ground plane.

3) Trace width (w) or target impedance (Z0)

• Impedance mode: Provide width and calculate Z0.
• Width mode: Provide target Z0 and calculate required width.

Equations used in this calculator

The tool calculates the width ratio u = w/h, then computes effective dielectric constant and characteristic impedance:

• a = 1 + (1/49)ln((u^4 + (u/52)^2)/(u^4 + 0.432)) + (1/18.7)ln(1 + (u/18.1)^3)
• b = 0.564 * ((εr - 0.9)/(εr + 3))^0.053
• εeff = (εr + 1)/2 + (εr - 1)/2 * (1 + 10/u)^(-ab)

Then impedance is computed piecewise:

• If u ≤ 1: Z0 = (60/√εeff) ln(8/u + 0.25u)
• If u > 1: Z0 = (120π)/(√εeff (u + 1.393 + 0.667 ln(u + 1.444)))

For width solving, this page uses numerical bisection to find w that matches your target impedance.

How to use it in design workflow

1. Get real stack-up values from your PCB fab notes.
2. Enter εr and h.
3. Choose either impedance mode or width mode.
4. Calculate and compare with your target.
5. Finalize dimensions with your fabricator's impedance control process.

Practical tips and caveats

• This is a fast engineering estimate and does not replace full-wave EM simulation.
• Trace thickness, solder mask, roughness, and frequency dispersion can shift actual impedance.
• For tight tolerances, use your board house field-solver-backed impedance tables.
• Always include manufacturing tolerance analysis (etch width, dielectric tolerance, plating).

Quick example

Suppose εr = 4.3 and h = 1.6 mm (common FR-4 context). If width is about 3.0 mm, the result lands near the 50 Ω region. Use this as a sanity check only; production geometry often differs due to copper thickness and stack-up details.

Bottom line

A microstrip impedance calculator is one of the fastest ways to move from concept to a manufacturable controlled-impedance trace. Use this page for early sizing and iteration, then lock dimensions with your PCB manufacturer’s process data.