Microstrip Transmission Line Tool
Use this calculator to either find characteristic impedance from geometry or solve required trace width for a target impedance. Assumes a thin conductor and homogeneous substrate.
What is a microstrip line?
A microstrip is one of the most common controlled-impedance transmission line structures on a PCB. It consists of a top copper trace over a dielectric substrate, with a continuous ground plane underneath. Because part of the electromagnetic field is in dielectric and part is in air, the line propagates with an effective dielectric constant instead of exactly the board dielectric constant.
In practical RF, high-speed digital, and mixed-signal design, getting the width right for a desired impedance (such as 50 Ω single-ended) is crucial for signal integrity, return loss, and predictable timing.
What this calculator computes
- Characteristic impedance (Z0) from known width and stackup.
- Required width (w) for a target impedance and substrate height.
- Effective dielectric constant (εeff).
- Guided wavelength and delay per millimeter when frequency is provided.
Equations used
This page uses classic closed-form microstrip approximations (Hammerstad/Jensen-style, thin conductor assumption). Let u = w/h:
Z0 = (120π)/(sqrt(εeff) * (u + 1.393 + 0.667 ln(u + 1.444))), for u > 1
When solving for width from target impedance, the script numerically inverts these equations with binary search.
How to use it
Mode 1: Find impedance from width
- Enter substrate dielectric constant (εr), substrate height (h), and trace width (w).
- Click Calculate to get Z0 and εeff.
- Optionally enter frequency to estimate guided wavelength.
Mode 2: Find width from target impedance
- Switch mode to Find Width from Target Z0.
- Enter desired impedance (for example, 50 Ω).
- The calculator returns the required width for your substrate height.
Design notes and practical tips
- FR-4 dielectric constant changes with frequency, resin content, and manufacturer process. Expect variation.
- Copper thickness, solder mask, and etch profile can shift impedance by several percent.
- For final products, verify with your PCB fabricator’s field solver and impedance coupons.
- Keep a solid reference plane directly beneath the trace to preserve microstrip behavior.
Example
For a typical 2-layer FR-4 board with εr = 4.3 and h = 1.6 mm, a 50 Ω microstrip will often require a fairly wide trace (around 3 mm in this simplified model). On thinner dielectrics, the required width reduces significantly.
Limitations
This tool is excellent for fast estimates and early layout decisions. It is not a replacement for full-wave EM simulation when you need tight tolerance RF designs, differential pair optimization, discontinuity modeling, or mmWave performance prediction.