PCB Trace Impedance Calculator

Work out pcb trace impedance instantly with clear inputs, formula shown and shareable results.

About PCB Trace Impedance Calculator

For a microstrip the standard approximation is Z0 = (87/sqrt(er + 1.41)) x ln(5.98h / (0.8w + t)). Impedance falls as the trace widens or the dielectric thins, so controlled-impedance stackups fix h and er and then specify w to hit 50 or 100 ohms.

Formula

Microstrip impedance

Z0 = (87 / sqrt(er + 1.41)) x ln(5.98 h / (0.8 w + t))

Worked example

With the default values below, PCB Trace Impedance Calculator returns Characteristic impedance of 53.52. Change any input above and every figure here is recalculated in your browser.

Inputs used

Trace width: 0.3 mm
Dielectric height: 0.2 mm
Dielectric constant: 4.3
Copper thickness: 35 µm

Results

Characteristic impedance: 53.52 Ω
Propagation delay: 6.37 ps/cm
Capacitance per centimetre: 1.19 pF/cm
Effective dielectric constant: 3.640

Step by step

1Values used
2Microstrip impedance
3Characteristic impedance= 53.52 Ω
4Propagation delay= 6.37 ps/cm
5Capacitance per centimetre= 1.19 pF/cm
6Effective dielectric constant= 3.640

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why does impedance matter on a PCB?

Once the trace is longer than about a sixth of the signal's rise-time length, reflections from impedance discontinuities cause ringing and timing errors.

Microstrip or stripline?

Microstrip is on an outer layer with one reference plane; stripline is buried between two planes, giving better shielding but slower propagation and lower impedance for the same width.