Microstrip Impedance Calculator

Track Width
The width of the track (microstrip). This is normally measured in mm or mils.
Track Thickness
The thickness of the track (microstrip). This is the same as the 'weight' of the copper layer the track is on. Usually measured in um or oz./sq foot.
Substrate Thickness
The thickness (height) of the substrate. This is also the distance between the track and the plane below it. On a two layer standard thickness PCB, this is usually about 1.6mm. Between two layers of a high-density PCB this value can be much smaller. Usually measured in mm or mils.
Substrate Dielectric
The dielectric of the substrate. For standard FR-4 PCB material, this value is around 4-4.7.
Track Impedance
The calculated impedance of the track (microstrip). This needs to match the impedance of what ever is connected to each end so that RF reflections do not occur. Value is usually between 20 and 150.

This calculator can find the impedance of various microstrip and stripline style PCB tracks.

To calculate the impedance of a microstrip, we need to know the following parameters of the PCB:

\( w \) = width of the track (in meters)
\( t \) = thickness of the track (in meters)
\( h \) = thickness (or height) of the substrate (in meters)
\( \epsilon_r \) = the dielectric constant of the PCB substrate

The following equations can then be used to calculate the impedance \(Z\) (in \(\Omega\)) of the microstrip:

$$ W = w + \frac{t}{\pi} \left[ ln\left(\frac{2h}{t}\right) = 1 \right] $$

$$ H = h - 2t $$

If \( \frac{W}{H} < 1 \)

$$ \epsilon_{eff} = \frac{\epsilon_r + 1}{2} + \frac{\epsilon_r - 1}{2}\left[\frac{1}{\sqrt{1 + 12\frac{H}{W}}} + 0.04\left(1 - \frac{W}{H}\right)^2\right] $$

$$ Z = \frac{60}{\sqrt{\epsilon_{eff}}} ln\left(\frac{8H}{W} + \frac{W}{4H}\right) $$

If \( \frac{W}{H} \geq 1 \)

$$ \epsilon_{eff} = \frac{\epsilon_r + 1}{2} + \frac{\epsilon_r - 1}{2\sqrt{1 + 12\frac{H}{W}}} $$

$$ Z = \frac{120\pi}{ \sqrt{\epsilon_{eff}} \left[ \frac{W}{H} + 1.393 + \frac{2}{3}ln(\frac{W}{H} + 1.444) \right] } $$

Equations are from http://www.rfcafe.com/references/electrical/microstrip-eq.htm.

More information on microstrips can be found at http://blog.mbedded.ninja/pcb-design/impedance-controlled-routing/microstrips.