How to Use PCB Impedance Calculator
- Choose the transmission-line type that best matches the PCB geometry: microstrip, stripline, coplanar waveguide, or differential variants.
- Enter Er, dielectric height H, trace width W, copper thickness T, and spacing S. Use one length unit consistently through the selector.
- Set a target impedance such as 50 Ω, 90 Ω, or 100 Ω to get a first-pass width estimate.
- Compare impedance, effective dielectric constant, propagation velocity, and delay. Treat all values as stackup-planning estimates, not fabrication sign-off.
Formula & Theory - PCB Impedance Calculator
Microstrip example:
Z0 = (60 / sqrt(eeff)) × ln(8H/W + W/(4H)) for W/H <= 1
eeff ≈ (Er + 1)/2 + (Er - 1)/(2 × sqrt(1 + 12H/W))
Differential estimate:
Zdiff ≈ 2 × Z0 × coupling_factor(S/H)Controlled impedance depends on geometry and dielectric properties. Wider traces usually lower impedance, thicker dielectric usually raises impedance, and higher Er lowers signal velocity.
The calculator uses closed-form approximations that are appropriate for quick comparison. Coplanar and differential results are simplified and cannot replace a field solver for tight tolerances.
Solder mask, copper roughness, trapezoidal etch, glass weave, plating, and manufacturer process limits can shift the real impedance. Final values should be confirmed with the PCB fabricator.
Use Cases for PCB Impedance Calculator
- Early stackup planning for 50 Ω RF traces.
- Estimating 90 Ω USB or 100 Ω Ethernet/LVDS differential routing before sending constraints to layout.
- Comparing the effect of dielectric height or trace width changes.
- Preparing rough impedance targets for discussion with a board house.