Microstrip PCB Impedance Calculator

PCB Trace Impedance Estimator

Input Fields

Dielectric constant (εr)

εᵣ

Relative permittivity of PCB substrate

Trace width (W)

W

mm

Width of the microstrip trace

Substrate height (H)

H

mm

Height from trace to ground plane

Copper thickness (T)

T

mm

Thickness of the copper trace

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Your Results

Characteristic Impedance

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Microstrip Line Impedance Formulas

$$Z_0 = \begin{cases} \frac{60}{\sqrt{\varepsilon_r}} \cdot \ln\left( \frac{8h}{w} + \frac{w}{4h} \right), & \text{if } \frac{w}{h} \leq 1 \\[1em] \frac{120\pi}{\sqrt{\varepsilon_r} \cdot \left( \frac{w}{h} + 1.393 + 0.667 \cdot \ln\left( \frac{w}{h} + 1.444 \right) \right)}, & \text{if } \frac{w}{h} > 1 \end{cases}$$

Where:

• $$Z_0$$ = characteristic impedance (Ω)
• $$w$$ = trace width (mm)
• $$h$$ = substrate height (mm)
• $$varepsilon_r$$ = elative dielectric constant of the substrate

These equations model the impedance of a microstrip trace above a ground plane.

Microstrip PCB Impedance – Calculation Example

Given:

• $$w$$ = 0.75 mm
• $$h$$ = 0.6 mm
• $$varepsilon_r$$ = 4.2

Calculation:

1. $$\frac{w}{h} = \frac{0.75}{0.6} = 1.25 > 1 \Rightarrow \text{Use second formula}$$
2. $$Z_0 = \frac{120\pi}{\sqrt{4.2} \cdot \left(1.25 + 1.393 + 0.667 \cdot \ln(1.25 + 1.444)\right)}$$
3. $$= \frac{376.99}{2.049 \cdot (2.643 + 0.667 \cdot 0.933)}$$
4. $$= \frac{376.99}{2.049 \cdot 3.266} ≈ \frac{376.99}{6.693} ≈ 56.32~\Omega$$

Accurate calculation of microstrip impedance is essential for PCB designers working with high-speed signals like USB, HDMI, or RF traces. This calculator allows engineers to estimate impedance quickly and optimize trace width for a given substrate height and material. Proper impedance control reduces signal reflection, distortion, and EMI, ensuring reliable and efficient signal transmission in complex circuits.