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Microstrip Impedance Calculator

This calculator determines the characteristic impedance of a microstrip transmission line based on trace width, substrate height, and dielectric constant. It’s essential for RF PCB design, signal integrity, and impedance-controlled routing.

Characteristic Impedance of Microstrip Line

Input Fields
εr
Relative permittivity of the substrate
h
mm
Height of dielectric between ground and conductor
w
mm
Width of the copper trace
t
mm
Thickness of the copper layer
If enabled, the result will update automatically when you change any value.

Microstrip Impedance Calculation Formula

Formula
$$\text{If } \frac{w}{h} \leq 1: \\ Z_0 = \frac{60}{\sqrt{\varepsilon_r}} \cdot \ln\left( \frac{8h}{w} + \frac{w}{4h} \right) \\[1em] \text{If } \frac{w}{h} \geq 1: \\ Z_0 = \frac{120\pi}{\sqrt{\varepsilon_r} \cdot \left( \frac{w}{h} + 1.393 + 0.667 \cdot \ln\left( \frac{w}{h} + 1.444 \right) \right)}$$

Where:

  • $$Z_0$$ = characteristic impedance (Ω)
  • $$w$$ = trace width (mm)
  • $$h$$ = dielectric height (mm)
  • $$varepsilon_r$$ = dielectric constant (unitless)

These equations estimate the impedance of a microstrip line based on geometry and dielectric properties, commonly used in PCB design.


Microstrip Impedance – Calculation Example

Given:

  • $$w$$ = 1.2 mm
  • $$h$$ = 0.8 mm
  • $$varepsilon_r$$ = 4.4

Calculation:

  1. $$\frac{w}{h} = \frac{1.2}{0.8} = 1.5 \Rightarrow \text{use 2nd formula}$$
  2. $$Z_0 = \frac{120\pi}{\sqrt{4.4} \cdot \left( 1.5 + 1.393 + 0.667 \cdot \ln(1.5 + 1.444) \right)}$$
  3. $$Z_0 = \frac{376.99}{2.097 \cdot (1.5 + 1.393 + 0.667 \cdot \ln(2.944))}$$
  4. $$Z_0 ≈ \frac{376.99}{2.097 \cdot (1.5 + 1.393 + 0.667 \cdot 1.079)}$$
  5. $$Z_0 ≈ \frac{376.99}{2.097 \cdot 4.115} = \frac{376.99}{8.631} ≈ 43.7~\Omega$$


The microstrip impedance calculator is crucial in PCB design for RF and high-speed digital circuits. Impedance mismatches cause signal reflections, noise, and reduced performance. By calculating the characteristic impedance based on substrate properties and trace dimensions, designers can ensure signal integrity and meet the specs of components and connectors. The calculator supports both narrow and wide trace designs.

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