Stripline Impedance Calculator

Stripline is an inner-layer trace sandwiched between two ground planes and fully surrounded by dielectric. It is a pure TEM line without dispersion and offers the best shielding and isolation for RF and high-speed signals on multilayer boards.

Analyze W → Z_oSynthesize Z_o → WCentered striplineThickness correctionε_eff = ε_r

① Inputs

Relative permittivity ε_rGround spacing bmmCopper thickness tµmFrequency fMHz

A. Analyze: known size → impedance

Trace width Wmm

B. Synthesize: known impedance → size

Target impedance Z_oΩ

② Results

Item	Value
Characteristic impedance Z_o	Ω
Synthesized width W	mm
Velocity factor k = 1/√ε_eff
Quarter wavelength λ/4 (guided)	mm

Cross-section

Signal conductorGroundDielectric

Symbols vs inputs: Relative permittivity εr　Ground spacing b　Copper thickness t　Trace width W

Impedance vs dimension (other inputs fixed)

Common laminates ε_r (click to apply)

Material	ε_r
FR-4 Low-cost general PCB laminate; higher loss above 1 GHz, εr varies with frequency and lot	4.3–4.6
Rogers RO4003C Hydrocarbon ceramic laminate, FR-4-compatible processing	3.38 (design 3.55)
Rogers RO4350B Common for power amplifiers, antennas and 5–6 GHz circuits	3.48 (design 3.66)
Rogers RO3003 PTFE ceramic, low loss and stable εr for mmWave	3.00
RT/duroid 5880 PTFE glass, very low loss, wider lines	2.20
RO3010 / RT6010 High permittivity for compact circuits	10.2
Alumina Al₂O₃ Thin/thick-film microwave substrate	≈ 9.8

How to use

Enter the dielectric ε_r and the total ground spacing b between the two planes (including the trace, centered).
Enter copper thickness t (µm) and frequency f.
Known width: enter W and press Analyze for Z_o.
Known target: enter Z_o and press Synthesize for the width W.

Theory and formulas

Wheeler's stripline equations with thickness correction, trace centered between the planes:

x = t/b　　m = 2 / [ 1 + (2/3) · x/(1 − x) ]
ΔW = (b − t) · x / (π(1 − x)) · { 1 − ½ ln[ (x/(2 − x))² + (0.0796x / (W/b + 1.1x))^m ] }
W′ = W + ΔW　　A = 4(b − t) / (πW′)
Z_o = 30/√ε_r · ln{ 1 + A · [ 2A + √( (2A)² + 6.27 ) ] }
k = 1/√ε_r　　λ/4 = k · c / (4f)

Stripline is entirely in dielectric, so the effective permittivity equals ε_r.

Design tips

50 Ω reference (from this tool): ε_r 4.4, b = 1.0 mm, t = 18 µm → W ≈ 0.42 mm; ε_r 4.2, b = 0.5 mm → W ≈ 0.20 mm. Stripline is much narrower than microstrip for the same impedance.
The formula assumes a centered trace. For asymmetric (offset) stripline treat the result as an estimate and confirm with the stack-up or simulation.
Core and prepreg may have different ε_r; use a weighted value or the fabricator's number.
Stitch the two ground planes with vias along the line, and add return vias next to every layer-change via.

FAQ

Stripline or microstrip?: Stripline is shielded, non-radiating and dispersion-free but narrower, slightly lossier and needs vias to reach. Use microstrip near antennas and parts, stripline for long routing.
How is b defined?: b is the total dielectric thickness between the two reference planes, including the copper thickness of the centered trace.
Why is there no ε_eff output?: The field is entirely in a homogeneous dielectric, so ε_eff = ε_r and the velocity factor is 1/√ε_r.

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