PCB Trace Impedance (Microstrip / Stripline)

Characteristic impedance and propagation delay of microstrip and stripline from permittivity, dielectric height, trace width and conductor thickness, including whether the approximation is valid.

Worked example

Input: Trace type Microstrip (surface), Relative permittivity εr 4.3, Dielectric height H (B for stripline) 1.6 mm, Trace width W 3 mm, Conductor thickness T 0.035 mm, Trace length 100 mm
Result: Characteristic impedance Z₀ 49.824 Ω, Effective permittivity ε_eff 2.7125, Propagation delay 5.49369 ps/mm, Total delay of the trace 549.369 ps, W/H (W/B for stripline) 1.875, Within the approximation range: Within the range

Formula

Microstrip: Z₀ = 87 / √(εr + 1.41) · ln(5.98·H / (0.8·W + T)) (W/H = 0.1 to 2)

Stripline: Z₀ = 60 / √εr · ln(4·B / (0.67π·(0.8·W + T))) (W < 0.35·B)

ε_eff = 0.475·εr + 0.67 (microstrip), ε_eff = εr (stripline)

t_pd = √ε_eff / c (≈ 85·√ε_eff ps/inch)

How it works

This uses the IPC-2141 approximations: microstrip Z₀ = 87/√(εr+1.41)·ln(5.98H/(0.8W+T)) and stripline Z₀ = 60/√εr·ln(4B/(0.67π(0.8W+T))). With the example values (FR-4 εr 4.3, H 1.6 mm, W 3 mm) the result is about 49.8 Ω, which matches the familiar rule that 50 Ω on a 1.6 mm board needs a 3 mm trace. Propagation delay uses ε_eff = 0.475εr + 0.67 for microstrip, giving about 5.49 ps/mm (139 ps/inch).

Practical tipThese are empirical approximations. Microstrip accuracy degrades outside W/H of 0.1 to 2.0, and the stripline form does not hold once W exceeds 0.35·B; the calculator flags both. For production controlled-impedance designs, confirm with a field solver and your fabricator’s stack-up table. From an EMC point of view, an unbroken reference plane under the trace matters more than the exact impedance figure.

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Results and summaries are for reference. For certification and test reports use the latest official standard text and calibrated instrument data. Last updated: 2026-10-10