Controlled Impedance Calculator

Hitting a target characteristic impedance — for a high-speed signal, a differential pair, or anything a fab house needs to hold to spec — means solving IPC-2141's microstrip or stripline geometry equations. This calculator handles both surface microstrip and symmetric stripline, solving for trace width from a target impedance or checking the impedance a given geometry actually produces.

Controlled Impedance Trace

Surface microstrip (IPC-2141)

Show your work

Geometry

Target Z₀

Dielectric height h

mil

Copper thickness t

mil

1 oz ≈ 1.4 milDielectric constant εr

FR-4 ≈ 4.2–4.5

Width for target Z₀

10.93 mil

Width for target Z₀

0.2777 mm

Check an existing width

Trace width w

mil

Resulting Z₀

47.06 Ω

Propagation delay

139.5 ps/inch

Formula

Z_{0} = \frac{87}{\sqrt{ε_{r} + 1.41}} \cdot \ln (\frac{5.98 h}{0.8 w + t})

t_{pd} = 84.72 \cdot \sqrt{ε_{eff}} ps/inch

ε_{eff} = 0.475 \cdot ε_{r} + 0.67

The width for your target Z₀ is found numerically from the same formula. IPC-2141's microstrip formula is fit for w between 0.1× and 2× h; the calculator warns above 2× h.

Z₀ = characteristic impedance (Ω)
w = trace width (mil)
t = copper thickness (mil)
h = dielectric height from the trace down to the plane (mil)
ε_r = dielectric constant of the board material
ε_eff = effective dielectric constant the signal sees
t_pd = propagation delay (ps/inch)

These closed-form IPC-2141 equations are an estimate and can be 5–10% off a field solver for common FR-4 stackups. Always confirm the final width with your fabricator's impedance-controlled stackup before release.