Microstrip Impedance Calculator determines PCB trace impedance from width, height, copper thickness and dielectric constant per IPC-2141 for controlled high-speed digital circuits.
Calculate PCB Microstrip Trace Impedance for Controlled-Impedance Board Design
This calculator computes the characteristic impedance of a copper trace running over a ground plane, along with effective dielectric constant, propagation delay, and per-unit-length inductance and capacitance. PCB design engineers, RF engineers, and signal integrity specialists use it to size trace width and dielectric height for controlled-impedance requirements — USB, HDMI, DDR, Ethernet, RF antenna feeds — before sending a board to fabrication.
How to Use
Enter Trace Width (W), Substrate Height (H), and Trace Thickness (T) in mm, mil, or inch, plus the Substrate Dielectric (er) of the board material. The tool returns Characteristic Impedance (Z0) in ohms, effective dielectric constant, propagation delay, and per-length inductance and capacitance.
Formula
Per IPC-2141A, IPC’s design guide for high-speed controlled-impedance circuit boards, characteristic impedance for a surface microstrip is:
$$Z_0 = \frac{87}{\sqrt{\varepsilon_r + 1.41}} \ln\left(\frac{5.98H}{0.8W + T}\right)$$
with $W$, $H$, and $T$ expressed in the same unit. The effective dielectric constant used for signal timing is $\varepsilon_{eff} = 0.475\varepsilon_r + 0.67$, giving a one-way propagation delay of $t_{pd} = 85\sqrt{\varepsilon_{eff}}$ ps/inch. Per-length inductance and capacitance follow from standard transmission-line relations, $L = Z_0\sqrt{\varepsilon_{eff}}/c$ and $C = \sqrt{\varepsilon_{eff}}/(Z_0 c)$.
The most common input mistake is mixing units — leaving W in mm while H and T are entered in mils. Because the equation takes a ratio of H to (0.8W+T), a stray unit doesn’t produce an obviously broken number; it just skews Z0 by roughly an order of magnitude, which is easy to miss on a first pass.
IPC-2141A’s equation is documented as accurate for W/H ratios between 0.1 and 2.0; outside that range error grows past the ~5-10% tolerance the standard itself assumes.
It also models bare copper — solder mask sitting over an outer-layer trace raises the dielectric loading around the conductor and typically pulls real-world impedance down a few ohms (commonly cited around 2-5 ohms) versus the bare-formula result, which is why many fabricators target a slightly higher bare-board impedance than the final spec calls for.
FAQ
What units does the microstrip impedance calculator accept?
Trace width, substrate height, and trace thickness can be entered in mm, mil, or inch. The tool converts internally, but the underlying formula only works correctly when W, H, and T share one consistent unit.
Why doesn’t my calculated impedance match my fabricator’s number?
IPC-2141A is a closed-form approximation, accurate for W/H between 0.1 and 2.0. Fabricators use field solvers with your actual stackup, copper roughness, and solder mask thickness, which shifts real impedance by a few ohms.
What is effective dielectric constant (eeff) and why is it lower than er?
Part of a microstrip’s electric field passes through air above the trace and part through the substrate below it. eeff blends the two, so it always falls between 1 and the substrate’s er.
Does trace thickness (T) matter much for impedance?
Yes, especially for thin, high-impedance traces. Thicker copper effectively widens the conductor, lowering Z0 slightly; IPC-2141A folds T into the denominator alongside 0.8W to capture this.
Should I account for solder mask when targeting a specific impedance?
Yes, on outer layers. Solder mask increases dielectric loading around the trace and typically lowers real impedance a few ohms versus the bare-board calculation, so many designers target slightly higher than their final spec.