Microstrip Impedance Calculator

Quickly estimate the characteristic impedance of an external-layer PCB trace using the IPC-2141 microstrip formula. A fast single-geometry check — for full stackup design, see our complete impedance calculator.

Microstrip Parameters

Trace Width (W) *

Dielectric Height (H) *

Copper Thickness (T) *

1oz ≈ 1.378 mil (35μm) finished copper.

Dielectric Constant (E_r) *

Uses the IPC-2141 simplified microstrip formula. See our Trace Width Calculator for current-carrying capacity, or the full stackup impedance tool for a manufacturable stackup match.

Estimated Microstrip Impedance

Characteristic Impedance

0.00 Ω

Z₀, single-ended microstrip

Effective Dielectric Constant (ε_eff) 0.00

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Calculation Reference

This calculator uses the IPC-2141 simplified closed-form formula for microstrip: a trace on an external layer, referenced to a ground plane below it, with the top of the trace exposed to air or solder mask.

Z₀ = 87/√(E_r+1.41) × ln( 5.98H / (0.8W+T) )

ε_eff = (E_r+1)/2 + (E_r−1)/2 × 1/√(1+12H/W)

Source: IPC-2141, "Controlled Impedance Circuit Boards and High Speed Logic Design."

Frequently Asked Questions

What is microstrip, and when is a trace a microstrip instead of a stripline?

Microstrip is a trace routed on an external (top or bottom) layer, referenced to a ground plane below it, with the other side of the trace exposed to air or solder mask rather than embedded in dielectric on both sides. That partial exposure to air is what distinguishes microstrip's impedance behavior from stripline, which is fully embedded in dielectric between two reference planes.

How is microstrip impedance calculated?

This calculator uses the IPC-2141 simplified microstrip formula, Z₀ = (87 / √(E_r + 1.41)) × ln(5.98H / (0.8W + T)), where H is the dielectric height to the reference plane, W is trace width, T is copper thickness, and E_r is the dielectric constant. It's a closed-form approximation, not a full field solver, so treat it as a fast first-pass estimate for early routing decisions.

Why does exposure to air matter for microstrip impedance?

Because part of the trace's electric field passes through air (dielectric constant near 1) instead of through the PCB laminate, microstrip's effective dielectric constant is lower than the laminate's bulk E_r, which is why the IPC-2141 microstrip formula uses E_r+1.41 rather than E_r alone. This also means microstrip is more sensitive to solder mask coverage than stripline, since a layer of solder mask over the trace raises the effective dielectric constant slightly and nudges impedance down.

What copper thickness (T) should I use?

Use your finished copper weight converted to thickness: roughly 1.4 mil (35 micrometers) for 1oz copper, or about 2.8 mil (70 micrometers) for 2oz copper, adjusted slightly for your fabricator's actual finished copper. Thicker copper very slightly lowers impedance for a given trace width, since it adds a bit of extra conductor perimeter for the field to couple to.

Should I use this or the full NextPCB stackup impedance tool?

Use this calculator for a quick single-geometry estimate when you already know roughly what stackup you're working with. If you're still deciding on layer count, board thickness, or copper weights, or you want trace width and spacing solved directly against one of NextPCB's real, manufacturable stackups, use the full PCB Controlled Impedance Calculator instead, which also returns a recommended stackup rather than just a geometry.

How accurate is this compared to a 2D field solver?

IPC-2141's closed-form equations are widely used for early-stage sizing and are generally within a few percent of a full field solver for typical PCB geometries, but they don't model the trapezoidal etch profile of real copper, solder mask precisely, or glass-weave effects in the laminate. For tightly toleranced designs, verify the final geometry with your fabricator's field solver or a TDR-tested impedance coupon before committing to production.