PCB Trace Impedance Calculator
Estimate controlled impedance for PCB traces including microstrip, stripline and differential pair routing from trace width, dielectric height, copper thickness, spacing and dielectric constant.
📐 PCB Rule: Controlled impedance is not decided by trace width alone. It depends on the full PCB stackup: trace width, dielectric height, copper thickness, dielectric constant and reference plane geometry. Use this with the PCB Trace Width Calculator, PCB Via Current Calculator, Reactance Calculator and LC Resonance Calculator.
📐 Trace Width + Dielectric Height + εr → Controlled Impedance
Trace Width
Dielectric Height
Copper Thickness
Dielectric Constant εr
Frequency Optional
Board Material
Microstrip means a trace on an outer PCB layer above a reference plane. This uses a practical closed-form approximation for quick design estimates.
Presets:
Trace Width
Plane-to-Plane Height
Copper Thickness
Dielectric Constant εr
Structure
Stripline means the trace is buried between two reference planes. The plane-to-plane spacing strongly affects impedance.
Presets:
Single Trace Width
Trace Spacing
Dielectric Height
Copper Thickness
Dielectric Constant εr
Pair Type
For USB, HDMI, Ethernet and high-speed digital lines, differential impedance is often more important than single-ended impedance.
Presets:
Target Impedance
Dielectric Height
Copper Thickness
Dielectric Constant εr
Trace Type
This reverse solver estimates the trace width required for a target impedance by numerical search. Confirm final values with your PCB fabricator’s stackup calculator.
Targets:
📐 Formula Reference
Microstrip Approx.
Z₀ ≈ 87 / √(εr + 1.41) × ln(5.98h / (0.8w + t))
Stripline Approx.
Z₀ ≈ 60 / √εr × ln(4b / (0.67π(0.8w + t)))
Differential Microstrip
Zdiff ≈ 2Z₀ × (1 − 0.48e−0.96s/h)
Signal Velocity
v ≈ c / √εeff
📋 Quick Reference
Common Targets
RF / coax style50Ω
Video / antenna75Ω
USB differential90Ω diff
LVDS / Ethernet pair100Ω diff
FR-4 Guide
εr rough range3.8–4.6
1 oz copper35 µm
0.5 oz copper17 µm
Design Notes
Higher widthlower Z
Higher heighthigher Z
Higher εrlower Z
📚 Engineering Notes
Use the actual PCB stackupControlled impedance depends on the manufacturer’s dielectric thickness and material data, not just the board’s total thickness.
50Ω is not universal50Ω is common for RF single-ended traces. Digital buses may need different values, especially differential pairs.
FR-4 εr changes with frequencyFor high-speed or RF work, ask the PCB fab for the exact dielectric constant and loss tangent for your stackup.
Differential pairs need spacing controlChanging pair spacing changes coupling and differential impedance, even if the single-ended width stays the same.
What is PCB trace impedance?
PCB trace impedance is the effective AC impedance seen by a fast signal travelling along a PCB conductor and its return path. For high-speed digital, RF, USB, HDMI, Ethernet and antenna circuits, the trace geometry should match a target impedance to reduce reflections.
Microstrip vs stripline
A microstrip trace is on an outer layer above a reference plane. A stripline trace is buried between reference planes. Stripline usually has better shielding, while microstrip is easier to access and manufacture.
Important limitation
This calculator is a practical estimate. For production controlled-impedance boards, always confirm the final width, spacing and stackup with your PCB manufacturer.
❓ Frequently Asked Questions
It depends mainly on dielectric height to the reference plane, copper thickness and εr. A 2-layer 1.6 mm board may need a very wide trace, while a 4-layer board with a thin prepreg can use a much narrower trace.
For slow signals and short traces, exact impedance is usually not critical. It becomes important when signal rise time is fast compared with trace delay, or for RF and high-speed differential interfaces.
50Ω usually refers to one single-ended trace referenced to ground. 90Ω differential refers to the impedance between two coupled traces carrying opposite signals.
Yes, solder mask can slightly lower outer-layer microstrip impedance. For critical work, include solder mask details in a manufacturer-controlled stackup calculation.
Use them for planning and sanity checks. For final production, use the PCB fabricator’s controlled impedance stackup because they know the exact materials, prepreg thickness and process compensation.