What is characteristic impedance and why does it matter?

Characteristic impedance (Z0) is the ratio of voltage to current for a wave traveling along a transmission line. When a PCB trace impedance does not match the driver or load (typically 50 Ω single-ended, 100 Ω differential), signal reflections occur, causing data errors, overshoot, and EMI. Controlled-impedance traces eliminate these issues at frequencies above ~100 MHz.

What is the difference between microstrip and stripline?

A microstrip is a trace on the outer layer of the PCB with air (or solder mask) above it and a dielectric + reference plane below. A stripline is a trace buried between two reference planes, fully surrounded by dielectric. Striplines have lower radiation and better EMI performance; microstrips are easier to route and inspect.

What formula does this calculator use?

Microstrip uses the IPC-2141A approximation: Z0 = (87 / sqrt(Er + 1.41)) × ln(5.98H / (0.8W + T)). Stripline uses the Wadell formula: Z0 = (60 / sqrt(Er)) × ln(4b / (0.67π × (0.8W + T))) where b is the distance between both reference planes. Both are accurate within ~1–2 % for typical PCB geometries.

What dielectric constant should I use for FR4?

Standard FR4 has Er between 4.0 and 4.5 at 1 MHz. At higher frequencies (1–10 GHz) the effective Er drops to roughly 3.8–4.2 due to dispersion. Many PCB fabs use Er = 4.3 as their nominal value. Check your fabricator's datasheet for the specific laminate you are using.

Why is my calculated impedance different from my fabricator's value?

Fabricators measure finished impedance on test coupons after etching, plating, and lamination. Actual trace width narrows slightly during etching (undercut), copper plating adds thickness, and resin fill compresses dielectric height. Expect ±5–10 % variation. Use the calculator to get close, then confirm with your fab's controlled-impedance service.

What trace width gives 50 ohms on a standard 4-layer FR4 board?

On a typical 4-layer board with 7-mil dielectric (top layer to L2 ground plane), 1 oz copper, and Er = 4.3, a microstrip trace of approximately 13–14 mils achieves 50 Ω. The exact value depends on your fab's stackup — always request the actual layer thickness from your manufacturer.

PCB Impedance Calculator

Calculate characteristic impedance of microstrip and stripline PCB traces for high-speed signal integrity design.

Trace Parameters

Trace Width (W)

Dielectric Height (H) — trace to reference plane

Plane Separation (b) — distance between both reference planes

Trace Thickness (T)

1 oz copper ≈ 1.4 mils (35 µm) | 2 oz ≈ 2.8 mils

Dielectric Constant (Er)

FR4 ≈ 4.0–4.5 | Rogers RO4003C ≈ 3.55 | Isola 370HR ≈ 4.04

Characteristic Impedance (Single-Ended)

Enter parameters and click Calculate

Common Impedance Targets

This calculator computes single-ended Z₀. For differential signals (USB, Ethernet, PCIe), each uncoupled trace targets roughly half the differential specification (~42–50 Ω).

Standard	Single-Ended	Differential
USB 2.0	—	90 Ω
USB 3.x / USB4	—	85 Ω
HDMI / DisplayPort	—	100 Ω
PCIe (all generations)	—	85 Ω
Gigabit Ethernet	—	100 Ω
RF / SMA / coax	50 Ω	—
75 Ω video (SDI)	75 Ω	—
DDR4 / DDR5 data	40–50 Ω	—

Summary

This calculator determines the characteristic impedance of PCB transmission lines using industry-standard formulas for microstrip (surface traces) and stripline (buried traces). Accurate impedance control is critical for high-speed signals — USB, Ethernet, PCIe, HDMI, and RF — to prevent reflections, signal degradation, and EMI. Enter your stackup parameters and get the impedance in ohms immediately.

How it works

Select the transmission line type: Microstrip (trace on outer layer) or Stripline (trace buried between reference planes).
Enter the trace width (W) in mils or millimeters.
Enter the dielectric height (H) — distance from trace to nearest reference plane.
Enter the trace thickness (T) and the dielectric constant (Er) of your PCB material.
Click Calculate — the tool applies IPC-2141A / Wadell formulas and returns the characteristic impedance Z0 in ohms.

Use cases

Design 50-ohm single-ended traces for RF, SMA connectors, and antenna feeds.
Size ~45-ohm traces as single-ended reference for 90-ohm differential pairs (USB 2.0).
Size ~50-ohm traces as single-ended reference for 100-ohm differential pairs (Ethernet, HDMI).
Verify existing stackup impedance before sending a board to fabrication.
Reverse-engineer required trace width for a target impedance.
Compare microstrip vs. stripline tradeoffs for a given stackup.
Teach PCB signal integrity concepts in an educational setting.
Cross-check fabricator impedance calculations independently.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu