IPC-2141A · controlled impedance

PCB Impedance Calculator

Find the characteristic impedance of a microstrip, stripline or grounded coplanar (CPWG) trace, the differential impedance of an edge-coupled pair, or the trace width that hits a target impedance. Microstrip uses the Hammerstad-Jensen equations, the accurate set, not the IPC-2141 microstrip form that most calculators still use.

Solve for

Trace width (W) ?

Dielectric height (H) ?

Dielectric constant (er) ?

Method based on IPC-2141A (2004, errata to 2014) · reviewed June 2026 · method rev Hammerstad-Jensen microstrip

Microstrip and stripline cross-sections

A microstrip sits on an outer layer over one reference plane, with part of its field in air; a stripline runs between two planes, fully inside the dielectric. H and B are measured from the trace to the plane, not the full board thickness.

Microstrip width for common targets on FR-4

Single-ended microstrip on FR-4 (er 4.3), Hammerstad-Jensen, computed by this tool. Example: at H = 4 mil, 50 ohm needs W about 7.8 mil. Differential pairs reach 90 or 100 ohm by coupling two of these traces; spacing sets the final value.
Target Z0	W / H	Common use
40 Ω	2.78	Some DDR
50 Ω	1.96	RF, digital
60 Ω	1.42	Single-ended
75 Ω	0.91	Video, RF
90 Ω	0.60	USB, HDMI pair
100 Ω	0.45	Ethernet, PCIe

When a trace becomes a transmission line

At low frequencies a trace is just a wire. Once the signal's rise time is short enough that the trace is an appreciable fraction of a wavelength, the trace behaves as a transmission line and its characteristic impedance starts to matter. Mismatched impedance reflects energy back along the line, which shows up as ringing, overshoot and, on fast serial links, a closed eye and intermittent failures.

Characteristic impedance is set by the cross-section geometry: the trace width, the height to the reference plane, the copper thickness and the dielectric constant. It does not depend on the trace length.

Microstrip versus stripline

A microstrip runs on an outer layer with a single reference plane beneath it, so part of its field is in the board and part is in the air. That makes it easy to route and probe, but it radiates more and is sensitive to whatever sits above it. A stripline runs on an inner layer between two planes, fully inside the dielectric: better shielded and quieter, but it needs inner-layer routing and its field is entirely in the laminate, so its effective permittivity equals the full dielectric constant.

The height that matters is the distance from the trace to its reference plane, not the overall board thickness. Getting that one number wrong is the most common reason a calculated impedance does not match the board.

Why this tool uses Hammerstad-Jensen for microstrip

Many online calculators use the IPC-2141 microstrip equation because it is simple. It is also known to be inaccurate: it drifts badly for wide traces and even returns a nonsense negative impedance once the width-to-height ratio passes about 7.5. The Hammerstad-Jensen equations, which this tool uses, stay within roughly one percent across the practical range.

We still use the IPC-2141 form for stripline, where it holds up well, and for the differential coupling factor. The microstrip result here also assumes thin copper; real copper thickness lowers impedance by a small amount that a field solver accounts for.

Differential pairs and field solvers

A differential pair carries equal and opposite signals on two coupled traces. The differential impedance comes from the single-ended impedance of one trace plus the coupling between the pair, set by the spacing. This tool reports the differential, odd, even and common-mode values for an edge-coupled microstrip pair.

Closed-form differential models disagree with each other by ten to twenty-five percent, more than the spread on single-ended formulas. Treat the differential number as a starting point for sizing and confirm the final geometry against your fabricator's field solver and stackup, which are authoritative.

How this relates to other standards

Standard / tool	Relationship	What it means
PCB trace width calculator	same cross-section	Sets width for current and heat; this tool sets width for impedance. High-speed traces have to satisfy both.
Via current capacity	same signal path	Vias break the controlled-impedance reference; keep them short and well-referenced on high-speed nets.
Trace resistance	same trace	Resistance governs DC drop and loss; impedance governs reflections. Different limits on the same copper.
IPC-2221	companion standard	IPC-2221 covers width for current; IPC-2141A covers controlled impedance. High-speed boards use both.

A first-pass estimate for sizing and stackup discussion, not a substitute for your fabricator's field solver. Microstrip uses Hammerstad-Jensen and assumes thin copper; the model does not include solder mask, copper surface roughness, trapezoidal etching or frequency dispersion. Differential results follow a closed-form coupling model and can differ from a 2D solver by ten to twenty-five percent. Confirm final geometry against the impedance-controlled stackup from your board house.

Where engineers use this

USB and HDMI routing

USB 2.0 and 3.0 and HDMI run as 90 ohm differential pairs. The tool sizes width and spacing for the target before the layout is committed.

Ethernet, PCIe and SATA

These high-speed serial links route as 100 ohm differential pairs; missing the target by even ten percent degrades the eye diagram at multi-gigabit rates.

RF and 50 ohm lines

Antennas, connectors and most RF building blocks expect a 50 ohm single-ended environment, so microstrip and stripline feeds get sized to 50 ohm.

Specifying a stackup to the fab

Turning a target impedance into a first-pass width and height gives a concrete starting point for the impedance-controlled stackup discussion with the fabricator.

Frequently asked questions

What is characteristic impedance on a PCB?

It is the impedance a fast signal sees as it travels along a trace, set by the trace's cross-section: width, height to the reference plane, copper thickness and dielectric constant. It does not depend on length. Matching it end to end prevents reflections that cause ringing and signal-integrity failures.

What width gives 50 ohm on FR-4?

For a microstrip on FR-4 (er about 4.3) the width is close to twice the dielectric height, a W/H ratio near 1.96. So over a 4 mil dielectric the trace is about 7.8 mil wide. The exact number depends on the laminate and copper, so use the Find width mode with your real stackup values.

What is the difference between microstrip and stripline?

A microstrip is on an outer layer over one plane, with part of its field in air; it is easy to route but radiates more. A stripline is buried between two planes, fully in the dielectric; it is better shielded and quieter but needs inner layers. For the same target impedance a stripline is narrower than a microstrip.

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

Closed-form equations assume thin copper and ignore solder mask, copper surface roughness, trapezoidal etching and frequency dispersion. A fabricator's 2D field solver includes those, so expect a few percent difference on single-ended lines and more on differential pairs. The fab's stackup and solver are authoritative.

Which dielectric height do I enter?

The distance from the trace to its nearest reference plane, taken from the stackup, not the overall board thickness. On a typical four-layer board the signal-to-plane height is far smaller than the board thickness, and using the wrong one is the most common cause of a wrong answer.

How accurate is the differential result?

It is a first-pass estimate. Single-ended microstrip from Hammerstad-Jensen is within about one percent, but closed-form differential models disagree by ten to twenty-five percent. Use the differential value to get close, then confirm width and spacing with the fabricator's field solver.

What is grounded coplanar waveguide (CPWG)?

CPWG places the signal trace between two coplanar ground pours on the same layer, with a full ground plane underneath. The fields are tightly confined, which helps at RF and microwave frequencies and lets you route without via transitions. The gap to the adjacent ground sets the impedance along with the width and height, so a tighter gap lowers it. This tool uses the conductor-backed CPW equations with exact elliptic integrals.

Guides

Microstrip vs striplineWhen to use each, and how to read the numbers.

Sources: IPC-2141A, Design Guide for High-Speed Controlled Impedance Circuit Boards (2004, errata to 2014) · Why Hammerstad-Jensen and Wheeler are more accurate than the IPC-2141 microstrip equation (Altium) · Coplanar waveguide impedance by conformal mapping (Steer, Microwave and RF Design II) · IEEE 370, electrical characterization of PCB interconnects (measurement reference). Verify against the current edition.