IPC-2141A · controlled impedance

PCB Impedance Calculator

Estimate microstrip, stripline, grounded coplanar or differential impedance, or solve a microstrip width for a target. Each mode uses a stated closed-form approximation; final geometry depends on the actual stackup. IPC currently lists IPC-2141 as “No Longer Maintained”, so it is used here only as a historical/reference formula source.

Maintained by Giorgi Gaprindashvili Published June 27, 2026 Updated September 18, 2026 Editorial check September 2026

Solve for

Trace width (W) ?

Dielectric height (H) ?

Dielectric constant (er) ?

Method based on IPC-2141A (2004; IPC lists the document as No Longer Maintained) · editorial check September 2026 · method Piecewise quasi-static microstrip approximation · revision 1.1

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 is the microstrip trace-to-plane height; B is the total separation between the stripline reference planes.

Microstrip width for common targets on FR-4

Single-ended microstrip estimates at relative permittivity 4.3 using this tool’s thin-conductor approximation. A single-ended target is not a differential-pair specification.
Target Z0	W / H	Interpretation
40 Ω	2.78	Single-ended example target
50 Ω	1.96	Single-ended example target
60 Ω	1.42	Single-ended example target
75 Ω	0.91	Single-ended example target
90 Ω	0.60	Single-ended example target
100 Ω	0.45	Single-ended example target

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.

For microstrip, H is the trace-to-reference-plane dielectric height. For the symmetric stripline approximation, B is the total spacing between the two planes. Neither dimension should be replaced with overall board thickness unless it actually matches the stackup.

The microstrip approximation used here

The microstrip engine uses piecewise thin-conductor equations, with a separate effective-permittivity approximation. It is not the complete Hammerstad–Jensen model.

The calculation omits finite-copper-thickness corrections, solder mask and frequency-dependent dispersion. The width solver accepts W/H from 0.1 to 10 and checks that the target is bracketed and the solution converges. These are implementation limits, not an accuracy guarantee.

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.

Worked examples

Real sizing calls, and the number that decides each one.

Scenario	Result	Why
50 Ω microstrip on FR-4 (er 4.3)	W/H about 1.95	The trace width needs to be roughly twice the height to the reference plane; effective permittivity is about 3.3, between air and the laminate.
That same 50 Ω line, signal delay	about 153 ps/inch	Propagation delay scales with the square root of effective permittivity; at about 3.3 a signal takes roughly 153 ps to cross each inch.
W/H = 1 microstrip on FR-4	about 72 Ω	Halving the width-to-height ratio from the 50 Ω point raises impedance to about 72 Ω. Impedance is set by the cross-section, not the length.

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-2221C is the actively revised generic board-design standard. IPC-2141A is a historical controlled-impedance guide that IPC currently lists as “No Longer Maintained”.

A preliminary stackup estimate. Microstrip uses a piecewise thin-conductor approximation, not the full Hammerstad–Jensen model. Solder mask, surface roughness, etching shape and dispersion are not modeled. Confirm final dimensions and impedance with the fabricator’s actual stackup and field solver.

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?

H is the microstrip trace-to-plane dielectric height. B in stripline mode is the total separation of both reference planes. Read these dimensions from the actual stackup.

How accurate is the differential result?

The differential mode is a preliminary coupling approximation. It does not justify a universal accuracy percentage. Confirm trace width and pair spacing against the required interface impedance using the fabricator’s actual stackup and 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 (2004), historical controlled-impedance guide; IPC revision table lists IPC-2141 as No Longer Maintained · Qucs technical documentation: full Hammerstad-Jensen model, for comparison with the simpler implementation used here · Coplanar waveguide impedance by conformal mapping (Steer, Microwave and RF Design II). Verify against the current edition.