Tools / PCB impedance calculator
PCB impedance calculator
Impedance of microstrip, stripline, differential pairs and grounded coplanar waveguide, or the trace width that hits your target. Every model is checked against a 2D field solver.
Pick a structure and enter your stackup. Switch to Trace width to solve for a target such as 50 Ω single-ended or 100 Ω differential.
Characteristic impedance
–
- Trace width
- Characteristic impedance
- Differential impedance
- Odd-mode impedance
- Single trace (uncoupled)
- Effective dielectric constant
- Propagation delay
- Capacitance
- Inductance
How the calculator works
Each structure uses a closed-form model from the transmission-line literature, checked against our own field solver.
We ran a 2D finite-difference field solver on 136 geometries across the five structures. The calculator agrees with it within 3%, and most results within 1.5%. Outside the checked range of widths, spacings and gaps, the calculator tells you so.
Microstrip
Hammerstad and Jensen (1980), including their correction for copper thickness. It is the standard reference model for a single trace over a plane.
Stripline
Wheeler’s formula for a trace centered between two planes, with copper thickness. At zero thickness it matches Cohn’s exact solution to within 0.5%.
Differential pairs
Edge-coupled microstrip uses Garg and Bahl’s capacitance model for the odd mode; edge-coupled stripline uses Cohn’s exact coupled-line solution. Both add a copper-thickness correction that we fitted to field-solver results. We did that because the common IPC-2141 shortcut was off by up to 10% on the same test set.
Zdiff = 2 × Zodd
Grounded coplanar waveguide
Conformal mapping for a trace with coplanar grounds and a solid plane below (Wadell), plus a fitted copper-thickness correction. It assumes wide coplanar grounds stitched to the plane with vias.
Delay, capacitance and inductance
tpd = √εeff / c C = √εeff / (c × Z0) L = Z0 × √εeff / c
| Structure | Geometries checked | Largest difference | Typical (RMS) |
|---|---|---|---|
| Microstrip | 16 | 2.2% | 1.4% |
| Stripline | 16 | 1.5% | 0.6% |
| Differential microstrip | 36 | 2.3% | 1.4% |
| Differential stripline | 36 | 0.7% | 0.4% |
| Grounded coplanar waveguide | 36 | 2.9% | 1.7% |
These are quasi-static results. They leave out soldermask, which lowers outer-layer impedance by a few ohms, along with copper roughness and the way the dielectric constant changes with frequency. Final controlled-impedance widths should come from your fabricator’s stackup and field solver.
Trace widths for common targets
Widths from the calculator for an FR-4 dielectric constant of 4.2. Differential pairs use a spacing equal to the dielectric height.
Microstrip, 1 oz copper
| Dielectric height | 50 Ω single-ended | 90 Ω differential | 100 Ω differential |
|---|---|---|---|
| 3 mil (0.076 mm) | 5.0 mil (0.13 mm) | 3.5 mil (0.088 mm) | 2.6 mil (0.065 mm) |
| 4 mil (0.10 mm) | 6.9 mil (0.18 mm) | 5.2 mil (0.13 mm) | 4.0 mil (0.10 mm) |
| 5 mil (0.13 mm) | 8.8 mil (0.22 mm) | 7.0 mil (0.18 mm) | 5.5 mil (0.14 mm) |
| 6 mil (0.15 mm) | 10.8 mil (0.27 mm) | 8.8 mil (0.22 mm) | 6.9 mil (0.18 mm) |
| 8 mil (0.20 mm) | 14.6 mil (0.37 mm) | 12.4 mil (0.31 mm) | 9.9 mil (0.25 mm) |
| 10 mil (0.25 mm) | 18.5 mil (0.47 mm) | 16.0 mil (0.41 mm) | 12.8 mil (0.33 mm) |
Stripline, 0.5 oz copper, dielectric to each plane
| Dielectric height | 50 Ω single-ended | 90 Ω differential | 100 Ω differential |
|---|---|---|---|
| 4 mil (0.10 mm) | 3.0 mil (0.075 mm) | 2.8 mil (0.070 mm) | 2.0 mil (0.051 mm) |
| 5 mil (0.13 mm) | 3.9 mil (0.098 mm) | 3.7 mil (0.095 mm) | 2.8 mil (0.071 mm) |
| 6 mil (0.15 mm) | 4.8 mil (0.12 mm) | 4.7 mil (0.12 mm) | 3.6 mil (0.091 mm) |
| 8 mil (0.20 mm) | 6.7 mil (0.17 mm) | 6.7 mil (0.17 mm) | 5.2 mil (0.13 mm) |
| 10 mil (0.25 mm) | 8.5 mil (0.22 mm) | 8.7 mil (0.22 mm) | 6.8 mil (0.17 mm) |
On a 62 mil (1.6 mm) two-layer board, a 50 Ω microstrip is about 121 mil (3.1 mm) wide. Grounded coplanar waveguide with 12 mil gaps brings that down to about 62 mil (1.6 mm), which is why RF feeds on two-layer boards often use it.
Common questions
What dielectric constant should I use?
Your fabricator’s value for the exact laminate and glass style, at your signal frequency. FR-4 runs from about 3.8 to 4.6; resin-rich prepreg sits at the low end, and the value drops a little as frequency rises.
Why does my fabricator quote a different width?
Fabricators tune widths to their own pressed dielectric thickness, resin content and etch profile. Etched traces are trapezoids, narrower on top than at the base. Use this calculator for the starting point and let the fabricator adjust within your tolerance, usually ±10%.
Does soldermask change the impedance?
Yes, on outer layers. Soldermask over a microstrip lowers its impedance by a few ohms, so outer-layer pairs are often drawn slightly narrower to compensate. Stripline is not affected.
Which structure do I need for USB, Ethernet or HDMI?
Those are differential pairs. USB 2.0 targets 90 Ω differential; Ethernet, LVDS and HDMI target 100 Ω. Use differential microstrip on outer layers and differential stripline on inner layers.
When should I use coplanar waveguide?
For RF feeds, especially on two-layer or thick boards where a microstrip would be too wide. Ground copper beside the trace pulls the impedance down and keeps the field contained. Stitch the coplanar ground to the plane with vias along both sides of the trace.
Does every trace need impedance control?
No. It matters once a trace is long compared with the signal’s edge rate: roughly when the one-way delay passes a sixth of the rise time. On FR-4, a 1 ns edge needs controlled impedance on traces longer than about an inch.
Can Evertech lay out my controlled-impedance board?
Yes. We plan stackups with the fabricator and lay out high-speed pairs and RF feeds and antennas as part of our PCB layout work, and our production team builds the boards here in Cedarburg. Send your files for a quote.
Designing a controlled-impedance board?
Our engineers plan stackups with the fabricator and lay out high-speed pairs and RF feeds alongside our own production team in Cedarburg, Wisconsin.