Free PCB impedance calculator
PCB Impedance Calculator: Microstrip, Stripline & Differential
Calculate characteristic and differential impedance for microstrip, stripline and edge-coupled pairs. It is free, runs in your browser and needs no sign-up.
Calculator
Impedance calculator.
Z₀ single-ended
- Single-ended Z₀
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- Propagation delay
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- Model check
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Microstrip: Hammerstad–Jensen with thickness correction. Stripline and differential: IPC-2141 approximations. Results are estimates; we confirm final geometry with a field solver and TDR coupons.
How to use
How to use this impedance calculator.
- Choose the structure: microstrip (outer layer) or stripline (inner layer), single-ended or differential.
- Enter trace width, copper weight and dielectric height (or plane spacing for stripline).
- Enter the laminate Dk at your frequency, for example about 4.2 for FR-4 or 3.48–3.66 for Rogers 4350B.
- Adjust width (and spacing) until you reach the target.
Structures
Microstrip vs stripline.
| Structure | Where | Notes |
|---|---|---|
| Microstrip | Outer layer over one plane | Field partly in air; lower effective Dk; solder mask lowers Z by ~1–3 Ω |
| Stripline | Inner layer between two planes | Fully in dielectric; better shielding |
| Differential pair | Two coupled traces | Spacing S sets coupling and Zdiff |
Common targets
Common impedance targets.
| Interface | Target |
|---|---|
| RF, general single-ended | 50 Ω |
| USB 2.0 / 3.x | 90 Ω differential |
| PCIe, Ethernet, SATA | 85–100 Ω differential |
| HDMI, LVDS, MIPI | 100 Ω differential |
| DDR4/5 single-ended | ≈ 40–50 Ω |
Transmission line configurations
Transmission line configurations.
Microstrip is a trace on an outer layer over one reference plane, with its field partly in air, so its effective Dk is lower than the substrate’s. Embedded or coated microstrip sits under solder mask or prepreg, which raises effective Dk and lowers impedance. Allow for mask Dk of about 3.5–4.5 on outer layers. Stripline sits between two planes with its field entirely in the dielectric, which gives better shielding and less radiation but needs two reference planes.
Edge-coupled differential pairs add a spacing term S. Closer spacing raises coupling and lowers differential impedance. Broadside-coupled pairs on adjacent layers are possible but sensitive to layer registration.
Using the results
How to use the results.
- Treat results as a starting width; final width comes from the stack-up we return
- Use the laminate’s Dk at your signal frequency, not a generic value
- Check model validity — outside the range, use a field solver
- Add 1–3 Ω margin on uncoated microstrip if the board will have solder mask
- Specify the target and tolerance on the drawing — see controlled impedance
Worked examples
Worked examples.
| Structure | Inputs | Approx. result |
|---|---|---|
| Microstrip, FR-4 | W 10 mil, H 5.3 mil, 1 oz, Dk 4.2 | ≈ 48 Ω |
| Microstrip, RO4350B | W 20 mil, H 10 mil, 1 oz, Dk 3.66 | ≈ 51 Ω |
| Stripline, FR-4 | W 5 mil, B 16 mil, 0.5 oz, Dk 4.2 | ≈ 50 Ω (estimate) |
Use these as sanity checks. If your design lands far from them with similar inputs, re-check units (mil vs mm) and copper weight.
Accuracy
How accurate is it?
Closed-form equations are typically within a few percent for normal geometries. They ignore etch trapezoid, copper roughness, resin-rich areas and frequency-dependent Dk. For production we model your actual stack-up and verify with TDR to ±8%. See controlled impedance.
FAQ
Tool questions.
How do I calculate PCB impedance?
Enter trace width, copper thickness, dielectric height and Dk; the calculator applies standard closed-form equations for the chosen structure.
What Dk should I use for FR-4?
About 4.0–4.5 depending on resin content and frequency; use the laminate datasheet value.
Why does my fabricator change the trace width?
Fabricators adjust width or dielectric to hit impedance on real materials; approve changes before fabrication.
Is this impedance calculator free?
Yes, free and browser-based with no sign-up.
Microstrip or stripline?
Microstrip on outer layers is easier to probe and tune; stripline on inner layers shields better and radiates less.
Does solder mask change impedance?
Yes, it typically lowers outer-layer microstrip impedance by about 1–3 Ω.
Ready when your files are
Done designing? Get it built.
Upload Gerber, ODB++ or IPC-2581 data. Every quote includes a written engineering DFM review, typically within one business day.