PCB Impedance Calculator
Characteristic impedance for microstrip, stripline and differential pairs — classic IPC-2141 formulas.
What it does: Calculate the characteristic (and differential) impedance of a PCB trace from your stackup.
When to use it: When routing 50Ω/90Ω/100Ω controlled-impedance traces — USB, HDMI, Ethernet, RF — before fabrication.
Disclaimer: This result is a reference estimate. For actual production, refer to the device datasheet / local regulations as authoritative.
⚠ W/H is outside 0.1–2.0, where this microstrip approximation is most accurate — treat the result as rough.
MEANS This trace has a characteristic impedance of about —. Tune W and H to hit your target (commonly 50Ω single-ended, 90–100Ω differential).
No history yet. Each calculation is automatically saved to this device.
How to use the impedance calculator
Pick the trace type and enter your stackup dimensions.
- 01
Pick the trace type
Microstrip = a signal on an outer layer over one reference plane; stripline = a signal on an inner layer between two planes; diff = a coupled pair of traces.
- 02
Enter the stackup dimensions
Trace width
W, dielectric heightH(orBfor stripline), copper thicknessT, and the dielectric constantEr(FR-4 ≈ 4.3). - 03
Read Z0 (and Zdiff for pairs)
For a differential pair also enter the edge-to-edge spacing
S, then click Calculate to read the characteristic impedance Z0 and, for pairs, the differential impedance Zdiff.
Typical dielectric constant (Er / Dk)
Er is the relative permittivity of the dielectric; it is material- and frequency-dependent.
| Material | Er (≈) | Note |
|---|---|---|
| FR-4 (standard) | 4.2 – 4.6 | Frequency-dependent; ask your fab for the Dk at your frequency |
| Rogers RO4350B | ~3.48 | Low-loss RF laminate |
| Polyimide | ~3.5 | Flex |
| PTFE / Teflon | ~2.1 | High-frequency |
Er is material- and frequency-dependent; enter your fab’s value. No fabricated stack data.
Common questions, answered in 3 minutes
What is the difference between microstrip and stripline?
Microstrip is a trace on an outer layer running over a single reference plane, with one side exposed to air. Stripline is a trace on an inner layer sandwiched between two reference planes, so it is more shielded and its impedance depends only on the dielectric, not the air above.
Why do I get 50Ω for some widths?
50Ω single-ended is the most common controlled-impedance target. To hit it, widen or narrow the trace width W, or change the dielectric height H — Z0 rises as the trace gets thinner or the dielectric thicker.
How accurate is this calculator?
It uses the classic IPC-2141A closed-form approximations, which are good first-pass estimates only. Real impedance depends on the exact stackup, copper roughness and solder mask, and needs a 2-D field solver plus the fab’s measured Dk — always confirm the impedance spec with your board house.
What spacing sets 100Ω differential?
Differential impedance Zdiff drops as the two traces get closer together, so tune the edge-to-edge spacing S (and W/H). USB, HDMI and Ethernet pairs typically target about 90–100Ω.
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| Microstrip Z0 | 87/√(Er+1.41)·ln(5.98H/(0.8W+T)) | IPC-2141A |
| Stripline Z0 | 60/√Er·ln(4B/(0.67π(0.8W+T))) | IPC-2141A |
| Differential | 2·Z0·(1−k·e^(−a·S/H)) | IPC-2141A edge-coupled |
Classic IPC-2141A closed-form approximations (first-pass estimates). For production use a 2-D field solver and confirm the impedance spec and stackup with your board house.